Balanced equations and conservation of mass

Atoms are never made or destroyed in a reaction. Balanced equations and mass measurements both show this.

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Counting atoms

In a formula, a small number counts the atom just before it: H2O has 2 H and 1 O.
A big number in front multiplies the whole formula: 2H2O means two water molecules, so 4 H and 2 O.
In a balanced equation, there are the same number of each type of atom on both sides.

Balancing an equation

Hydrogen burns in oxygen: H2 + O2 → H2O
Count: left has 2 O, right has 1 O. Put a 2 in front of H2O: H2 + O2 → 2H2O
Now the right has 4 H, so put a 2 in front of H2: 2H2 + O2 → 2H2O
Never change the small numbers in a formula: that would make a different substance.

Mass in reactions

Conservation of mass: the total mass of products equals the total mass of reactants.
In an open container, mass can seem to change:
Mass goes down if a gas escapes, for example when calcium carbonate is heated and carbon dioxide leaves.
Mass goes up if a gas from the air joins on, for example when magnesium burns and gains oxygen.
In a sealed container, the mass stays exactly the same.
Worked example

10.0 g of calcium carbonate is heated. It breaks down into 5.6 g of calcium oxide and some carbon dioxide gas. What mass of carbon dioxide is made?

  1. Mass of reactants = mass of products
  2. 10.0 = 5.6 + mass of carbon dioxide
  3. 10.0 − 5.6

Answer: 4.4 g

Key idea

A balanced equation has the same number of each atom on both sides: change only the big numbers in front. Total mass is conserved; mass seems to change in an open container only when a gas escapes or joins.

Key words
Conservation of mass
In a physical or chemical change, the total mass stays the same, because atoms are not created or destroyed.
Mass
The amount of matter in an object, measured in kilograms (kg). It is the same everywhere.
Atom
The smallest particle of an element that can exist.
Water
Pure water melts at 0 °C and boils at 100 °C. Its density is 1 g/cm³. Ice is less dense than water, so it floats.

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Extra practice

2 more questions

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  1. How many hydrogen atoms are shown in 2NH3?

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    Answer: 6

    The big number multiplies the whole formula: 6 atoms.

  2. How many oxygen atoms are shown in 3H2O?

    Show the answer

    Answer: 3

    The big number multiplies the whole formula: 3 atoms.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.