A chemical reaction never creates or destroys atoms. The atoms simply let go of their old partners and bond with new ones, and that is what makes a new substance.
So the kinds and numbers of atoms are the same before and after, and so is the total mass.
A chemical equation writes this down, putting coefficients in front of formulas so both sides hold the same atoms.
In the diagram, two hydrogen molecules and one oxygen molecule become two water molecules, which shows at a glance why the coefficient 2 is needed.
A Reaction Rearranges Atoms
⚗️ Like Rebuilding with LEGO Bricks
①In a reaction, atoms break their bonds and join new partners
②The atoms themselves do not change, so the kinds and numbers of atoms stay the same
③New partners make a substance with completely different properties, the way gaseous hydrogen and oxygen become liquid water
④The starting substances are reactants; the substances formed are products
Reactions in Molecular Models
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Molecular models showing atoms regrouping before and after a reaction
💡 Law of Conservation of Mass
①The kinds and numbers of atoms do not change, so the total mass does not change either
②Lavoisier first showed this by weighing reactions in sealed containers
③When a gas escapes from an open container the mass only seems to drop; counting the escaped gas, it is the same
④This law is the reason we balance atoms on both sides of an equation
Writing Chemical Equations
Forming water
2H₂ + O₂ → 2H₂O
Two hydrogen molecules and one oxygen molecule form two water molecules (4 H and 2 O on each side)
Burning carbon
C + O₂ → CO₂
One carbon atom and one oxygen molecule form one carbon dioxide molecule (a coefficient of 1 is not written)
Burning magnesium
2Mg + O₂ → 2MgO
Two magnesium atoms and one oxygen molecule form two units of magnesium oxide (Mg and O combine 1 : 1)
Balancing Step by Step
📝 Balancing in Three Steps
①Write the reactants and products as formulas joined by an arrow
②Count each kind of atom on both sides
③Starting with an atom that does not match, raise the coefficients until both sides agree, then reduce to the simplest whole numbers
Coefficients Are Not Subscripts
Coefficient = number of molecules (may change) / subscript = atoms in one molecule (changing it makes a new substance)
In 2H₂O the leading 2 means two water molecules, and ₂ means each molecule has two H atoms. Changing H₂O to H₂O₂ would give hydrogen peroxide, a different substance
Summary
Chemical Equations in One Line
Reactants → products (same kinds and numbers of atoms on both sides)
Atoms are conserved, so total mass is conserved too
🎯 Common Test Points
①A reaction rearranges atoms; no atom appears or disappears
②Write reactants → products and balance the atoms on both sides
③Use the simplest whole-number coefficients and leave out 1
④The coefficient ratio equals the ratio of molecules, and for gases at the same temperature and pressure it also equals the volume ratio
⑤Never change a subscript to balance
Examples and Unit Review
Example 1
In the equation □H₂ + O₂ → □H₂O for hydrogen and oxygen forming water, what goes in the two boxes?
1
O₂ on the left has 2 oxygen atoms, so the right side also needs 2 O, which takes 2 H₂O.
2
Then the right side has 4 H, so the left side needs 2 H₂ as well.
▸ 2H₂ + O₂ → 2H₂O (both boxes are 2)
In the water model you can count 4 H and 2 O both before and after the reaction.
Example 2
Why does the total mass stay the same before and after a chemical reaction, and what is this law called?
1
In a reaction, atoms only change partners; their kinds and numbers stay the same.
2
Each atom keeps its own mass and the count does not change, so the total mass is the same. This is the law of conservation of mass.
▸ Because the kinds and numbers of atoms do not change; it is the law of conservation of mass
Even when gas escapes from an open container and the mass seems to drop, collecting the escaped gas brings the total back to the same value.
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).