Conservation of MassOxford AQA IGCSE Chemistry: Revision notes
Section 1
How are chemical reactions represented?
Chemical reactions can be represented in two main ways:
- A word equation, which names the reactants and products, e.g. magnesium + oxygen -> magnesium oxide
- A balanced symbol equation, which uses chemical formulae and balancing numbers to show the exact atoms involved, e.g. 2Mg + O2 -> 2MgO
Both forms must show reactants on the left and products on the right, separated by an arrow.
Section 2
How are state symbols used?
State symbols show the physical state of each reactant and product at the temperature of the reaction:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous (dissolved in water)
State symbols are written in brackets immediately after each formula, e.g. HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l).
Always include state symbols where the question gives you enough information to work them out - marks are often lost for omitting them.
Section 3
What does the law of conservation of mass state?
During a chemical reaction, no atoms are lost or made - atoms are simply rearranged into new substances.
- This means the total mass of the products must equal the total mass of the reactants
- A balanced symbol equation must show equal numbers of each type of atom on both sides, reflecting this conservation of mass
- This principle underlies all reacting-mass calculations in chemistry
If an experiment appears to show a mass change (e.g. a mass increase when magnesium burns, or a mass decrease when a carbonate is heated), it is because a reactant or product is a gas entering or leaving an open container - mass is still conserved overall, it is just not all measured in the container.
Must Know
- Reactions can be shown as word equations or balanced symbol equations
- State symbols show physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water)
- No atoms are lost or made in a chemical reaction - atoms are only rearranged
- The mass of the products always equals the mass of the reactants
- Apparent mass changes in open-container experiments are usually due to a gas entering or leaving the system, not a breach of conservation of mass
That's the notes covered.
Carry on to the next subtopic.