seegongsik
Saved words
Grade 8 / Middle 2 (age 13-14)

Chemical Equations

Chemical Reaction Equation

A chemical reaction does not create or destroy atoms; it rearranges how they bond. Before and after, the kinds and counts of atoms stay the same, which is the law of conservation of mass. In a chemical equation we choose coefficients so both sides have equal atom counts, using the simplest integers. Here you can watch reactions with molecular models and practice balancing equations such as the formation of water.

What is a Chemical Reaction?
⚗️ Atoms Rearrange Like LEGO Bricks
①Chemical reaction: atoms rearrange into new combinations
②The type and count of atoms don't change before vs. after
③Only the bonding patterns change, producing entirely different substances
Reactions via Molecular Models
💡 Law of Conservation of Mass
①Same atom counts before and after → mass is conserved
②This is Lavoisier's law of conservation of mass
③That's why we balance coefficients in chemical equations!
Writing Chemical Equations
Water Synthesis
2H₂ + O₂ → 2H₂O
2 hydrogen molecules + 1 oxygen molecule → 2 water molecules
Carbon Combustion
C + O₂ → CO₂
1 carbon atom + 1 oxygen molecule → 1 CO₂ molecule
Magnesium Oxidation
2Mg + O₂ → 2MgO
2 magnesium atoms + 1 oxygen molecule → 2 magnesium-oxide molecules
Balancing Coefficients
📝 Three-Step Balancing
①Step 1: write the formulas for reactants and products
②Step 2: count atoms on each side
③Step 3: adjust coefficients (numbers in front) so atom counts match
Coefficient vs Subscript
Coefficient: number of molecules (can change) / Subscript: atom ratio (cannot change)
In 2H₂O, 2 = coefficient (2 waters), ₂ = subscript (2 H atoms)
Summary
Chemical-Equation Core
Reactants → Products (same atom counts on both sides)
Conservation of mass: total mass is the same before and after
🎯 Exam Key Points
①Reactions = atom rearrangement (no atoms appear or disappear)
②Equation: reactants → products with equal atom counts
③Coefficients = number of molecules; use the simplest integer ratio
④Conservation of mass
⑤Distinguish exothermic (combustion) vs endothermic (photosynthesis) reactions
Examples and Unit Test
Example 1
In the reaction where hydrogen and oxygen form water, □H₂ + O₂ → □H₂O, what coefficients go in the boxes?
1
A chemical equation is balanced so that the number of each kind of atom is the same on both sides.
2
To match the 2 oxygen atoms in O₂, you need 2 H₂O; that gives 4 H, so you need 2 H₂. So 2H₂ + O₂ → 2H₂O.
▸ 2H₂ + O₂ → 2H₂O (the coefficients are 2 and 2)
A chemical equation is balanced so the number of atoms is equal on both sides.
Example 2
Why does the total mass stay the same before and after a chemical reaction, and what is the name of this law?
1
In a chemical reaction the atoms are only rearranged; the kinds and numbers of atoms do not change.
2
Since the kinds and numbers of atoms stay the same, the total mass does not change. This is called the law of conservation of mass.
▸ Because the kinds and numbers of atoms do not change / the law of conservation of mass
The number of atoms is conserved across a reaction, so the total mass stays constant (conservation of mass).
Unit-test style
Which statement about chemical equations and the conservation of mass is correct?
①When a chemical reaction occurs, the kinds of atoms change
②The total number of atoms does not change before and after a chemical reaction
③When balancing, you change the subscripts
④The total mass after a reaction is always less than before
⑤The arrow in a chemical equation points to the reactants
▸ ② The total number of atoms does not change before and after a chemical reaction
1
A chemical reaction is a rearrangement of atoms, so the kinds and total number of atoms do not change.
2
The kinds of atoms stay the same (①), you balance by adjusting coefficients, not subscripts (③), mass is conserved and does not change (④), and the arrow points toward the products (⑤).
← Previous
Magnetic Field
Next →
Photosynthesis
Was this helpful? Support seegongsik