Chemical Equations and Conservation of Mass: a full term of science
Ten ready-to-teach lessons for Year 9 Chemical sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards or exit tickets, an assessment kit and worked answers.
Start here: five minutes to Monday
- Skim the term at a glance on the next page.
- Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards or tickets.
- Gather the few items under “You need” on the plan. Most come from the science prep room or the supermarket.
- Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
- Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.
For any Year 9 science teacher
The pack suits a chemistry specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: in a chemical reaction the atoms of the reactants are not made or destroyed; they come apart and join up again in new arrangements, the products. Students show this in models, word equations, balanced symbol equations, particle pictures and masses, and every one of them tells the same story: the total mass of the reactants equals the total mass of the products.
Two ways to run each lesson
Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 1 needs modelling clay in four colours, Lesson 5 needs two solutions made up the day before, and Lesson 6 is a teacher demonstration that needs a heatproof mat and a fire blanket nearby.
Safety and care in one look
Eye protection goes on for every practical. Sodium carbonate and calcium chloride solutions irritate eyes and skin, so gloves are offered and spills are wiped up. The steel wool in Lesson 6 is heated only by the teacher, on a heatproof mat, away from paper, with a fire blanket nearby. Nothing in the science room is tasted, including the effervescent tablets.
The term at a glance
One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.
| # | Lesson | Students learn and do | You need (in short) |
|---|---|---|---|
| 1 | Reading formulas | Count the atoms in eight formulas, and build three molecules from modelling clay | Modelling clay in four colours, toothpicks |
| 2 | Word equations | Write reactants and products for six reactions, and find the patterns of acid reactions | The worksheet |
| 3 | Rearranging atoms | Model methane burning with atom tokens, and find why two oxygen molecules are needed | The atom tokens |
| 4 | Balancing equations | Balance eight equations by counting atoms on each side | The worksheet |
| 5 | Nothing in, nothing out | Weigh two solutions before and after they react to make a solid | A balance, sodium carbonate and calcium chloride solutions |
| 6 | A reaction that gains mass | Watch steel wool gain mass as it burns, account for the oxygen, and sort twelve reactions by what the balance shows | Steel wool, a balance, a heatproof mat (teacher); the balance cards |
| 7 | Masses in equations | Use reacting masses to find a missing mass in four reactions | The worksheet |
| 8 | Four ways to show a reaction | Group twelve cards into three reactions shown four ways each | The representation cards |
| 9 | Faster, but no heavier | Test whether warmer water changes the mass of gas a tablet gives off, or only the speed | Effervescent tablets, a balance, timers |
| 10 | Show what we know | Balance and check the masses for a camping stove, sit the final check and play the class quiz | The task and check, the interactive unit |
How the sequence builds
Lessons 1 to 3 build the language of reactions: formulas, word equations, and atoms rearranged with tokens. Lesson 4 balances equations. Lessons 5 and 6 weigh reactions, one that stays the same and one that seems to gain mass, and Lesson 7 uses the masses in equations. Lesson 8 links the four ways of showing a reaction, Lesson 9 tests whether speeding a reaction up changes its mass, and Lesson 10 brings it together.
Curriculum links (Australian Curriculum V9)
The whole term teaches the Science Understanding descriptor AC9S9U07 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:
Assessment in this pack
- Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 9 voice, with working.
- The assessment sheet near the front has a class observation checklist and a three-level rubric.
- Lesson 10 is the summative pair: the camping stove task, plus the “Show what we know” check.
Materials for the whole term
One gathering session covers all ten lessons, and most of it is already in the science prep room. Three things are worth doing ahead: buy modelling clay in four colours for Lesson 1, make up the two solutions for Lesson 5 the day before, and check the fire blanket and heatproof mat for the Lesson 6 demonstration.
| Lesson | You need |
|---|---|
| 1 | for each pair: modelling clay in four colours (for hydrogen, oxygen, carbon and nitrogen) and a handful of toothpicks |
| 2 | the worksheet only |
| 3 | the atom tokens from this pack, one sheet per pair; scissors |
| 4 | the worksheet; the atom tokens from Lesson 3 if students want them |
| 5 | for each group: a balance reading to 0.1 g, two small beakers, 25 mL of sodium carbonate solution and 25 mL of calcium chloride solution, eye protection and gloves |
| 6 | fine steel wool, a heatproof mat, a balance reading to 0.01 g, a 9 V battery, tongs, eye protection and a fire blanket, for the teacher only; the balance cards from this pack, one set per pair; scissors |
| 7 | the worksheet only |
| 8 | the representation cards from this pack, one set per pair; scissors |
| 9 | for each group: effervescent tablets, three cups, a measuring cylinder, a thermometer, cold, room-temperature and warm (not hot) water, a balance reading to 0.01 g, a timer and eye protection |
| 10 | the task and the final check; the interactive unit open for the class quiz |
The one-trip list
- From the prep room: balances, small beakers, measuring cylinders, thermometers, sodium carbonate, calcium chloride, eye protection, gloves, a heatproof mat, tongs and a fire blanket.
- From the supermarket: modelling clay, toothpicks, fine steel wool and effervescent tablets.
- From a cupboard: a 9 V battery, cups and timers.
- Printed from this pack: the atom tokens, the balance cards and the representation cards.
Safety and care in one look
- Eye protection for every practical; gloves offered for the solutions in Lesson 5.
- The steel wool is heated by the teacher only, on a heatproof mat, away from paper, with a fire blanket nearby.
- Warm water only in Lesson 9, never hot from a kettle.
- Nothing in the science room is tasted, including the effervescent tablets.
Assessment without extra work
The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the task, in which students balance the equation for a camping stove, draw it and check its masses, and the final check. This sheet is the place to note what you see along the way.
The three levels
| Idea | Working towards | At standard | Beyond |
|---|---|---|---|
| Rearranging atoms | says substances change | models a reaction as atoms breaking apart and joining in new ways | explains with a model why the numbers of each kind of atom cannot change |
| Equations | names reactants and products | writes word equations and balances simple symbol equations | balances harder equations and explains why only the coefficients may change |
| Conservation of mass | says mass stays the same | shows with measurements that mass is conserved in a closed system | explains apparent gains and losses by gases entering or leaving |
| Representations | uses one way to show a reaction | links word and symbol equations, particle pictures and masses for one reaction | chooses the most useful representation for a purpose and explains why |
| Working scientifically | records masses | measures masses carefully and controls variables | identifies sources of error in a mass measurement and how to reduce them |
Class observation checklist
| Name | Atoms | Equations | Mass | Representations | Investigating |
|---|---|---|---|---|---|
A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.
Word wall cards
Cut out the cards and build the wall as the words arrive: element, compound, molecule, formula and subscript in Lesson 1; reactant, product, word equation and salt in Lesson 2; symbol equation in Lesson 3; coefficient and balanced equation in Lesson 4; law of conservation of mass, closed system and precipitate in Lesson 5; open system and oxide in Lesson 6; particle picture in Lesson 8.
a substance made of only one kind of atom
a substance made of two or more elements joined together
a group of atoms joined together
symbols and numbers that show the atoms in a substance, such as H₂O
the small number in a formula: the number of that atom in one molecule
a substance that changes in a chemical reaction
a new substance made in a chemical reaction
reactants, an arrow, then products, in words
a chemical equation written with formulas
the big number in front of a formula: how many molecules take part
an equation with the same number of each kind of atom on both sides
the total mass of the products equals the total mass of the reactants
a container that nothing can get into or out of
a container that gases can escape from or enter
a solid made when two solutions react
a compound of an element with oxygen
a compound made when an acid reacts with a base or a metal
a drawing of the atoms and molecules in a reaction
Dear families
This term in science, Year 9 studies chemical equations. In every chemical reaction the atoms of the starting substances come apart and join up again in new ways, and none are made or lost, so the total mass stays the same. Students show reactions in models, words, formulas, drawings and masses, and learn to balance simple equations.
Along the way your student will build molecules from modelling clay, rearrange atom tokens to model burning, balance equations, weigh two solutions as they react to make a solid, watch steel wool gain mass as it burns, and test whether a fizzing tablet loses the same mass in cold and warm water.
Try this at home
- Read the ingredients on a packet. Can you find any chemical names, and guess the elements in them?
- Drop an effervescent tablet into water in a glass. Where does the gas come from, and where does it go?
- Look at a rusty nail. What has the iron joined with, and is the rusty nail heavier or lighter than before?
- Count the atoms in the formula for sugar, C₆H₁₂O₆.
What to ask your scientist
- What happens to the atoms in a chemical reaction?
- Why can you only change the big numbers when you balance an equation?
- If mass is never lost, why does a burnt log seem to lose mass?
- Why does steel wool get heavier when it burns?
A small note on care: students wear eye protection for practical work, the steel wool is heated only by the teacher, and nothing in the science room is tasted.
Warm regards,
The Year 9 science team
Printed from the free seegongsik Chemical Equations and Conservation of Mass teaching pack · seegongsik.com/au/y9/chemical/AC9S9U07/pack
Reading formulas
A chemical formula is a short way of showing which atoms make up a substance, and how many of each. Each element has a symbol of one or two letters, and a small number written low after a symbol, a subscript, gives the number of that atom; no number means one. So water, H₂O, has two hydrogen atoms and one oxygen atom in each molecule. Elements are made of one kind of atom; compounds join two or more elements. Students read eight formulas, count the atoms, and build three molecules from modelling clay, ready to rearrange them in later lessons.
We are learning to
- read a chemical formula and count the atoms of each element,
- tell an element from a compound by its formula,
- build a model of a molecule from its formula.
Success criteria
- I can count every atom in all eight formulas.
- I can build and draw a model of water, carbon dioxide and methane.
You need
- for each pair: modelling clay in four colours (for hydrogen, oxygen, carbon and nitrogen) and a handful of toothpicks,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Codes Write H₂O and CO₂ on the board. Ask: “These are short codes that tell a chemist exactly what something is made of. What could each part mean?” |
| 10 min | On the board Show the board (see the box). Add element, compound, molecule, formula and subscript to the word wall. |
| 20 min | Counting atoms Students complete the table in Part A. Ask: “In CaCO₃, which number belongs to which atom?” |
| 25 min | Clay molecules Pairs choose a colour for each element, build the three molecules in Part B, joining atoms with toothpicks, and draw them. Then Part C. |
Running two half-sessions instead? Do the opening, the board and the table in Session A, and the models and Part C in Session B.
Watch for these ideas
- “The 2 in CO₂ belongs to the carbon.” A subscript belongs to the symbol just before it: two oxygen atoms.
- “Co and CO are the same.” Co is cobalt, one element; CO is carbon and oxygen, a compound.
- “O₂ is a compound.” It has only one kind of atom, so it is an element made of molecules.
Make it easier, make it bigger
- Easier: start with the four formulas with the fewest atoms, then the rest.
- Bigger: write the formula for a molecule with two carbon atoms and six hydrogen atoms, then build it.
Answers and look-fors
- Part A: H₂O: 2 H, 1 O; 3 in all. CO₂: 1 C, 2 O; 3 in all. CH₄: 1 C, 4 H; 5 in all. O₂: 2 O; 2 in all. NH₃: 1 N, 3 H; 4 in all. NaCl: 1 Na, 1 Cl; 2 in all. CaCO₃: 1 Ca, 1 C, 3 O; 5 in all. C₆H₁₂O₆: 6 C, 12 H, 6 O; 24 in all.
- Part B: water, one oxygen with two hydrogens joined to it; carbon dioxide, one carbon with an oxygen on each side; methane, one carbon with four hydrogens.
- Part C: 1. Oxygen gas, O₂: only one kind of atom. 2. It shows the number of the atom just before it. 3. C₆H₁₂O₆: 24 atoms. 4. Changing it would make a different substance: H₂O₂ is hydrogen peroxide, not water.
- Bigger: C₂H₆ (ethane).
- Exit tickets: 1. 1 carbon and 2 oxygen. 2. 4 hydrogen. 3. A substance made of two or more elements joined together.
Reading formulas
A. Counting atoms
| Substance | Formula | Atoms of each element | Atoms in all |
|---|---|---|---|
| Water | H₂O | ||
| Carbon dioxide | CO₂ | ||
| Methane | CH₄ | ||
| Oxygen gas | O₂ | ||
| Ammonia | NH₃ | ||
| Sodium chloride (table salt) | NaCl | ||
| Calcium carbonate (chalk) | CaCO₃ | ||
| Glucose | C₆H₁₂O₆ |
B. Clay molecules
Build each molecule, then draw it. Use a key to show which colour is which element.
C. Questions
Exit tickets
Cut out one ticket for each student.
1. What atoms are in one molecule of CO₂?
2. How many hydrogen atoms are in one molecule of CH₄?
3. What is a compound?
1. What atoms are in one molecule of CO₂?
2. How many hydrogen atoms are in one molecule of CH₄?
3. What is a compound?
1. What atoms are in one molecule of CO₂?
2. How many hydrogen atoms are in one molecule of CH₄?
3. What is a compound?
1. What atoms are in one molecule of CO₂?
2. How many hydrogen atoms are in one molecule of CH₄?
3. What is a compound?
Word equations
A word equation names the substances in a reaction: the reactants on the left, an arrow meaning “react to make”, and the products on the right. It does not show the atoms, but it is the first step to a balanced equation, and it already carries one rule: every element in the reactants must turn up somewhere in the products. Many reactions follow patterns. An acid and a metal make a salt and hydrogen; an acid and a carbonate make a salt, water and carbon dioxide; an acid and a base make a salt and water. Students match six reactions to their products and use the patterns to predict new ones.
We are learning to
- write word equations with the reactants and products in the right places,
- check a word equation by following each element from left to right,
- use the patterns of acid reactions to predict products.
Success criteria
- I can match the six reactions to their products and write each word equation.
- I can predict the products of an acid reaction I have not seen before.
You need
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Reverse Ask: “Photosynthesis and respiration use the same four substances. How can that be?” |
| 10 min | On the board Show the board (see the box). Add reactant, product, word equation and salt to the word wall. |
| 25 min | Match the products Students match each reaction in Part A to its products by letter, then check each match by following the elements across the arrow. Ask: “Where does the carbon in the carbon dioxide come from, in each reaction that makes it?” |
| 20 min | Patterns Students use the patterns in Part B to predict the products, then answer Part C. |
Running two half-sessions instead? Do the opening, the board and Part A in Session A, and Parts B and C in Session B.
Watch for these ideas
- “The arrow means equals.” It means “react to make”: the substances change.
- “Salt means table salt.” In chemistry a salt is any compound made this way; table salt, sodium chloride, is one of many.
- “An element can appear in the products from nowhere.” Every element in the products was in the reactants.
Make it easier, make it bigger
- Easier: underline the elements in each reactant before matching.
- Bigger: write the word equation for burning propane, a fuel made of carbon and hydrogen, in plenty of oxygen.
Answers and look-fors
- Part A: 1 C, 2 F, 3 A, 4 E, 5 B, 6 D.
- Part B: 1. Zinc chloride + hydrogen. 2. Magnesium sulfate + water + carbon dioxide (from sulfuric acid). 3. Potassium chloride + water.
- Part C: 1. Photosynthesis and respiration are the same reaction run in opposite directions. 2. Carbon from the carbonate or the fuel; the acids hold no carbon. 3. Every element in the products came from the reactants: atoms are only rearranged.
- Bigger: propane + oxygen → carbon dioxide + water.
- Exit tickets: 1. Before the arrow. 2. Salt + hydrogen. 3. Carbon dioxide and water.
Word equations
A. Match the products
Write the letter of the products next to each reaction.
| 1 | Natural gas burns on a stove | methane + oxygen → | |
| 2 | Cells release energy from food | glucose + oxygen → | |
| 3 | Leaves make sugar in sunlight | carbon dioxide + water → | |
| 4 | Magnesium ribbon fizzes in acid | magnesium + hydrochloric acid → | |
| 5 | Chalk fizzes in acid | calcium carbonate + hydrochloric acid → | |
| 6 | An antacid settles an acid | hydrochloric acid + sodium hydroxide → |
B. Patterns
| acid + metal | → salt + hydrogen |
| acid + carbonate | → salt + water + carbon dioxide |
| acid + base | → salt + water |
C. Questions
Exit tickets
Cut out one ticket for each student.
1. In a word equation, where are the reactants written?
2. What does an acid and a metal make?
3. What are the products of burning methane?
1. In a word equation, where are the reactants written?
2. What does an acid and a metal make?
3. What are the products of burning methane?
1. In a word equation, where are the reactants written?
2. What does an acid and a metal make?
3. What are the products of burning methane?
1. In a word equation, where are the reactants written?
2. What does an acid and a metal make?
3. What are the products of burning methane?
Rearranging atoms
In a chemical reaction the bonds in the reactant molecules break, and the same atoms join up again in a new pattern, the products. No atom is made, lost or turned into another kind. A model makes this concrete: with atom tokens, students build a methane molecule and some oxygen molecules, take them apart, and rebuild them as carbon dioxide and water. They soon find that one oxygen molecule is not enough: the four hydrogen atoms need two water molecules, which need two oxygen atoms, and the carbon dioxide needs two more. That is why the balanced equation has two oxygen molecules: CH₄ + 2O₂ → CO₂ + 2H₂O.
We are learning to
- model a reaction as atoms rearranging, using tokens,
- count the atoms of each element before and after a reaction,
- explain why a reaction may need more than one molecule of a reactant.
Success criteria
- I can rebuild methane and oxygen into carbon dioxide and water with no tokens left over.
- I can explain why two oxygen molecules are needed.
You need
- the atom tokens (third sheet), one sheet per pair, with scissors,
- the worksheet (next sheet), one per student.
Lesson flow (about 60 minutes)
| 5 min | Same blocks, new build Ask: “If you pull apart a toy-brick car and build a boat from the same bricks, how many bricks does the boat have?” |
| 10 min | On the board Show the board (see the box). Add symbol equation to the word wall. |
| 30 min | Token reactions Pairs cut out the tokens, build one methane molecule and one oxygen molecule, and try to rebuild them as carbon dioxide and water. When they get stuck, they add a second oxygen molecule. They record the counts in Part A. Ask: “Which atoms were left over, or missing, with only one oxygen molecule?” |
| 15 min | Draw it Students draw the particle picture in Part B and answer Part C. Keep the tokens for Lesson 4. |
Running two half-sessions instead? Do the opening, the board and the tokens in Session A, and Parts B and C in Session B.
Watch for these ideas
- “Carbon turns into oxygen.” Atoms never change kind in a chemical reaction; they only change partners.
- “The water comes from the oxygen.” Water is made from hydrogen of the methane and oxygen of the air.
- “Spare atoms disappear.” With the wrong amounts, the extra reactant is simply left over.
Make it easier, make it bigger
- Easier: start with hydrogen and oxygen making water, which needs fewer tokens, before methane.
- Bigger: use the tokens to model magnesium burning in oxygen to make magnesium oxide, MgO. How many magnesium atoms react with one oxygen molecule?
Answers and look-fors
- Part A: with one oxygen molecule, 1 C, 4 H, 2 O, too little oxygen; with two, 1 C, 4 H, 4 O before and after.
- Part B: reactants CH₄ + 2O₂; products CO₂ + 2H₂O, as in the picture below.
- Part C: 1. The same atoms, in the same numbers, are there before and after. 2. Carbon dioxide needs two oxygen atoms and each of the two water molecules needs one: four in all, from two oxygen molecules. 3. The bonds between atoms break and new bonds form.
- Bigger: two magnesium atoms react with one oxygen molecule to make two units of magnesium oxide: 2Mg + O₂ → 2MgO.
Rearranging atoms
A. Counting atoms
| Build | C atoms | H atoms | O atoms |
|---|---|---|---|
| One methane + one oxygen | |||
| One methane + two oxygen | |||
| The products we built |
B. The particle picture
Draw the reactant molecules on the left of the arrow and the product molecules on the right. Count the atoms of each kind on both sides.
C. Questions
Atom tokens
Cut out the tokens. Keep them in an envelope: you will use them again in Lesson 4.
H hydrogen, O oxygen, C carbon, Mg magnesium.
Balancing equations
A symbol equation is balanced when each side has the same number of atoms of every element, because atoms are not made or destroyed. To balance one, change only the coefficients, the big numbers in front of the formulas, which say how many molecules take part. Never change a subscript: that would turn the substance into a different one. A good method is to tally each element on both sides, fix one element at a time, starting with the one that appears in the fewest formulas, and tally again after every change. Students balance eight equations, from hydrogen burning to iron rusting.
We are learning to
- balance a symbol equation by changing coefficients only,
- check a balanced equation by tallying each element,
- explain why balancing shows that atoms are conserved.
Success criteria
- I can balance at least six of the eight equations and show my tallies.
- I can explain why a subscript must never be changed.
You need
- the atom tokens from Lesson 3, for anyone who wants to build an equation,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Count both sides Write H₂ + O₂ → H₂O on the board. Ask: “Count the oxygen atoms on each side. What has gone wrong?” |
| 10 min | On the board Show the board (see the box), then balance the first equation together with a tally. Add coefficient and balanced equation to the word wall. |
| 30 min | Eight equations Students balance the equations in Part A, writing 1 where a single molecule is needed. Ask: “Which element did you fix first? Why that one?” |
| 15 min | Check and explain Pairs swap and check each other’s tallies, then answer Part B. |
Running two half-sessions instead? Do the opening, the board and the first four equations in Session A, and the rest and Part B in Session B.
Watch for these ideas
- “Change H₂O to H₂O₂ to balance it.” That makes hydrogen peroxide, a different substance.
- “A coefficient only multiplies the first atom.” It multiplies every atom in the formula after it.
- “Any coefficients that balance are wrong if they are not the smallest.” They balance, but we write the smallest whole numbers.
Make it easier, make it bigger
- Easier: balance the first five equations, building each with the tokens.
- Bigger: balance the burning of ethane: C₂H₆ + O₂ → CO₂ + H₂O.
Answers and look-fors
- Part A: 1. 2H₂ + O₂ → 2H₂O; 2. 2Mg + O₂ → 2MgO; 3. C + O₂ → CO₂; 4. N₂ + 3H₂ → 2NH₃; 5. 2Na + Cl₂ → 2NaCl; 6. 2H₂O₂ → 2H₂O + O₂; 7. CH₄ + 2O₂ → CO₂ + 2H₂O; 8. 4Fe + 3O₂ → 2Fe₂O₃. (Equation 3 is already balanced.)
- Part B: 1. It would make a different substance. 2. Every element has the same number of atoms on both sides. 3. For equation 8: 4 Fe and 6 O on each side. 4. Atoms are not made or destroyed, so the same atoms must be on both sides.
- Bigger: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
- Exit tickets: 1. The coefficients, the numbers in front. 2. 2, 1, 2. 3. It would change the substance.
Balancing equations
A. Balance these
Write a coefficient in every blank (write 1 if one molecule is needed). Tally each element on both sides to check.
| # | Reaction | Equation |
|---|---|---|
| 1 | Hydrogen burns | ___ H₂ + ___ O₂ → ___ H₂O |
| 2 | Magnesium burns | ___ Mg + ___ O₂ → ___ MgO |
| 3 | Carbon burns | ___ C + ___ O₂ → ___ CO₂ |
| 4 | Ammonia is made | ___ N₂ + ___ H₂ → ___ NH₃ |
| 5 | Sodium meets chlorine | ___ Na + ___ Cl₂ → ___ NaCl |
| 6 | Hydrogen peroxide breaks down | ___ H₂O₂ → ___ H₂O + ___ O₂ |
| 7 | Methane burns | ___ CH₄ + ___ O₂ → ___ CO₂ + ___ H₂O |
| 8 | Iron rusts to iron oxide | ___ Fe + ___ O₂ → ___ Fe₂O₃ |
B. Questions
Exit tickets
Cut out one ticket for each student.
1. When balancing, which numbers may you change?
2. Balance: ___ H₂ + ___ O₂ → ___ H₂O
3. Why may you not change a subscript?
1. When balancing, which numbers may you change?
2. Balance: ___ H₂ + ___ O₂ → ___ H₂O
3. Why may you not change a subscript?
1. When balancing, which numbers may you change?
2. Balance: ___ H₂ + ___ O₂ → ___ H₂O
3. Why may you not change a subscript?
1. When balancing, which numbers may you change?
2. Balance: ___ H₂ + ___ O₂ → ___ H₂O
3. Why may you not change a subscript?
Nothing in, nothing out
The law of conservation of mass says that in a chemical reaction the total mass of the products equals the total mass of the reactants, because the same atoms are there before and after. It is easiest to see when nothing can get in or out. When a solution of sodium carbonate is poured into a solution of calcium chloride, a cloudy white solid forms at once: calcium carbonate, a precipitate, the same compound as chalk. No gas is made, so even in open beakers no atoms leave. Students weigh both beakers together before and after mixing, and find the reading unchanged, although a new substance has clearly been made.
We are learning to
- measure the total mass before and after a reaction with care,
- write the word and balanced equations for the reaction,
- use the result to state the law of conservation of mass.
Success criteria
- I can show that the mass stayed the same, and explain why with atoms.
- I can write the balanced equation and check it with a tally.
You need
- for each group: a balance reading to 0.1 g, two small beakers, 25 mL of sodium carbonate solution and 25 mL of calcium chloride solution, eye protection and gloves,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Safety: eye protection on; both solutions irritate eyes and skin, so gloves are offered; wipe up spills and wash hands afterwards.
Lesson flow (about 60 minutes)
| 5 min | Where would mass go? Ask: “When two clear liquids react to make a solid, could the total mass go up, go down or stay the same? Why?” |
| 10 min | On the board Show the board (see the box). Add law of conservation of mass, closed system and precipitate to the word wall. |
| 25 min | Weigh, mix, weigh Put both beakers on the balance together and record the mass. Pour one solution into the other, put the empty beaker back on the balance beside the full one, and record the mass again. Record what you see in Part A. Ask: “Why must the empty beaker go back on the balance?” |
| 20 min | Equations Students complete the equations and the tally in Part B and answer Part C. |
Running two half-sessions instead? Do the opening, the board and the weighing in Session A, and Parts B and C in Session B.
Watch for these ideas
- “A solid appeared, so the mass must have gone up.” The solid is made from atoms that were already in the solutions.
- “Mass is only conserved in sealed containers.” Mass is always conserved; a sealed container stops gases carrying some away.
- “A small change in the reading proves mass changed.” A reading can wobble by 0.1 g; drops left in a beaker matter too.
Make it easier, make it bigger
- Easier: give the word equation, and ask students to label the reactants and products.
- Bigger: suggest a reaction where an open beaker would lose mass, and explain how you would change the method to show that mass is still conserved.
Answers and look-fors
- Part A, example: 152.4 g before and 152.4 g after; a cloudy white solid formed.
- Part B: sodium carbonate + calcium chloride → calcium carbonate + sodium chloride; Na₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl, with 2 Na, 1 C, 3 O, 1 Ca, 2 Cl on each side.
- Part C: 1. The same: no atoms entered or left. 2. Calcium carbonate, from calcium in one solution and carbonate in the other. 3. No gas is made, so nothing escapes even from open beakers. 4. In a chemical reaction the total mass of the products equals the total mass of the reactants.
- Bigger: for example, vinegar and baking soda give off carbon dioxide; do it in a sealed bottle, or catch the gas in a balloon on the bottle.
- Exit tickets: 1. It stays the same. 2. A solid made when two solutions react. 3. The atoms are only rearranged.
Nothing in, nothing out
A. Weigh, mix, weigh
| Reading | Example (g) | Ours (g) |
|---|---|---|
| Both beakers, before mixing | 152.4 | |
| Both beakers, after mixing | 152.4 |
B. The equations
The formulas are Na₂CO₃, CaCl₂, CaCO₃ and NaCl.
| Element | Na | C | O | Ca | Cl |
|---|---|---|---|---|---|
| Atoms on the left | |||||
| Atoms on the right |
C. Questions
Exit tickets
Cut out one ticket for each student.
1. What happens to the total mass when two solutions react to make a solid?
2. What is a precipitate?
3. Why is mass conserved in a chemical reaction?
1. What happens to the total mass when two solutions react to make a solid?
2. What is a precipitate?
3. Why is mass conserved in a chemical reaction?
1. What happens to the total mass when two solutions react to make a solid?
2. What is a precipitate?
3. Why is mass conserved in a chemical reaction?
1. What happens to the total mass when two solutions react to make a solid?
2. What is a precipitate?
3. Why is mass conserved in a chemical reaction?
A reaction that gains mass
When steel wool burns it glows orange, throws a few sparks and leaves a dark, crumbly solid that looks like less than it was. Yet the balance reads more afterwards. Steel wool is mostly iron, and as it burns, oxygen from the air joins the iron to make iron oxide; the extra mass is the oxygen. The law of conservation of mass still holds, as long as the oxygen is counted. On a balance open to the air there are three possible results: heavier when a gas from the air joins in, lighter when a gas made in the reaction escapes, and the same when no gas joins or leaves. The teacher burns the steel wool on the balance, and students sort twelve reactions by what the balance would show.
We are learning to
- explain a gain in mass by the atoms that joined from the air,
- predict whether a reaction in an open container gets heavier, lighter or stays the same,
- explain why all three results agree with the law of conservation of mass.
Success criteria
- I can say where the extra mass of the burnt steel wool came from.
- I can sort the twelve reactions and name the gas that joins or escapes in each.
You need
- for the teacher: a small pad of fine steel wool (about 2.5 g), a heatproof mat, a balance reading to 0.01 g, a 9 V battery, tongs, eye protection and a fire blanket,
- the worksheet (next sheet), one per student, and the cards (third sheet), one set per pair, with scissors.
Safety: a teacher demonstration only. Everyone wears eye protection, students stand at least 2 m back, and the bench is cleared of paper and anything else that burns. Keep the battery away from the steel wool until the moment of lighting, and store it with its terminals covered. Let the mat and wool cool before anyone touches them.
Lesson flow (about 60 minutes)
| 5 min | Heavier or lighter? Ask: “A log burns down to a small pile of ash. When steel wool burns, will it end up heavier, lighter or the same?” Students circle a prediction in Part A and give a reason. |
| 15 min | Burn the steel wool Put the mat on the balance and zero it. Tease the steel wool out loosely, put it on the mat and read the mass. Touch both terminals of the battery to the wool and let it burn, turning it once with the tongs. When it has stopped glowing and cooled, read the mass again. Students record both readings in Part B. Ask: “The wool looks thinner and crumbles. So why does the balance read more?” |
| 10 min | On the board Show the board (see the box), then write the word and symbol equations together. Add open system and oxide to the word wall. |
| 30 min | Sort the cards Pairs cut out the cards, set out the three heading cards and sort the reactions under them, then complete Part C. Ask: “For each card, which gas joins in or escapes?” |
Running two half-sessions instead? Do the opening, the demonstration and the board in Session A, and the cards and Part C in Session B.
Watch for these ideas
- “Burning destroys things, so they always get lighter.” Burning joins a fuel with oxygen; a solid product is heavier, and gas products escape.
- “The extra mass came from the battery or the flame.” They give energy, not atoms; the extra atoms are oxygen from the air.
- “Mass is not conserved in an open container.” It is; the balance just does not weigh the gases that come in or go out.
Make it easier, make it bigger
- Easier: for each card, first answer one question: does a gas join in, escape, or neither?
- Bigger: if all 2.5 g of iron had become iron oxide, Fe₂O₃, what would it weigh? Why was the real mass less?
Answers and look-fors
- Part A: any prediction with a reason; many students expect lighter, as with the log.
- Part B, example: 2.50 g before and 2.86 g after, a gain of 0.36 g.
- Part C: 1. Oxygen from the air joined the iron. 2. Iron + oxygen → iron oxide; 4Fe + 3O₂ → 2Fe₂O₃. 3. 0.36 g, the gain in mass. 4. Heavier: B, E, I, K; Lighter: A, D, F, G, L; The same: C, H, J. 5. No: the ash is lighter because carbon dioxide and water vapour escaped. Weigh them too, with the oxygen that joined, and the total stays the same.
- Bigger: about 3.57 g. Some iron inside the pad, or under the oxide, did not get enough oxygen to burn.
- Cards, for checking: A carbon dioxide and water vapour escape; B oxygen from the air joins the iron; C no gas joins or escapes; D carbon dioxide escapes; E oxygen and water from the air join the iron; F carbon dioxide escapes; G carbon dioxide escapes; H the tube is sealed, so nothing gets in or out; I oxygen from the air joins the magnesium; J the products are a salt and water, with no gas; K oxygen from the air joins the copper; L carbon dioxide and water vapour escape.
A reaction that gains mass
A. Predict
B. The readings
| Reading | Example (g) | Ours (g) |
|---|---|---|
| Steel wool, before | 2.50 | |
| After burning and cooling | 2.86 | |
| Change in mass | +0.36 |
C. Questions
4. Sort the cards (third sheet), then write their letters under each heading.
| Heavier | Lighter | The same |
|---|---|---|
What does the balance show?
Each reaction happens on a balance, open to the air unless the card says otherwise. Cut out the cards and sort them under the three headings by what the balance shows when the reaction has finished.
Masses in equations
Atoms of different elements have different masses. Chemists compare them with relative atomic masses, which give each atom’s mass compared with a hydrogen atom: hydrogen 1, carbon 12, oxygen 16. Adding these up gives the mass of each formula, and multiplying by the coefficient gives the mass of everything that takes part. Because a balanced equation has the same atoms on both sides, the two sides always add to the same total. Read in grams, the equation also tells us the masses that react: 12 g of carbon burns with 32 g of oxygen to make 44 g of carbon dioxide, and half as much carbon makes half as much carbon dioxide. Students add up the masses of four equations and use them to find missing masses.
We are learning to
- work out the mass of a formula from relative atomic masses,
- show that both sides of a balanced equation have the same total mass,
- use the law of conservation of mass to find a missing mass.
Success criteria
- I can add up the masses on each side of all four equations.
- I can find the missing mass in a reaction, and check that both sides add to the same total.
You need
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Count, then weigh Ask: “Our balanced equations count atoms. Last lesson we weighed. How could we get from counting atoms to grams?” |
| 15 min | On the board Show the board (see the box). Then work the first equation together: C + O₂ → CO₂ gives 12 + 32 → 44. |
| 20 min | Four equations Students work the formula masses in Part A and the masses on each side of the equations in Part B. Ask: “Which side came out heavier? What does that tell you about your working?” |
| 20 min | Missing masses Students find the missing masses in Part C, check each with a total, then answer Part D. |
Running two half-sessions instead? Do the opening, the board and Parts A and B in Session A, and Parts C and D in Session B.
Watch for these ideas
- “The mass of 2H₂O is 2 + 18.” The coefficient multiplies the whole formula: 2 × 18 = 36.
- “More molecules on one side means more mass.” The number of molecules can change; the mass cannot, as the atoms are the same.
- “Equal masses of reactants react.” Atoms differ in mass: 12 g of carbon reacts with 32 g of oxygen, not 12 g.
Make it easier, make it bigger
- Easier: do the first two equations, and give the formula masses for Part C.
- Bigger: what mass of carbon dioxide does 100 kg of carbon make when it burns?
Answers and look-fors
- Part A: C 12, O₂ 32, CO₂ 44, Mg 24, MgO 40, CaCO₃ 100, CaO 56, H₂ 2, H₂O 18.
- Part B: 1. 12 + 32 → 44, 44 on each side; 2. 48 + 32 → 80, 80 on each side; 3. 100 → 56 + 44, 100 on each side; 4. 4 + 32 → 36, 36 on each side.
- Part C: 1. 88 g; 2. 8 g; 3. 22 g; 4. 2 g.
- Part D: 1. The same atoms are on both sides, and each atom keeps its mass. 2. 10 − 5.6 = 4.4 g of carbon dioxide. 3. When magnesium burns, oxygen from the air joins it, so the solid gains mass; when limestone is heated, carbon dioxide leaves, so the solid loses mass.
- Bigger: 100 × 44 ÷ 12 ≈ 367 kg, more than three times the mass of the carbon, as the oxygen is counted too.
- Exit tickets: 1. 36 g. 2. 6 g. 3. The atoms on the two sides are the same, so their masses are too.
Masses in equations
A. Formula masses
Relative atomic masses: H = 1, C = 12, O = 16, Mg = 24, Ca = 40. Add up the atoms in each formula.
B. Both sides
| # | Balanced equation | Mass of reactants | Mass of products |
|---|---|---|---|
| 1 | C + O₂ → CO₂ | ||
| 2 | 2Mg + O₂ → 2MgO | ||
| 3 | CaCO₃ → CaO + CO₂ | ||
| 4 | 2H₂ + O₂ → 2H₂O |
C. Missing masses
Each reaction is one from Part B. Find the mass marked ?, then check that both sides add to the same total.
D. Questions
Exit tickets
Cut out one ticket for each student.
1. In 2H₂ + O₂ → 2H₂O, 4 g of hydrogen burns with 32 g of oxygen. What mass of water forms?
2. Heating 10 g of a solid gives off 4 g of gas. What mass of solid is left?
3. Why does each side of a balanced equation have the same mass?
1. In 2H₂ + O₂ → 2H₂O, 4 g of hydrogen burns with 32 g of oxygen. What mass of water forms?
2. Heating 10 g of a solid gives off 4 g of gas. What mass of solid is left?
3. Why does each side of a balanced equation have the same mass?
1. In 2H₂ + O₂ → 2H₂O, 4 g of hydrogen burns with 32 g of oxygen. What mass of water forms?
2. Heating 10 g of a solid gives off 4 g of gas. What mass of solid is left?
3. Why does each side of a balanced equation have the same mass?
1. In 2H₂ + O₂ → 2H₂O, 4 g of hydrogen burns with 32 g of oxygen. What mass of water forms?
2. Heating 10 g of a solid gives off 4 g of gas. What mass of solid is left?
3. Why does each side of a balanced equation have the same mass?
Four ways to show a reaction
One reaction can be shown in several ways, and each shows something the others hide. A word equation names the substances. A symbol equation gives their formulas and, once balanced, how many of each molecule take part. A particle picture shows the atoms themselves, so you can see them come apart and join up again. A line of masses shows the law of conservation of mass in numbers: both sides add to the same total. Students sort twelve cards into three reactions, each shown four ways, then show a fourth reaction all four ways themselves.
We are learning to
- match a word equation, a symbol equation, a particle picture and a line of masses for the same reaction,
- show a reaction in all four ways,
- say what each way shows best.
Success criteria
- I can group the twelve cards into three reactions and explain each match.
- I can show carbon burning in all four ways, with the same atoms and the same mass on both sides.
You need
- the representation cards (third sheet), one set per pair, with scissors,
- the worksheet (next sheet), one per student.
Lesson flow (about 60 minutes)
| 5 min | One story, four tellings Ask: “A footy match can be told as a score, a match report, a replay or a stats sheet. What does each show best?” |
| 10 min | On the board Show the board (see the box). Add particle picture to the word wall. |
| 25 min | Group the cards Pairs cut out the cards and group them into three reactions of four cards each, then record the letters in Part A. Ask: “Which card was hardest to place? What did you check to place it?” |
| 20 min | Your own set Students show carbon burning in all four ways in Part B, then answer Part C. |
Running two half-sessions instead? Do the opening, the board and the cards in Session A, and Parts B and C in Session B.
Watch for these ideas
- “The masses card with the biggest numbers must be methane, as it has the most molecules.” Work the masses from the formulas instead.
- “A particle picture is just decoration.” It is a model: the atoms in it can be counted, like the symbols in the equation.
- “The word equation shows how much reacts.” It names the substances only; the numbers come from the balanced equation.
Make it easier, make it bigger
- Easier: first find the three word equations, then build each group from them.
- Bigger: make a set of four cards for a reaction of your choice from earlier lessons, and swap it with another pair.
Answers and look-fors
- Part A: Hydrogen burns: B, K, F, I; Methane burns: D, L, A, G; Magnesium burns: H, E, J, C.
- Part B: carbon + oxygen → carbon dioxide; C + O₂ → CO₂; one carbon atom and an oxygen molecule above the arrow, a carbon dioxide molecule below, with 1 C and 2 O on each side; 12 g + 32 g → 44 g.
- Part C: 1. The particle picture, or the balanced symbol equation: the atoms can be counted on both sides. 2. The masses. 3. How many molecules of each substance take part, and which atoms are in them.
Four ways to show a reaction
A. Group the cards
Write the letters of the four cards for each reaction.
| Reaction | Word equation | Symbol equation | Particle picture | Masses |
|---|---|---|---|---|
| Hydrogen burns | ||||
| Methane burns | ||||
| Magnesium burns |
B. Carbon burns: your own set of four
Particle picture: the reactants above an arrow, the products below.
C. Questions
Representation cards
Cut out the twelve cards. Group them into three reactions, each shown four ways. The masses use H = 1, C = 12, O = 16 and Mg = 24.
Each group has one card of each kind: word equation, symbol equation, particle picture, masses.
Faster, but no heavier
An effervescent tablet fizzes in water because two substances in it react to make carbon dioxide, which bubbles out into the air. On a balance, the cup and tablet together read less once the fizzing stops; the mass lost is the carbon dioxide. Warmer water makes the tablet fizz much faster, as its particles move faster and collide more often. But the tablet holds the same atoms whatever the water temperature, so it gives off the same mass of gas: about 0.42 g. Students run the test at three temperatures, measure both the mass lost and the time to stop fizzing, and graph the time.
We are learning to
- plan and run a fair test with one variable changed and two measured,
- find the mass of a gas from the mass lost on a balance,
- explain why temperature changes the speed of a reaction but not the mass of its products.
Success criteria
- I can record the mass lost and the time at three temperatures, and graph the times.
- I can explain why the mass lost stayed about the same.
You need
- for each group: three effervescent tablets of one kind, three cups, a measuring cylinder, a thermometer, cold, room-temperature and warm (not hot) water, a balance reading to 0.01 g and a timer, with eye protection,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Safety: warm water from the tap, never hot from a kettle; eye protection on; nothing is tasted, including the tablets; wipe up spills.
Lesson flow (about 60 minutes)
| 5 min | More gas, or faster gas? Ask: “In warm water a tablet fizzes harder. Does it make more gas, or the same gas faster? How could a balance tell us?” |
| 10 min | On the board Show the board (see the box), then plan the test together in Part A. |
| 30 min | Three cups Pour 100 mL of water into a cup and take its temperature. Put the cup on the balance with a tablet beside it on the pan, and read the mass. Drop the tablet in, start the timer, and stop it when the fizzing stops. Read the mass again. Repeat with the other two temperatures. Ask: “Your mass lost will not match the example exactly. What could make it differ?” |
| 15 min | Graph and explain Students graph the time against temperature in Part C and answer Part D. |
Running two half-sessions instead? Do the opening, the board, the plan and the test in Session A, and the graph and Part D in Session B.
Watch for these ideas
- “Faster fizzing means more gas.” The same atoms make the same mass of gas; warm water only makes it come off sooner.
- “The tablet disappears, so its mass is destroyed.” Part dissolves and stays in the cup; the gas leaves, and its mass is what is lost.
- “A difference of 0.01 g means warm water made more gas.” That is the size of one step on the balance, and of the spray from a fizzing cup.
Make it easier, make it bigger
- Easier: use the example results to draw the graph, and run the test at two temperatures only.
- Bigger: use the graph to predict how long a tablet takes to stop fizzing in water at 30 °C.
Answers and look-fors
- Part A: change the water temperature; measure the mass lost and the time to stop fizzing; keep the same volume of water, kind of tablet, cup and balance.
- Part B, example: mass lost 0.42 g at 10 °C, 0.43 g at 22 °C, 0.42 g at 40 °C; times 95 s, 50 s, 25 s.
- Part C: a curve falling from 95 s at 10 °C to 25 s at 40 °C, steepest at the cold end.
- Part D: 1. Into the air, as carbon dioxide gas. 2. Warmer water made the reaction faster; the mass of gas given off stayed about the same. 3. The same tablet holds the same atoms whatever the temperature, so it can make only the same mass of gas. 4. No: the dissolved part stays in the cup, and the lost mass is gas that can be accounted for.
- Bigger: about 39 s or a little less, as the curve sits just under a straight line between the 22 °C and 40 °C results.
- Exit tickets: 1. Into the air, as carbon dioxide. 2. No: the mass lost was about the same. 3. We changed the temperature and measured the mass lost and the time.
Faster, but no heavier
A. The plan
B. Results
| Water | Example (°C) | Example lost (g) | Example time (s) | Our °C | Our mass before (g) | Our mass after (g) | Our time (s) |
|---|---|---|---|---|---|---|---|
| Cold | 10 | 0.42 | 95 | ||||
| Room temperature | 22 | 0.43 | 50 | ||||
| Warm | 40 | 0.42 | 25 |
C. Graph
Plot the time to stop fizzing against the water temperature, for our results or the example, and draw a smooth curve.
D. Questions
Exit tickets
Cut out one ticket for each student.
1. A fizzing tablet loses mass. Where does the mass go?
2. In warm water the tablet fizzed faster. Did it give off more gas? How do you know?
3. In our test, which variable did we change, and which two did we measure?
1. A fizzing tablet loses mass. Where does the mass go?
2. In warm water the tablet fizzed faster. Did it give off more gas? How do you know?
3. In our test, which variable did we change, and which two did we measure?
1. A fizzing tablet loses mass. Where does the mass go?
2. In warm water the tablet fizzed faster. Did it give off more gas? How do you know?
3. In our test, which variable did we change, and which two did we measure?
1. A fizzing tablet loses mass. Where does the mass go?
2. In warm water the tablet fizzed faster. Did it give off more gas? How do you know?
3. In our test, which variable did we change, and which two did we measure?
Show what we know
The term comes together. In the task, students balance the equation for a camping stove burning propane, draw it, check that the masses balance and explain why the canister gets lighter. Then they sit the eight-question final check, and the class plays the unit quiz as a game on the board.
We are learning to
- show one reaction as a word equation, a balanced symbol equation, a particle picture and masses,
- use the law of conservation of mass to find an unknown mass,
- explain changes in mass in open and closed systems with atoms.
Success criteria
- I can balance the propane equation and show the same atoms and the same mass on both sides.
- I can explain why a fuel canister gets lighter without breaking the law of conservation of mass.
You need
- the task (next sheet) and the final check (third sheet), one of each per student,
- the interactive unit open on the board for the quiz; mini whiteboards if you have them.
Lesson flow (about 60 minutes)
| 5 min | Dinner on the stove Ask: “After dinner the gas canister of a camping stove is lighter. Where has that mass gone?” |
| 25 min | The task Students work alone on the task sheet. It is the summative task for the term: mark it with the rubric on the assessment sheet. |
| 20 min | Final check Students complete the eight-question final check alone. |
| 10 min | Class quiz Run the unit quiz on the board as a class game (see the box). |
Running two half-sessions instead? Do the opening and the task in Session A, and the final check and the quiz in Session B.
Watch for these ideas
- Students who change C₃H₈ or CO₂ to make the atoms fit: only the coefficients may change.
- Students who expect the products to weigh only as much as the propane: the oxygen from the air adds its mass too.
- Students who say the gas is destroyed as it burns: it becomes carbon dioxide and water vapour in the air.
Make it easier, make it bigger
- Easier: give the numbers 1, 3, 4 and 5 to place in Question 2.
- Bigger: some stoves burn butane, C₄H₁₀, instead. Write and balance the equation for butane burning.
Answers and look-fors
- Task: 1. Propane + oxygen → carbon dioxide + water. 2. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (1, 5, 3, 4). 3. One propane molecule and five oxygen molecules above the arrow, three carbon dioxide and four water molecules below; 3 C, 8 H, 10 O on each side. 4. 44 + 160 = 204 and 132 + 72 = 204: the same. 5. 88 g is 2 × 44 g, so 2 × 132 = 264 g of carbon dioxide, using 320 g of oxygen from the air. 6. No: the propane leaves the canister and joins with oxygen from the air, and the products escape as gases; counting the oxygen and the products, the total mass is unchanged.
- Final check: 1. 24. 2. 2H₂ + O₂ → 2H₂O. 3. The coefficients. 4. 1, 3, 2. 5. More. 6. 20 − 11.2 = 8.8 g. 7. It stays the same. 8. In a reaction the bonds between atoms break and the atoms join up in new ways; no atom is made, destroyed or turned into another kind, so the total mass of the products equals the total mass of the reactants.
- Bigger: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.
The camping stove
A camping stove burns propane gas, C₃H₈, from a small canister. With plenty of air, propane burns to make carbon dioxide and water. Use H = 1, C = 12 and O = 16.
3. Draw a particle picture of your balanced equation, then tally the atoms of each element on both sides.