GCSE Chemistry revision

Conservation of mass and balanced chemical equations

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

AQA 4.3.1.1 Foundation & Higher

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Key knowledge

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What is a Chemical Equation?

A chemical equation is a way of showing what happens during a chemical reaction, identifying which substances react and which are produced.
The substances on the left-hand side of the arrow are called the reactants, and the substances on the right-hand side are called the products.
A single arrow ( ) separates the reactants from the products and indicates that the reactants react to form the products.

1. What is a Chemical Equation? AQA

Notes

A chemical equation is a way of showing what happens during a chemical reaction, identifying which substances react and which are produced.
The substances on the left-hand side of the arrow are called the reactants, and the substances on the right-hand side are called the products.
A single arrow ( ) separates the reactants from the products and indicates that the reactants react to form the products.

Word Equations

A word equation uses the full names of substances to describe a chemical reaction, for example: methane + oxygen carbon dioxide + water.
Word equations are a simple way to represent reactions but do not show the chemical formulae or the number of atoms involved.

Symbol Equations

A symbol equation uses chemical formulae to represent each substance in the reaction, for example: CH₄ + O₂ → CO₂ + H₂O.
Some elements exist as diatomic molecules, meaning they naturally occur as pairs of atoms, so they must be written as such in equations.
Oxygen must be written as O₂, chlorine as Cl₂, and nitrogen as N₂ because these elements exist as diatomic molecules.

3. Symbol Equations AQA

Notes

A symbol equation uses chemical formulae to represent each substance in the reaction, for example: CH₄ + O₂ → CO₂ + H₂O.
Some elements exist as diatomic molecules, meaning they naturally occur as pairs of atoms, so they must be written as such in equations.
Oxygen must be written as O₂, chlorine as Cl₂, and nitrogen as N₂ because these elements exist as diatomic molecules.

Why Do We Balance Equations?

Chemical equations must be balanced because atoms cannot be created or destroyed during a chemical reaction — this is the law of conservation of mass.
A balanced equation has the same number of each type of atom on both the left-hand side and the right-hand side of the arrow.
For example, CH₄ + O₂ → CO₂ + H₂O is unbalanced because there are 2 oxygen atoms on the left but 3 on the right.

4. Why Do We Balance Equations? AQA

Notes

Chemical equations must be balanced because atoms cannot be created or destroyed during a chemical reaction — this is the law of conservation of mass.
A balanced equation has the same number of each type of atom on both the left-hand side and the right-hand side of the arrow.
For example, CH₄ + O₂ → CO₂ + H₂O is unbalanced because there are 2 oxygen atoms on the left but 3 on the right.

Rules for Balancing Equations

You must never change the small subscript numbers in a chemical formula, as this would change the identity of the substance entirely (e.g. changing O₂ to O₃ turns oxygen into ozone).
To balance an equation, you can only change the large coefficients (big numbers) written in front of chemical formulae, which represent how many molecules of that substance are present.
Coefficients must always be whole numbers — fractions such as ½O₂ are not permitted in balanced equations.

5. Rules for Balancing Equations AQA

Notes

You must never change the small subscript numbers in a chemical formula, as this would change the identity of the substance entirely (e.g. changing O₂ to O₃ turns oxygen into ozone).
To balance an equation, you can only change the large coefficients (big numbers) written in front of chemical formulae, which represent how many molecules of that substance are present.
Coefficients must always be whole numbers — fractions such as ½O₂ are not permitted in balanced equations.

Balancing by Trial and Error

Balancing equations is usually a process of trial and error, where you try different coefficients until the number of each atom is equal on both sides.
A useful strategy is to balance the least common elements first, leaving hydrogen and oxygen until last as they often appear in multiple compounds.
After making any change to a coefficient, always recount all atoms on both sides to check whether the equation is now balanced.

6. Balancing by Trial and Error AQA

Notes

Balancing equations is usually a process of trial and error, where you try different coefficients until the number of each atom is equal on both sides.
A useful strategy is to balance the least common elements first, leaving hydrogen and oxygen until last as they often appear in multiple compounds.
After making any change to a coefficient, always recount all atoms on both sides to check whether the equation is now balanced.

Worked Example 1: Methane Combustion

Starting with CH₄ + O₂ → CO₂ + H₂O, the oxygen and hydrogen atoms are not balanced.
Adding a coefficient of 2 in front of O₂ gives 4 oxygen atoms on the left, and adding a coefficient of 2 in front of H₂O balances both the hydrogen and oxygen atoms.
The fully balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O, with 1 carbon, 4 hydrogens, and 4 oxygens on each side.

7. Worked Example 1: Methane Combustion AQA

Notes

Starting with CH₄ + O₂ → CO₂ + H₂O, the oxygen and hydrogen atoms are not balanced.
Adding a coefficient of 2 in front of O₂ gives 4 oxygen atoms on the left, and adding a coefficient of 2 in front of H₂O balances both the hydrogen and oxygen atoms.
The fully balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O, with 1 carbon, 4 hydrogens, and 4 oxygens on each side.

Worked Example 2: Sulfuric Acid and Sodium Hydroxide

The unbalanced equation is H₂SO₄ + NaOH → Na₂SO₄ + H₂O, where sodium is not balanced (1 on the left, 2 on the right).
Placing a coefficient of 2 in front of NaOH balances the sodium atoms, giving 2 on each side.
Adding a coefficient of 2 in front of H₂O balances the remaining hydrogen and oxygen atoms, giving the fully balanced equation: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

8. Worked Example 2: Sulfuric Acid and Sodium Hydroxide AQA

Notes

The unbalanced equation is H₂SO₄ + NaOH → Na₂SO₄ + H₂O, where sodium is not balanced (1 on the left, 2 on the right).
Placing a coefficient of 2 in front of NaOH balances the sodium atoms, giving 2 on each side.
Adding a coefficient of 2 in front of H₂O balances the remaining hydrogen and oxygen atoms, giving the fully balanced equation: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

Checking Your Balanced Equation

Once you believe an equation is balanced, always verify by counting every type of atom on both sides of the arrow.
For H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O, there are 4 hydrogens, 1 sulfur, 6 oxygens, and 2 sodium atoms on each side, confirming it is balanced.
Checking your work at the end is an essential habit to avoid errors in exams.

9. Checking Your Balanced Equation AQA

Notes

Once you believe an equation is balanced, always verify by counting every type of atom on both sides of the arrow.
For H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O, there are 4 hydrogens, 1 sulfur, 6 oxygens, and 2 sodium atoms on each side, confirming it is balanced.
Checking your work at the end is an essential habit to avoid errors in exams.

Summary / Key Terms

Reactants are the starting substances in a chemical reaction, written on the left-hand side of the equation.
Products are the substances formed during a chemical reaction, written on the right-hand side of the equation.
A coefficient is the large whole number placed in front of a chemical formula to indicate how many molecules of that substance are involved in the reaction.

10. Summary / Key Terms AQA

Notes

Reactants are the starting substances in a chemical reaction, written on the left-hand side of the equation.
Products are the substances formed during a chemical reaction, written on the right-hand side of the equation.
A coefficient is the large whole number placed in front of a chemical formula to indicate how many molecules of that substance are involved in the reaction.