Conservation of mass and balanced equations · GCSE Chemistry

Conservation of mass

No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.
This means chemical reactions can be shown by balanced symbol equations, with the same number of each type of atom on both sides.

Only change the big numbers in front of formulae, never the small numbers inside them.
Example: H₂ + O₂ → H₂O becomes 2H₂ + O₂ → 2H₂O.
Example: Mg + O₂ → MgO becomes 2Mg + O₂ → 2MgO.
Check by counting the atoms of each element on each side.

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of the atoms in its formula.
Example: Mr of CO₂ = 12 + (2 × 16) = 44. Mr of CaCO₃ = 40 + 12 + (3 × 16) = 100.
In a balanced equation, the total Mr of the reactants (in the quantities shown) equals the total Mr of the products.

If a reactant is a gas from the air, the mass of solid increases. For example, when magnesium burns, oxygen from the air joins it, so the magnesium oxide is heavier than the magnesium.
If a product is a gas that escapes, the mass decreases. For example, when a metal carbonate is heated, carbon dioxide escapes (thermal decomposition).
In a closed system, where nothing can get in or out, the mass stays the same.

Whenever a measurement is made there is some uncertainty about the result.
The range of a set of repeat measurements about the mean gives an estimate of the uncertainty: uncertainty = range ÷ 2.

Conservation of mass: No atoms are lost or made in a reaction, so the total mass stays the same.
Balanced equation: A symbol equation with the same number of each type of atom on both sides.
Relative atomic mass (Ar): The average mass of an element's atoms compared with 1/12 of the mass of a carbon-12 atom.
Relative formula mass (Mr): The sum of the relative atomic masses of the atoms in a formula.
Thermal decomposition: Breaking down a substance using heat.
Closed system: A system where no substances can get in or out.
Uncertainty: How far a measurement might be from the true value; estimated as half the range.