GCSE Chemistry revision

Relative formula mass

Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

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The conservation of mass principle states that in a chemical reaction, mass is always conserved — no atoms are created or destroyed, only the bonds between atoms change.
This means the total mass of the reactants always equals the total mass of the products in any chemical reaction.

1. The Law of Conservation of Mass AQA

The conservation of mass principle states that in a chemical reaction, mass is always conserved — no atoms are created or destroyed, only the bonds between atoms change.
This means the total mass of the reactants always equals the total mass of the products in any chemical reaction.

Because atoms are neither created nor destroyed, chemical equations must be balanced so the number of each type of atom is equal on both sides.
For example, in the reaction of sodium with chlorine: 2Na + Cl₂ → 2NaCl, two sodium atoms and two chlorine atoms appear on each side.
Comparing relative formula masses confirms this: the left side gives 2 × 23 + 2 × 35.5 = 117 and the right side gives 2 × (23 + 35.5) = 117.

2. Balancing Equations Using Conservation of Mass AQA

Because atoms are neither created nor destroyed, chemical equations must be balanced so the number of each type of atom is equal on both sides.
For example, in the reaction of sodium with chlorine: 2Na + Cl₂ → 2NaCl, two sodium atoms and two chlorine atoms appear on each side.
Comparing relative formula masses confirms this: the left side gives 2 × 23 + 2 × 35.5 = 117 and the right side gives 2 × (23 + 35.5) = 117.

The conservation of mass can also be verified using actual masses in grams during an
experiment.
For example, reacting 2.3 g of sodium with 3.5 g of chlorine gas produces 5.8 g of sodium chloride, since 2.3 + 3.5 = 5.8 g.
If a reaction is carried out on a balance, the total mass should remain constant throughout the entire reaction.

3. Conservation of Mass Using Actual Masses AQA

The conservation of mass can also be verified using actual masses in grams during an
experiment.
For example, reacting 2.3 g of sodium with 3.5 g of chlorine gas produces 5.8 g of sodium chloride, since 2.3 + 3.5 = 5.8 g.
If a reaction is carried out on a balance, the total mass should remain constant throughout the entire reaction.

If a reactant is a gas (such as oxygen from the air), the scales may not measure it, making it appear as though the products weigh more than the reactants.
For example, when 1 g of magnesium burns in air, it reacts with oxygen to form approximately 1.6 g of magnesium oxide, because the oxygen from the air is not weighed on the scales.
The reaction can be written as: 2Mg + O₂ → 2MgO, and the increase in mass is due to the oxygen being absorbed from the surrounding air.

4. When Mass Appears to Increase AQA

If a reactant is a gas (such as oxygen from the air), the scales may not measure it, making it appear as though the products weigh more than the reactants.
For example, when 1 g of magnesium burns in air, it reacts with oxygen to form approximately 1.6 g of magnesium oxide, because the oxygen from the air is not weighed on the scales.
The reaction can be written as: 2Mg + O₂ → 2MgO, and the increase in mass is due to the oxygen being absorbed from the surrounding air.

If a product is a gas (such as carbon dioxide), it may escape into the air and not be measured, making it appear as though mass has been lost.
For example, when calcium carbonate decomposes:
CaCO₃_(s) → CaO_(s) + CO₂_(g)
the gaseous CO₂_(g) floats away, so the remaining solid appears lighter than the original reactant.

5. When Mass Appears to Decrease AQA

If a product is a gas (such as carbon dioxide), it may escape into the air and not be measured, making it appear as though mass has been lost.
For example, when calcium carbonate decomposes:
CaCO₃_(s) → CaO_(s) + CO₂_(g)
the gaseous CO₂_(g) floats away, so the remaining solid appears lighter than the original reactant.

Carrying out a reaction in a sealed container traps any gases produced or consumed, ensuring all substances are accounted for on the balance.
In a sealed container, the mass of reactants will always equal the mass of products, confirming the conservation of mass principle regardless of whether gases are involved.

6. Using a Sealed Container to Confirm Conservation of Mass AQA

Carrying out a reaction in a sealed container traps any gases produced or consumed, ensuring all substances are accounted for on the balance.
In a sealed container, the mass of reactants will always equal the mass of products, confirming the conservation of mass principle regardless of whether gases are involved.

Mass is always conserved in a chemical reaction, whether measured in grams or by counting atoms.
An apparent increase in mass suggests a gaseous reactant (e.g. oxygen) was not measured, whilst an apparent decrease suggests a gaseous product (e.g. CO₂) has escaped.
Performing experiments in a sealed container prevents gases from entering or escaping, ensuring accurate mass measurements that confirm conservation of mass.

7. Summary: Key Takeaways on Conservation of Mass AQA

Mass is always conserved in a chemical reaction, whether measured in grams or by counting atoms.
An apparent increase in mass suggests a gaseous reactant (e.g. oxygen) was not measured, whilst an apparent decrease suggests a gaseous product (e.g. CO₂) has escaped.
Performing experiments in a sealed container prevents gases from entering or escaping, ensuring accurate mass measurements that confirm conservation of mass.