Physical and Chemical Changes: a full term of science

Ten ready-to-teach lessons for Year 8 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, an evidence sheet or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards, an evidence sheet or tickets.
3. Gather the few items under “You need” on the plan. Most come from the science prep room, the kitchen or the supermarket.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 8 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: a physical change leaves the same substance in a new form, while a chemical change rearranges atoms into new substances. We cannot see the atoms, so we read the clues, including the heat and light that show energy flowing out or in, and we check for the new substance, because a clue alone can fool us. Through it all, no atom is made or lost.

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 4 needs red cabbage juice made beforehand, Lesson 6 needs 3% hydrogen peroxide from a pharmacy, and the Lesson 9 nails need a week before the class looks at them.

Safety and care in one look

Eye protection goes on for every practical. Candles stand in trays of sand, long hair is tied back, and limewater and lit splints are for the teacher only. Glow sticks, cold packs and hand warmers are never cut open, only 3% hydrogen peroxide is used, and nothing in the science room is tasted.

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.

How the sequence builds

Lessons 1 to 3 draw the line between physical and chemical changes, in everyday life, in a burning candle and at the particle level. Lessons 4 and 5 read the clues of a reaction, and learn when they mislead. Lessons 6 and 7 measure what flows: energy out or in, and mass that is never lost. Lesson 8 meets the reactions that run our lives, Lesson 9 investigates rusting and how to stop it, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S8U07 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 8 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 weekend task, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons. Most of it comes from the science prep room, the kitchen and the supermarket. Three things are worth doing ahead: make the red cabbage juice for Lesson 4, buy 3% hydrogen peroxide for Lesson 6, and leave a week for the Lesson 9 nails.

The one-trip list

• From the prep room: thermometers, balances, test tubes, heatproof beakers, tongs, limewater, calcium chloride, eye protection and gloves.
• From the supermarket: milk, vinegar, baking soda, citric acid, dried yeast, a red cabbage, salt, cooking oil, steel wool, candles, balloons, foam cups and glow sticks.
• From a pharmacy: 3% hydrogen peroxide, an instant cold pack and antacid tablets.
• From a hardware store: plain iron nails, a galvanised nail and sandpaper.

Safety and care in one look

• Eye protection for every practical; gloves for steel wool and calcium chloride.
• Candles in trays of sand, long hair tied back; limewater and lit splints for the teacher only.
• Only 3% hydrogen peroxide; glow sticks, cold packs and hand warmers are never cut open.
• Nothing in the science room is tasted, even the milk and the vinegar.

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 camping weekend task, in which students classify six changes, account for mass and draw a reaction, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

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: physical change, chemical change and chemical reaction in Lesson 1; combustion in Lesson 2; reactant and product in Lesson 3; indicator and precipitate in Lesson 4; exothermic, endothermic and energy level diagram in Lesson 6; law of conservation of mass and closed system in Lesson 7; word equation, respiration and photosynthesis in Lesson 8; corrosion and galvanised in Lesson 9.

Dear families

This term in science, Year 8 studies change: the physical changes that leave a substance the same, such as melting and dissolving, and the chemical changes that make something new, such as burning, rusting and cooking. Students learn to read the clues that a reaction has happened, to measure the energy it gives out or takes in, and to show that no matter is ever lost.

Along the way your student will watch a candle closely, turn milk into curds with vinegar, change the colour of red cabbage juice, measure a reaction that gets cold, weigh a fizzing reaction in a sealed bottle, and find out what makes iron rust.

Try this at home

• While cooking, sort what happens into physical changes (melting butter) and chemical changes (browning toast or onions).
• Cut an apple and leave half in the air and half in lemon juice. Which browns, and why?
• Look for rust around your home or street. Where is it worst, and what protects the things that do not rust?
• Read the label of a cold pack or a hand warmer in the first aid kit. Is it a chemical or a physical change inside?

What to ask your scientist

• What is the one test for a chemical change?
• Why can a clue such as bubbles or heat fool you?
• What is the difference between exothermic and endothermic?
• If a log burns away to ash, where does the rest of it go?

A small note on care: students wear eye protection for practical work, candles stand in trays of sand, and nothing in the science room is tasted, even when it comes from the kitchen.

Warm regards,

The Year 8 science team

Printed from the free seegongsik Physical and Chemical Changes teaching pack · seegongsik.com/au/y8/chemical/AC9S8U07/pack

Physical or chemical?

Matter changes all the time, in two different ways. In a physical change the substance stays the same substance: melting, freezing, boiling, dissolving, cutting and breaking change its state, shape or size, but nothing new is made. In a chemical change, also called a chemical reaction, new substances form with new properties: burning, rusting, cooking and souring all make something that was not there before. The test is the new substance, not whether the change can be undone: cut hair stays cut but is still hair, and some chemical changes can be run backwards, as when a battery is recharged. Pairs sort twelve everyday changes and give the evidence.

We are learning to

• tell physical changes from chemical changes,
• use the making of a new substance as the test,
• explain why “can it be undone?” is not a reliable test.

Success criteria

• I can sort the twelve changes and name the new substance in each chemical change.
• I can give an example of a physical change that cannot be undone.

You need

• the change cards (third sheet), one set per pair, and the worksheet (next sheet), one per student,
• to show: an ice cube on a saucer, a piece of toast and a rusty nail.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and six cards in Session A, and the other six and Part B in Session B.

Watch for these ideas

• “If it cannot be undone, it is chemical.” Cut hair and smashed glass cannot be undone, but they are still hair and glass.
• “Dissolving makes the sugar disappear, so it is a reaction.” The sugar is still there; evaporate the water and it is left behind.
• “It looks completely different, so it must be chemical.” Melted butter looks different but is still butter.

Make it easier, make it bigger

• Easier: sort cards C, B, F and A first, two of each kind, then the rest.
• Bigger: a candle is lit. Find one physical change and one chemical change happening at the same time.

Answers and look-fors

• Part A: physical C, F, G, I, L; chemical A, B, D, E, H, J, K.
• Evidence: A: the brown crust is new substances made by the heat; B: iron reacts with oxygen and water to make rust; C: still butter, only melted; it sets again as it cools; D: bacteria turn the milk sugar into an acid that tastes and smells different; E: heat makes new substances; the cake rises with a new gas and sets; F: still sugar, spread through the tea; evaporating the water would leave it; G: smaller pieces of the same glass; H: a new gas, carbon dioxide, is made; I: water vapour condensing into liquid water, the same substance; J: burning makes new substances and gives out light, heat and sound; K: the cut surface reacts with oxygen in the air to make brown compounds; L: shorter pieces of the same hair; it cannot be undone, but nothing new is made.
• Part B: 1. A new substance is made. 2. The hair is still hair; only its length changed, and no new substance was made. 3. For example, melting butter (can be undone) and smashing glass (cannot). 4. For example, rust: a flaky orange-brown solid, unlike shiny iron.
• Bigger: the wax melts and sets again (physical); the wax vapour burns in the flame, making new gases (chemical). Lesson 2 looks closely at this.

Physical or chemical?

A. Twelve changes

B. Questions

Physical or chemical?

Cut out the cards. Lay down the two grey heading cards, read each card, and sort it.

Teacher note: the piles are not the same size; the answer is on the plan.

A candle: two changes at once

A burning candle shows both kinds of change at once. Near the flame the wax melts, and a drip sets solid again down the side: physical changes. The liquid wax climbs the wick, turns to vapour, and the vapour burns: a chemical change that makes new substances, water and carbon dioxide, and gives out light and heat. A cold glass held over the flame mists up with the water; limewater turns milky with the carbon dioxide. The candle gets lighter as it burns, but the wax is not destroyed: its atoms leave as gases. Groups watch a candle closely, sort what they see into physical and chemical, and account for the mass the candle loses.

We are learning to

• identify physical and chemical changes happening at the same time,
• use observations as evidence that a new substance has formed,
• explain where mass goes when something burns.

Success criteria

• I can name two physical changes and one chemical change in a burning candle, with evidence.
• I can explain why the candle gets lighter without any matter being destroyed.

You need

• for each group: a short candle standing in a tray of sand, a heatproof glass beaker kept cold in the fridge, tongs or a peg to hold it, and a balance,
• for the teacher: a little limewater, the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Safety: tie back long hair and loose sleeves; the candle stays in its sand tray; hold the beaker with tongs for a few seconds only, as it gets hot and sooty. The limewater test is done by the teacher.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Weigh and light the candles, show the board and watch in Session A; weigh again and do Parts B and C in Session B.

Watch for these ideas

• “The wick is what burns.” Mostly it is the wax vapour that burns; the wick carries liquid wax up to the flame.
• “The mist is smoke.” It is water, a new substance made when the wax burns.
• “The candle gets lighter, so the wax is destroyed.” Its atoms leave as carbon dioxide and water vapour, which we cannot see.

Make it easier, make it bigger

• Easier: sort the six observations in Part A as a class first, then write the evidence in pairs.
• Bigger: if you could catch every gas the candle gives off, would the gases weigh more or less than the wax that burned? Why?

Answers and look-fors

• Part A: melting wax, physical (still wax); the drip setting, physical (still wax); the flame, chemical (light and heat, wax burning); mist in the glass, chemical (water, a new substance); milky limewater, chemical (carbon dioxide, a new substance); getting lighter, evidence that burnt wax has left as gases.
• Part B, example class: 21.6 − 19.4 = 2.2 g of wax burned in 20 minutes.
• Part C: 1. The wax melting near the flame and setting again in the drip. 2. The flame, the mist and the milky limewater: new substances made with light and heat given out. 3. No: the wax’s atoms joined with oxygen from the air and left as carbon dioxide and water vapour. 4. Chemical energy stored in the wax becomes light and heat.
• Bigger: more: the gases hold the atoms of the wax and of the oxygen it joined with from the air.
• Exit tickets: 1. Wax melting (or setting again). 2. Burning, shown by the flame, the mist or the limewater. 3. Into the air as carbon dioxide and water vapour.

A candle: two changes at once

A. What we saw

B. The candle’s mass

C. Questions

Exit tickets

Cut out one ticket for each student.

Inside a change

The particle model shows the difference between the two kinds of change. In a physical change the particles stay exactly the same particles: they spread apart, pack together or mix, but every molecule is still the molecule it was. In a chemical change the bonds between atoms break and the atoms join up again in new arrangements, making new molecules: a new substance. In both kinds of change every atom is still there afterwards; none is made and none is destroyed. Pairs match six before pictures to their after pictures, decide which changes are physical and which chemical, and count the atoms of each kind before and after.

We are learning to

• use the particle model to explain physical and chemical changes,
• recognise a chemical change as atoms rearranging into new particles,
• show that atoms are conserved in every change.

Success criteria

• I can match all six pairs and say which are chemical changes, and why.
• I can count the atoms before and after a change and show they are the same.

You need

• the particle cards (third sheet), one set per pair, and the worksheet (next sheet), one per student,
• a few coloured counters or blocks to model the first change at the front, if you like.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and three pairs in Session A, and three pairs and Part B in Session B.

Watch for these ideas

• “In a chemical change, atoms change into other atoms.” The atoms stay the same atoms; only how they are joined changes.
• “When water boils, the molecules break up.” Boiling only spreads the same water molecules apart.
• “Burning destroys atoms.” The atoms of the fuel and the oxygen are all in the products.

Make it easier, make it bigger

• Easier: count the atoms of every card first, and match cards with the same counts.
• Bigger: draw before and after pictures for a change of your own, one physical and one chemical, and swap with a partner.

Answers and look-fors

• Part A: 1 with B, a metal melting, physical: 9 metal; 2 with E, water boiling, physical: 10 hydrogen, 5 oxygen; 3 with D, hydrogen and chlorine reacting, chemical: 6 chlorine, 6 hydrogen; 4 with F, two gases mixing, physical: 6 nitrogen, 6 oxygen; 5 with A, methane burning, chemical: 2 carbon, 8 hydrogen, 8 oxygen; 6 with C, water being split, chemical: 8 hydrogen, 4 oxygen.
• Part B: 1. In 3, 5, 6: new molecules were made. 2. Card 2 has five water molecules and its after card has the same five, spread out; card 6 has four, and its after card has hydrogen and oxygen molecules instead. 3. The number of atoms of each kind. 4. The atoms are joined in new ways, so the particles, and the substance, are new.

Inside a change

Atom colours: hydrogen white, carbon black, oxygen red, nitrogen blue, chlorine green, metal grey.

A. Six pairs

B. Questions

Before and after

Cut out the cards. Match each numbered before card to the lettered card that shows the same atoms after the change.

Atom colours: hydrogen white, carbon black, oxygen red, nitrogen blue, chlorine green, metal grey.

Before

After

Teacher note: the after cards are printed in a different order; the matches are on the plan.

Clues that a reaction has happened

We cannot see atoms rearranging, so we look for clues, called indicators, that a new substance has formed: a colour change, a gas (bubbles), a precipitate (a solid forming in a liquid), a temperature change, light given off. A temperature change or light shows that energy is being given out or taken in, and every chemical reaction involves an energy change. No single clue is proof on its own, because some physical changes give one of these signs too (Lesson 5), but two or more together make a strong case. Groups visit five clue stations, record what they see and name the indicators.

We are learning to

• observe and name the indicators of a chemical reaction,
• recognise a temperature change or light as a sign of an energy change,
• use more than one clue to argue that a reaction has happened.

Success criteria

• I can name the indicator I saw at each of the five stations.
• I can explain why two clues make a stronger case than one.

You need

• five stations, each with its station card (third sheet): warm milk, vinegar and a teaspoon; vinegar, baking soda and cups; red cabbage juice, vinegar, baking soda solution and two cups; steel wool, vinegar, a thermometer and a jar; glow sticks and cups of warm and cold water,
• eye protection for everyone, gloves for the steel wool, and the worksheet (next sheet), one per student.

Before the lesson, make the cabbage juice: chop a quarter of a red cabbage, cover it with hot water for ten minutes and strain. Safety: eye protection on throughout; gloves for the steel wool; glow sticks are never cut open; nothing is tasted.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and three stations in Session A, and two stations and Part B in Session B.

Watch for these ideas

• “Bubbles always mean a reaction.” Boiling water bubbles too; the bubbles must be a new gas.
• “A precipitate is anything that sinks.” It is a new solid that forms in a liquid, like the lumps in the milk.
• “If it got colder, energy was lost.” The reaction took heat in from its surroundings; that is an energy change too.

Make it easier, make it bigger

• Easier: give each group the list of five indicators to tick at every station.
• Bigger: at the cabbage station, what else could you test to find out whether a liquid turns the juice pink or green?

Answers and look-fors

• Part A: 1 milk and vinegar: a solid forms (white lumps, a precipitate); 2 baking soda and vinegar: bubbles of a new gas, and the cup gets colder; 3 red cabbage juice: a colour change (pink with vinegar, green with baking soda); 4 steel wool and vinegar: the temperature rises as the iron rusts quickly; 5 glow stick: light is given off, brighter in warm water.
• Part B: 1. Station 2: a gas, and the cup got colder. 2. Stations 4 and 5 (the temperature rose; light was given off), and station 2 (it got colder). 3. A new solid that forms when two liquids or solutions react. 4. Some physical changes give one clue (boiling water bubbles), so two or more clues, or testing for the new substance, make a stronger case.
• Bigger: for example lemon juice, soap, toothpaste or cola: acids turn the juice pink and bases turn it green, so the juice is an indicator of a different kind.

Clues that a reaction has happened

Indicators: a colour change; a gas (bubbles); a precipitate (a solid forming in a liquid); a temperature change; light given off.

A. Five stations

B. Making the case

Clue stations

Cut out the cards and put one at each station with its equipment. Eye protection on at every station.

When a clue fools you

Every indicator of a reaction can also turn up in a physical change. Boiling water bubbles, an instant cold pack gets cold as a solid dissolves, a reusable hand warmer gets hot as crystals form, food colouring changes the colour of water, and a mint dropped in cola makes a fountain of gas that was already dissolved in the drink. So a clue is a reason to look closer, not a verdict. The deciding question is always the same: has a new substance been made? Students read the evidence for eight cases, decide reaction or not, and design a test that would settle one of them.

We are learning to

• recognise that a physical change can show an indicator too,
• use evidence of a new substance to decide whether a reaction happened,
• design a test to check a claim.

Success criteria

• I can sort the eight cases and give the deciding evidence for each.
• I can describe a test that would show whether a gas is a new substance.

You need

• the worksheet (next sheet) and the evidence sheet (third sheet), one each per student,
• to show if you have them: an instant cold pack from the first aid kit (squeezed, never opened), a reusable click hand warmer, an antacid tablet and a glass of water.

Safety: never cut open a cold pack or a hand warmer; if you show the lit-match test, it is done by the teacher.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and four cases in Session A, and four cases, Part B and Part C in Session B.

Watch for these ideas

• “It got hot, so it must be a reaction.” The click hand warmer gets hot as crystals form, a physical change.
• “Fizzing always means a new gas.” The gas in a cola fountain was in the drink all along.
• “Two clues prove it.” Two clues make a strong case, but the proof is the new substance.

Make it easier, make it bigger

• Easier: do the first four cases as a class, then the rest in pairs.
• Bigger: the click hand warmer is reset by boiling. Does being reversible prove it is physical? Use Lesson 1 to explain.

Answers and look-fors

• Part A: 1 water boiling in a kettle: not a reaction, the bubbles are water vapour, the same substance as the liquid; 2 an antacid tablet in a glass of water: reaction, the tablet reacts with the water to make a new gas, carbon dioxide; 3 squeezing an instant cold pack: not a reaction, a salt dissolves in the water inside, and dissolving is a physical change; 4 food colouring dropped into water: not a reaction, the colouring spreads through the water but is still the same dye; 5 a disposable hand warmer: reaction, iron powder inside reacts with oxygen from the air, quickly rusting; 6 a reusable click hand warmer: not a reaction, a dissolved salt crystallises; boiling the pack dissolves it again, ready to reuse; 7 bread toasting: reaction, heat makes new brown substances in the bread; 8 a mint dropped into a bottle of cola: not a reaction, carbon dioxide already dissolved in the cola escapes onto the rough mint; no new substance is made.
• The 5 physical cases are the ones where the clue fools us: bubbles, it gets very cold, a colour change, it gets hot and crystals appear, a fountain of bubbles.
• Part B, a good design: collect the gas from the tablet in a test tube and hold a lit match at the top (teacher); if it goes out, the gas is carbon dioxide, a new substance. Compare with steam from boiling water, which only condenses into water. Look for: one thing changed, the same amounts, a clear sign of a new substance.
• Part C: 1. An instant cold pack, a click hand warmer, boiling water, food colouring or a cola fountain. 2. Whether a new substance is made. 3. The disposable warmer makes iron oxide, a new substance, and never works again; the click warmer only forms crystals of the same salt, and boiling dissolves them back.
• Bigger: no; reversibility is not the test. It is physical because the crystals are the same salt that was dissolved; no new substance forms.

When a clue fools you

A. Reaction or not?

B. Settle it with a test

C. Questions

The inside story

Read the evidence for each case before you decide.

Heat out or heat in

Every chemical reaction involves an energy change, because breaking bonds takes energy in and making bonds gives energy out. In an exothermic reaction more energy is given out than taken in, so the surroundings warm up: burning, respiration and rusting are exothermic. In an endothermic reaction more is taken in, so the surroundings cool down: photosynthesis and the fizz of citric acid with baking soda are endothermic. Groups measure the temperature of two reactions in foam cups every 30 seconds, graph them, and draw simple energy level diagrams.

We are learning to

• measure the temperature change of a reaction,
• classify reactions as exothermic or endothermic,
• represent an energy change with a graph and an energy level diagram.

Success criteria

• I can graph both reactions and say which is exothermic and which endothermic, with evidence.
• I can explain where the energy goes in one reaction and where it comes from in the other.

You need

• for each group: two foam cups, a thermometer, a measuring cylinder, a teaspoon, 50 mL of 3% hydrogen peroxide (pharmacy strength), a sachet of dried yeast, citric acid, baking soda, water, a timer and eye protection,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Safety: eye protection on; use only 3% hydrogen peroxide; citric acid stings eyes; wipe up spills; nothing is tasted. The yeast cup foams up, so stand it in a tray.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and both reactions in Session A, and Parts B to D in Session B.

Watch for these ideas

• “Endothermic reactions make cold.” They take heat in from their surroundings; the surroundings, including the thermometer, cool down.
• “Exothermic reactions create energy.” The energy was stored in the substances and is given out as they react.
• “Only reactions that burn are exothermic.” Rusting and respiration give out heat without a flame.

Make it easier, make it bigger

• Easier: read the temperature only at the start and after 3 minutes, and use the example readings for the graph.
• Bigger: plan a fair test to find out whether more baking soda gives a bigger temperature drop. What would you change, measure and keep the same?

Answers and look-fors

• Part A, example class: yeast and hydrogen peroxide rose 9 degrees; citric acid and baking soda fell 8 degrees, lowest at 90 s.
• Part B: the yeast line climbs and levels off near 29 °C; the citric acid line drops, bottoms out near 12 °C and then creeps back up.
• Part C: exothermic: the products drawn lower than the reactants, the gap labelled energy given out; endothermic: the products higher than the reactants, the gap labelled energy taken in.
• Part D: 1. Cup A: its temperature rose, so energy was given out to the surroundings. 2. In A, chemical energy stored in the hydrogen peroxide is given out as heat that warms the water and the cup; in B, heat is taken in from the water and the cup, so they cool. 3. Foam insulates, so little heat moves through the cup walls and the thermometer shows the reaction’s own energy change. 4. Once the reaction has finished, heat flows in from the warmer room.
• Bigger: change the amount of baking soda; measure the biggest temperature drop; keep the same water volume, citric acid, cup, starting temperature and stirring.
• Exit tickets: 1. Exothermic. 2. For example citric acid and baking soda, or baking soda and vinegar. 3. Out into the surroundings, mostly as heat.

Heat out or heat in

A. Temperature (°C)

B. Graph

Plot both reactions on the same axes, join each set of points, and add a key.

C. Energy level diagrams

Draw a line for the reactants and a line for the products in each frame, and label the energy given out or taken in.

D. Questions

Exit tickets

Cut out one ticket for each student.

Is anything lost?

In a chemical reaction the atoms are rearranged, but none is made and none is destroyed, so the total mass of the substances stays the same: the law of conservation of mass. It can look as if mass is lost when a reaction makes a gas in an open container, because the gas escapes into the air, or gained when something takes oxygen from the air, as rusting iron does. In a closed container nothing can get in or out, and the mass stays the same. Groups weigh baking soda and vinegar before and after they react, first in an open cup and then in a bottle with a balloon over its neck.

We are learning to

• measure the mass of substances before and after a reaction,
• explain the law of conservation of mass using atoms,
• explain why an open container seems to lose mass.

Success criteria

• I can use the results to show that a closed container keeps its mass.
• I can explain where the mass went in the open cup, and spot an odd result.

You need

• for each group: a balance that reads to 0.1 g, a plastic cup, a small plastic bottle, a balloon, a funnel, vinegar, baking soda, a teaspoon, a measuring cylinder and eye protection,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Safety: eye protection on; point the bottle away from faces in case the balloon pops off; wipe up spills.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the prediction, the board and the open cup in Session A, and the closed bottle, Part B and Part C in Session B.

Watch for these ideas

• “Gases have no mass.” The gas made in the open cup has mass; it left, so the cup got lighter.
• “The reaction destroyed some of the substances.” Every atom is still there, in the products, wherever they went.
• “Things that rust get lighter.” Rust gains mass: it is iron joined with oxygen from the air.

Make it easier, make it bigger

• Easier: run the closed bottle as a class demonstration and do only the open cup in groups.
• Bigger: steel wool is weighed, then burned on a heatproof mat (teacher). Predict whether the ash weighs more or less than the steel wool, and explain.

Answers and look-fors

• Part B, example class: open cup changes -1.1, -1.2, -1.1 g, mean -1.1 g; closed bottle changes 0, -0.1, 0 g, mean 0 g.
• Part C: 1. The reaction made carbon dioxide gas, which escaped into the room, taking its mass with it. 2. The balloon trapped the gas, so all the atoms stayed on the balance. 3. Group 2’s bottle dropped 0.1 g: a small leak at the balloon, or the swollen balloon pushing aside a little air, which pushes up on it; 0.1 g is also the smallest step the balance can show. 4. The atoms of the baking soda and vinegar are rearranged into new substances; none are made or destroyed, so the total mass cannot change.
• Bigger: more: the iron joins with oxygen from the air as it burns, so the product holds extra oxygen atoms.
• Exit tickets: 1. 10 g. 2. Into the air, as carbon dioxide and water vapour. 3. Otherwise the gas escapes and its mass is not weighed.

Is anything lost?

A. Open cup

Closed bottle

B. The means

C. Questions

Exit tickets

Cut out one ticket for each student.

Reactions all around us

Chemical reactions run our lives: burning fuels (combustion), respiration in every living cell, photosynthesis in plants, rusting (corrosion) and cooking. A word equation names the reactants, an arrow, and the products: methane + oxygen → carbon dioxide + water. Some reactions give energy out and some take it in. Photosynthesis and respiration are a striking pair: the products of one are the reactants of the other, and the energy flows the opposite way. Pairs sort twelve everyday reactions by their energy change, then write word equations.

We are learning to

• classify everyday reactions as exothermic or endothermic,
• write and read word equations,
• compare photosynthesis and respiration.

Success criteria

• I can sort the twelve reactions and say where the energy goes or comes from in each.
• I can complete four word equations and say whether each gives energy out or takes it in.

You need

• the reaction cards (third sheet), one set per pair, and the worksheet (next sheet), one per student,
• a few large cards with the words of a word equation for the opening (methane, oxygen, carbon dioxide, water, and an arrow).

Lesson flow (about 60 minutes)

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

• “Rusting cannot be exothermic, because a rusty gate is not warm.” The heat is given out so slowly that it spreads away unnoticed.
• “Plants respire only at night.” Plants respire all the time; in daylight photosynthesis also runs, faster.
• “Cooking an egg is exothermic because it is hot.” The stove puts the heat in; the egg only changes while it takes heat in.

Make it easier, make it bigger

• Easier: sort the cards that involve a flame or heat you can feel first, then the rest.
• Bigger: a phone battery takes energy in as it charges. Write what happens to the energy when you then use the phone, and say which direction that reaction goes.

Answers and look-fors

• Part A: exothermic A, C, D, G, I, J, L; endothermic B, E, F, H, K.
• Reasons: A: gives out heat and light; B: takes in energy from sunlight; C: releases energy from glucose to move and keep warm; D: iron powder reacting with oxygen gives out heat; E: only changes while the stove keeps putting heat in; F: takes in heat, so the water gets colder; G: gives out energy as light; H: takes in electrical energy and stores it as chemical energy; I: gives out light, heat and sound; J: gives out heat, too slowly to notice; K: only breaks down while the oven puts heat in, giving off carbon dioxide; L: gives out heat and light.
• Part B: gas burning in a stove: methane + oxygen → carbon dioxide + water, energy out; photosynthesis: carbon dioxide + water → glucose + oxygen, energy in; respiration: glucose + oxygen → carbon dioxide + water, energy out; rusting: iron + oxygen + water → rust (hydrated iron oxide), energy out.
• Part C: 1. Respiration uses glucose and oxygen and makes carbon dioxide and water, giving energy out; photosynthesis uses carbon dioxide and water and makes glucose and oxygen, taking energy in from light. 2. It gives out its heat very slowly, so the heat spreads into the air as fast as it is made. 3. Charging stores energy as chemical energy in the battery; using the phone runs the reaction the other way and gives the energy out as electricity.
• Bigger: using the phone releases the stored chemical energy as electrical energy (and some heat): the reverse reaction, which gives energy out.

Reactions all around us

A. Twelve reactions

B. Word equations

C. Questions

Energy out, or energy in?

Cut out the cards. Lay down the two grey heading cards, read each card, and sort it.

Teacher note: the piles are not the same size; the answer is on the plan.

Why does iron rust?

Rusting is a slow, exothermic chemical reaction: iron + oxygen + water → rust, a flaky orange-brown hydrated iron oxide. Iron needs both oxygen and water to rust; salt speeds the reaction up, which is why things rust fastest near the sea. Rust is weak and flakes off, letting the reaction eat further in, so we protect iron and steel with paint, oil and grease, a coat of zinc (galvanising), or by mixing in chromium to make stainless steel. Groups set up five test tubes of iron nails to see which conditions cause rust; while those run for a week, they analyse an example class’s results and read about rust in the real world.

We are learning to

• plan a fair test of the conditions that make iron rust,
• analyse results, including an odd result, to draw a conclusion,
• link ways of preventing rust to the conditions rust needs.

Success criteria

• I can use the results to show that rusting needs both water and oxygen, and that salt speeds it up.
• I can choose a way to protect three steel objects and explain each choice.

You need

• for each group: five test tubes in a rack, four plain iron nails rubbed with sandpaper and one galvanised nail, tap water, water that has been boiled and cooled, cooking oil, salt, a little calcium chloride with cotton wool and a stopper, and labels,
• the worksheet (next sheet) and the evidence sheet (third sheet), one each per student.

Safety: calcium chloride irritates the skin and eyes: use gloves or a spatula and eye protection. Leave the tubes in a safe place for a week, then look at the nails with the class.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the set-up in Session A, and the results, conclusions and evidence sheet in Session B.

Watch for these ideas

• “Rust is a kind of dirt on the iron.” It is a new substance made from the iron itself, so the iron is used up as it rusts.
• “Water alone makes iron rust.” Without oxygen (tube B) it barely rusts; it needs both.
• “Stainless steel cannot corrode at all.” It resists rust very well, but salt and scratches can still mark it.

Make it easier, make it bigger

• Easier: set up tubes A, B and C only, and use the example results for D and E.
• Bigger: plan a test to find out whether hot water makes iron rust faster than cold water. What would you change, measure and keep the same?

Answers and look-fors

• Part B, example means: A 2.7, B 0.3, C 0, D 3.7, E 0.3.
• Part C: 1. Water and oxygen: tube A (both) rusted; tube B (no air) and tube C (no water) hardly rusted at all. 2. Salt speeds rusting up: tube D rusted most. 3. Tube B: one group scored 1, probably because the oil layer had a gap or the water took in some air again as it cooled; the other two scored 0. 4. The zinc coating keeps water and air off the iron, and corrodes first where it is scratched.
• Evidence sheet: for example, a bike chain: oil, which keeps water and air off and does not rub off like paint; a boat trailer: galvanising, because salt water attacks bare or painted steel and the zinc protects even where scratched; cutlery: stainless steel, which stays shiny and safe in food and water.
• Bigger: change the water temperature; measure the rust score after the same number of days; keep the same nails, water volume, air and container.

Why does iron rust?

A. Five tubes

B. Results after seven days

Rust score: 0 none, 1 a few spots, 2 some, 3 a lot, 4 covered. Three groups of an example class.

C. Conclusions

Rust in the real world

Show what we know

The last lesson brings the term together on a camping weekend. Students classify six changes as physical or chemical with their evidence and energy change, account for the mass of a camp stove’s gas, draw the particles of the gas burning, and answer a claim about a cold pack. Then each student sits the final check on their own, and the class finishes with the unit quiz on the board.

We are learning to

• compare physical and chemical changes using evidence,
• identify the energy change in a reaction,
• show what we know about changes on our own.

Success criteria

• I can classify all six camping changes and back each with evidence.
• I can back each answer on the final check with a clue, a particle picture or a reason.

You need

• the camping weekend task (next sheet) and the final check (third sheet), one each per student,
• the interactive unit open for the closing quiz.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the task and the partner check in Session A, and the final check and the class quiz in Session B.

Watch for these ideas

• Calling the cold pack a reaction because it got cold: the change is a solid dissolving, with no new substance.
• Calling dew a chemical change because water appears from nowhere: it is water vapour from the air condensing.
• Drawing the burnt gas with fewer atoms than it started with: every atom of the gas and the oxygen is in the products.

Make it easier, make it bigger

• Easier: give the first column of the table already filled in as physical or chemical, and ask for the evidence and energy.
• Bigger: the tin mug gains mass as it rusts. Explain where the extra mass comes from.

Answers and marking guide

• Task A: 1 physical: still butter; it sets again when cool; takes in heat. 2 chemical: a colour change and new smells; it cannot be undone; takes in heat from the pan. 3 chemical: light, heat and new gases; gives out heat and light. 4 physical: water vapour condensing into liquid water; gives out heat as it condenses. 5 physical: a salt dissolving; the cold alone does not prove a reaction; takes in heat. 6 chemical: a new flaky orange-brown substance; gives out heat, too slowly to notice.
• Task B: 230 g: the products hold every atom of the gas and the oxygen, so their total mass is the same.
• Task C: two carbon dioxide molecules and four water molecules (2 black, 8 white and 8 red atoms, the same as before), drawn spread out as gases.
• Task D: no. The cold comes from a solid dissolving in water inside the pack (a salt dissolving; the cold alone does not prove a reaction); no new substance is made, so it is a physical change. A temperature change is a clue, not a proof.
• Check: 1. toast burning. 2. Bubbles of a new gas, and the mixture getting colder. 3. endothermic. 4. The particles stay the same particles; heating only lets them move apart and slide past each other, and cooling packs them back.
• Check: 5. stays the same. 6. A reusable click hand warmer, crystals forming as it heats up (or dew forming, or butter setting). 7. Wood + oxygen → carbon dioxide + water; the energy was stored in the wood as chemical energy, from the Sun through photosynthesis. 8. Oil, paint or grease keeps water and air off; or galvanising (a zinc coat) protects the steel.
• Bigger: the iron joins with oxygen from the air (and water), so the rust holds extra atoms.
• Scoring: 1 mark each for 1, 3 and 5; 2 marks each for 2, 4, 6, 7 and 8. The rubric near the front of the pack turns the task and the check into a level.

A camping weekend

A. Six changes

B. The camp stove

C. Draw the burning

The left box shows the gas (methane) and oxygen before burning. Draw the products in the right box, using every atom. Colours: hydrogen white, carbon black, oxygen red, nitrogen blue, chlorine green, metal grey.

D. Answer the claim

Show what we know

Show what you know about physical and chemical changes. Read each item, then circle or write.

Teacher note: the marking guide is on this lesson’s plan; the rubric near the front of the pack turns the score into a level.