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Conservation of Mass and Balanced EquationsAQA GCSE Chemistry: Revision notes

Section 1

What is the law of conservation of mass?

The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.

Atoms are simply rearranged into new substances during a reaction — none are created or destroyed. This is why chemical equations must always be balanced.

Key termslaw of conservation of mass

Section 2

How do you write and balance symbol equations?

A balanced symbol equation shows exactly how many atoms of each element are on each side of a reaction, so it matches the law of conservation of mass.

To balance an equation, use multipliers:

  • A normal script number in front of a formula multiplies the whole formula, e.g. 2H2O means two molecules of water
  • A subscript number within a formula shows the number of atoms of that element in one unit, e.g. the '2' in H2O is fixed and cannot be changed to balance an equation

Only the numbers in front of formulae can be changed when balancing — never the subscripts within a formula, as that would change the substance itself.

Key termsmultiplierbalanced equation
Common mistake

Never change a subscript to balance an equation — e.g. changing H2O to H2O2 turns water into hydrogen peroxide, a completely different substance.

Example

Balancing the combustion of methane: CH4 + 2O2 → CO2 + 2H2O. There are 1 C, 4 H and 4 O atoms on each side.

Section 3

Why does mass sometimes appear to change during a reaction?

In a non-enclosed (open) system, mass can appear to increase or decrease even though no atoms are truly lost or gained overall. This happens when a reactant or product is a gas:

  • If a gas escapes into the air during the reaction (e.g. carbon dioxide released when a carbonate reacts with acid in an open flask), the measured mass will decrease, even though total mass is conserved
  • If a gas from the air (e.g. oxygen) joins the reaction (e.g. a metal being burned), the measured mass will increase

In a closed (sealed) system, no mass changes are observed, because no matter can enter or leave — this confirms the law of conservation of mass.

Key termsnon-enclosed systemclosed system
Example

Heating a metal carbonate in an open test tube releases carbon dioxide gas, so the solid remaining has a lower mass than the starting carbonate — even though total mass, including the escaped gas, is conserved.

Section 4

How can masses be used to find balancing numbers? (HT)

If you know the masses of reactants and products in a reaction, you can work out the balancing numbers for the equation:

  1. Convert each mass to moles (moles = mass ÷ Mr)
  2. Compare the moles of each substance to find the simple whole number ratio between them
  3. Use this ratio as the balancing numbers in the symbol equation

This approach links the law of conservation of mass directly to the mole concept, since equal ratios of moles correspond to equal ratios of atoms reacting.

Key termsmole ratio from masses

Must Know

  • Law of conservation of mass: no atoms are lost or made in a reaction, so mass of products = mass of reactants
  • Balance equations using multipliers in front of formulae, never by changing subscripts
  • In non-enclosed systems, apparent mass changes occur when a gas escapes or joins the reaction from the air
  • In a closed system, no mass change is observed, confirming conservation of mass
  • (HT) Convert masses to moles and find the simple whole number ratio to balance an equation

That's the notes covered.

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