GCSE · Chemistry · AQA · Spec 8462

Conservation of mass and balanced equations

Atoms are never created or destroyed — so every equation must balance perfectly.

What you need to know

  • The law of conservation of mass: atoms are conserved in every chemical reaction, so total mass of reactants = total mass of products.
  • A balanced symbol equation has the same number of each type of atom on both sides of the arrow.
  • Coefficients (in front of a formula) multiply the whole formula; subscripts (inside a formula) are fixed and must never be changed when balancing.
  • The relative formula mass (Mr) of a compound is found by adding the relative atomic masses (Ar) of all atoms in its formula, accounting for subscripts.

The big picture

The law of conservation of mass tells us that atoms are neither made nor destroyed during a chemical reaction. Because no atoms are lost, the total mass of the products always equals the total mass of the reactants. We represent reactions using balanced symbol equations, where the number of each type of atom is identical on both sides. Balancing is done by changing the coefficients (the big numbers in front of formulas) — never by changing subscripts inside a formula.

TONIGHT'S REVISION

Conservation of Mass & Balanced Equations

Atoms are never created or destroyed — so every reaction must balance perfectly.

How a balanced equation shows conservation of mass

Follow one reaction step by step — watch the atom count stay the same throughout.

Predict, then check

Read the equation carefully before committing — count every atom on both sides.

A student writes: H₂ + O₂ → H₂O to represent the formation of water. They say the equation is balanced because both sides contain H and O. Are they correct?

Conservation of mass

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Reason it through

Why does the total mass of products always equal the total mass of reactants?

Link 1 of 3

First link · your turn

Think about what a chemical reaction actually does to atoms — does it create new ones or destroy existing ones?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Where it appearsWhat it multipliesChange to balance?Effect on atom count
Coefficiente.g. the 2 in 2H₂O
Subscripte.g. the 2 in H₂O

Each cell hides a short answer and the reason behind it. Predict before you tap.

AQA 8462 · Paper 1

Mark scheme practice

AQA-style short-answer — check your answer against the mark scheme

Question

Balance the following equation: _Fe + _O₂ → _Fe₂O₃

Student answer

4Fe + 3O₂ → 2Fe₂O₃

Method marks0/2

Key points

1No atoms are created or destroyed — mass is always conserved.
2Balance equations by adjusting big numbers (coefficients) in front of formulas only.
3Never change a subscript to balance — that would change the substance entirely.
4Mr = sum of all Ar values for each atom in the formula.
5Percentage by mass = (Ar × number of atoms of that element ÷ Mr) × 100.

Worked example

Problem

Calculate the relative formula mass (Mr) of calcium carbonate, CaCO₃. Then find the percentage by mass of calcium in the compound. (Ar values: Ca = 40, C = 12, O = 16)

🧠

Memory hook

Think of atoms as Lego bricks: you can rearrange them but you can never magic a new brick into existence or make one vanish. Count the bricks on each side — they must always match.

★ Exam tip

In AQA Paper 1, balancing questions award a mark for the correct coefficients AND a mark for showing atom counts. Write a quick atom tally (e.g. 'H: left 2, right 2 ✓') in your working — examiners reward it and it stops careless errors.

⚠ Watch out

Changing a subscript (e.g. writing H₃O instead of H₂O) to make an equation balance — this changes the chemical entirely. Only ever change the coefficient in front.

✓

Check yourself

Without looking — what is the relative formula mass of water (H₂O), and what percentage by mass is oxygen? (Ar: H = 1, O = 16)

Flashcards

(24)
State the law of conservation of mass.
No atoms are created or destroyed in a chemical reaction, so the total mass of products equals the total mass of reactants.
What does a balanced symbol equation show?
The same number of each type of atom on both sides of the reaction arrow.
What is a coefficient in a chemical equation?
A large number written in front of a chemical formula that multiplies the whole formula (e.g. 2H₂O means two molecules of water).
What is a subscript in a chemical formula?
A small number written after an element symbol that shows how many atoms of that element are in one formula unit (e.g. the 2 in H₂O).
When balancing an equation, which number can you change?
Only the coefficient (in front of the formula). Subscripts must never be changed.
Why can you never change a subscript to balance an equation?
Changing a subscript changes the identity of the substance entirely, making the equation chemically wrong.
What does Mr stand for?
Relative formula mass — the sum of all relative atomic masses of atoms in a formula.
What does Ar stand for?
Relative atomic mass — the weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
How do you calculate the Mr of a compound?
Add together the Ar of every atom in the formula, multiplying each Ar by how many times that atom appears (given by its subscript).
Calculate Mr of CO₂ (C=12, O=16).
Mr = 12 + (2 × 16) = 12 + 32 = 44.
Calculate Mr of H₂O (H=1, O=16).
Mr = (2 × 1) + 16 = 2 + 16 = 18.
Calculate Mr of NaCl (Na=23, Cl=35.5).
Mr = 23 + 35.5 = 58.5.
What is the formula for percentage by mass of an element?
% by mass = (Ar × number of atoms of that element ÷ Mr of compound) × 100.
Calculate the percentage by mass of H in H₂O (H=1, Mr=18).
% H = (1 × 2 ÷ 18) × 100 = 11.1%.
Calculate the percentage by mass of O in CO₂ (O=16, Mr=44).
% O = (16 × 2 ÷ 44) × 100 = 72.7%.
In a balanced equation, what must be true about the sum of Mr values?
The sum of the Mr values of reactants (in the amounts shown by coefficients) equals the sum of the Mr values of products.
Balance this equation: H₂ + O₂ → H₂O
2H₂ + O₂ → 2H₂O (H: 4 each side, O: 2 each side).
Balance this equation: Mg + O₂ → MgO
2Mg + O₂ → 2MgO (Mg: 2 each side, O: 2 each side).
Balance this equation: CH₄ + O₂ → CO₂ + H₂O
CH₄ + 2O₂ → CO₂ + 2H₂O (C:1, H:4, O:4 each side).
What is the Mr of CaCO₃ (Ca=40, C=12, O=16)?
Mr = 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100.
If 10 g of reactants are used in a closed reaction, what is the total mass of products?
10 g — because mass is conserved; no atoms are lost.
What does the coefficient 3 in '3CO₂' mean?
Three molecules of CO₂ — it multiplies the entire formula, meaning 3 carbon atoms and 6 oxygen atoms in total.
Calculate Mr of MgO (Mg=24, O=16).
Mr = 24 + 16 = 40.
What is the percentage by mass of Ca in CaCO₃ (Ca=40, Mr=100)?
% Ca = (40 × 1 ÷ 100) × 100 = 40%.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn Conservation of mass and balanced equations properly — interactive practice, marked questions and flashcards.

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