2 · Atoms and conservation of mass

Nothing is created, nothing is destroyed

When paper burns, it looks like the paper "disappears." It doesn't — every atom that was in the paper is still there, just rearranged into new molecules (carbon dioxide, water vapour, and the ash left behind). A chemical reaction is a rearrangement of atoms into new combinations. No atoms are created, and none are destroyed.

Why this matters Because the exact same atoms exist before and after — just grouped differently — the total mass can't change either. This is conservation of mass: the total mass of the reactants always equals the total mass of the products.

Word equations

At this level, you describe a reaction with a word equation — the names of what you start with, an arrow, and the names of what you end up with. No symbols or numbers are needed.

reactant(s) → product(s)

For example, magnesium burning in oxygen:

magnesium + oxygen → magnesium oxide

Worked example — conservation of mass

4 g of magnesium reacts completely with 3 g of oxygen. What mass of magnesium oxide forms?

Mass in = mass out: 4 g + 3 g = 7 g of magnesium oxide. Nothing was added or lost — the atoms just combined.

Try it: watch the atoms rearrange

Pick a reaction. The left box shows what you start with; the right box shows what you end up with. Count the atoms in each — they always match.

Quick check

Q 7 g of iron reacts completely with 4 g of sulfur to form iron sulfide. What mass of iron sulfide forms, and why?

Mass is conserved — the atoms are just rearranged, none are added or lost.

Mass of product = 7 + 4 = 11 g

Atoms and conservation of mass

Nothing is created, nothing is destroyed

Word equations

Try it: watch the atoms rearrange

Before (reactants)

After (products)

Quick check