← Chemical Changes

Lesson 2 of 8 · Textbook 11.1

Word equations and atoms

A reaction looks like substances vanishing and new ones appearing. Nothing vanishes — the same atoms simply change partners.

Writing a word equation

A word equation shows which substances are involved in a reaction. Reactants go on the left, products on the right, and an arrow points from one to the other. Any condition the reaction needs — usually heat, light or electricity — is written above the arrow.

reactants → products

Two examples straight from your textbook:

methane + oxygen  --heat-->  carbon dioxide + water

calcium carbonate  --heat-->  calcium oxide + carbon dioxide

Take note The number of reactants and the number of products may not be the same. The first equation above has two reactants and two products; the second has one reactant and two products. Don't assume they must match.

Atoms rearrange — they are never created or destroyed

During a chemical reaction the atoms of the reactants rearrange themselves and combine in different ways to form the products. Not a single atom is lost, and not a single new one appears. They simply end up in different combinations.

That single fact has a direct consequence. If every atom that went in is still there at the end, the total mass cannot have changed either:

total mass of reactants = total mass of products

This is the conservation of mass, and it was proved by the French scientist Antoine Lavoisier (1743–1794). He carefully weighed the reactants and products of reactions he studied — including burning a wooden log in oxygen — and found the ashes and gases produced had exactly the same mass as the original log and the oxygen used.

Count the atoms yourself

Pick a reaction below. Count how many atoms of each element appear on the left, then on the right, and type them in. This is exactly the check your textbook asks you to do in Figures 11.4 and 11.5.

Rearrangement of atoms

Reactants — before
→
Products — after
ElementIn reactantsIn productsMatch?

Grey = carbon, red = oxygen, white = hydrogen — the same colours your textbook uses.
What the counts prove Every reaction gives the same result: the number of atoms of each element is identical before and after. The atoms have been rearranged into different molecules, but none were made and none were lost. That is why mass is conserved in every chemical reaction.

Balanced chemical equations Extension

Beyond the core syllabus — useful if you want to see where word equations lead next.

Chemists usually write reactions with symbols rather than words. A balanced chemical equation uses chemical formulae plus numbers in front, chosen so the count of each type of atom is the same on both sides — exactly the check you just did by hand:

CH4 + 2O2 → CO2 + 2H2O

2H2 + O2 → 2H2O

2Na + Cl2 → 2NaCl

Balanced equations let chemists calculate quantities — for instance, what mass of a reactant is needed to produce a certain mass of product.

Check yourself