Key Stage 4 science · Chemistry – Chemical changes

GCSE chemistry: formulae, equations and gas tests

In Key Stage 4 chemistry, students move from describing reactions in words to writing them in the shorthand chemists use. They work out empirical formulae from experimental masses, balance symbol equations, add state symbols, write simple ionic equations and learn the standard tests for common gases. This is the language of the whole chemistry course, so confidence here makes almost every later topic easier.

Formulae, equations and gas tests, explained

A chemical formula says which atoms are in a substance and in what ratio. In water, H₂O, the small 2 means two hydrogen atoms for every oxygen atom. The empirical formula is the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual numbers in one molecule. Ethane is C₂H₆, but its empirical formula is CH₃. Giant structures such as salt or magnesium oxide are only ever written as empirical formulae, because they are not made of separate molecules.

Students find an empirical formula from experimental results, usually the masses of two elements that combined. The method is always the same: divide each mass by that element’s relative atomic mass, read from the periodic table, then compare the results as a ratio and scale it to whole numbers. Teachers often call the amounts that come out “moles”, and many schools use that word here, but the statutory requirement is only that students can determine the formula from the ratio of atoms.

A balanced symbol equation shows the same atoms before and after a reaction, because atoms are rearranged and never created or destroyed. Methane burning is CH₄ + 2O₂ → CO₂ + 2H₂O: one carbon, four hydrogen and four oxygen atoms on each side. Students balance by putting numbers in front of formulae, never by changing the small numbers inside them. State symbols are added in brackets: (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water.

An ionic equation leaves out the ions that do not change, called spectator ions, and shows only what actually happens. Mixing silver nitrate solution with sodium chloride solution gives a white solid, and the ionic equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l). The charges must balance as well as the atoms. Ionic equations are part of the required content, but schools often teach them later in the course and treat the harder ones as stretch material, so ask your child’s teacher how far the school goes.

Finally, students learn how to identify four common gases. Hydrogen burns with a squeaky pop when a lit splint is held at the mouth of the test tube. Oxygen relights a glowing splint. Carbon dioxide turns limewater cloudy, sometimes described as milky. Chlorine bleaches damp blue litmus paper, which first turns red, and it is toxic, so it is only ever shown by a teacher in a fume cupboard. These tests are best learned together with the reaction that makes each gas, such as a metal reacting with acid for hydrogen.

Words your child will hear

Empirical formula: The simplest whole-number ratio of the atoms of each element in a compound, such as CH₂ for any alkene.
Molecular formula: The actual number of each kind of atom in one molecule, such as C₂H₆ for ethane.
Relative atomic mass: A number on the periodic table comparing the mass of an atom with others, for example 56 for iron and 16 for oxygen.
State symbol: A bracketed letter after a formula showing its state: (s), (l), (g) or (aq).
Aqueous: Dissolved in water. Salty water is sodium chloride (aq), but pure water is (l).
Spectator ion: An ion present in a mixture that takes no part in the reaction and is left out of the ionic equation.
Ionic equation: An equation showing only the ions and particles that change, with charges balanced.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about determination of empirical formulae from the ratio of atoms of different kinds
Students should be taught about balanced chemical equations, ionic equations and state symbols
Students should be taught about identification of common gases

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

When 5.6 g of iron is heated in oxygen, it combines with 2.4 g of oxygen. Find the empirical formula of the iron oxide, then write the balanced equation for its formation. (Relative atomic masses: Fe = 56, O = 16.)

Write down the masses of each element that combined: 5.6 g of iron and 2.4 g of oxygen.
Divide each mass by the relative atomic mass. This turns grams into amounts of atoms that can be compared fairly.
```
Fe:  5.6 ÷ 56 = 0.10
O:   2.4 ÷ 16 = 0.15
```
Write the results as a ratio and divide both by the smaller number, 0.10.
```
Fe : O = 0.10 : 0.15
       = 1 : 1.5
```
1.5 is not a whole number, so do not round it. Multiply both sides by 2 to get whole numbers.
```
Fe : O = 2 : 3
```
Write the formula from the ratio: 2 iron atoms to 3 oxygen atoms is Fe₂O₃.
Now the equation. Iron is a solid, oxygen is a diatomic gas, and the product is a solid. Start with Fe + O₂ → Fe₂O₃, then balance with big numbers only: 3 O₂ gives 6 oxygen atoms to match 2 Fe₂O₃, and 4 Fe then matches the 4 iron atoms.
```
4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)
Fe: 4 = 4     O: 6 = 6
```

The empirical formula is Fe₂O₃, and the balanced equation is 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s), with four iron atoms and six oxygen atoms on each side.

Common mistakes and how to help

Balancing an equation by changing the small numbers in a formula

Writing H₂O₂ instead of H₂O would fix the oxygen count in seconds, so it feels like a neat trick. But H₂O₂ is hydrogen peroxide, a completely different substance.

What helps: Explain that the formula is the substance’s name tag and cannot be edited. Only the big numbers in front of a formula can change. Counting atoms in a table, one row per element, keeps the work tidy.

Using the masses themselves as the ratio of atoms

The masses 5.6 g and 2.4 g look like a ready-made ratio. But 5.6 : 2.4 simplifies to 7 : 3, which is a ratio of mass, not of atoms, and atoms of different elements have different masses.

What helps: Always divide by the relative atomic mass first. Ask “is this a count of atoms or a weight?” before writing the ratio. Heavier atoms such as iron are fewer in number for the same mass.

Rounding a ratio such as 1 : 1.5 to 1 : 1 or 1 : 2

Students have been taught to give sensible answers, so they tidy up an awkward number. In formulae, a ratio of 1 : 1.5 is exactly 2 : 3 and rounding gives the wrong compound.

What helps: Rounding is only safe when a value is very close to a whole number, such as 2.98. For a clear half, multiply through by 2. For a third, multiply by 3.

Mixing up the gas tests, or thinking a lit splint identifies any gas

Hydrogen, oxygen and carbon dioxide are all tested with a splint or limewater, so the details blur. Carbon dioxide puts a lit splint out, but so do several other gases, so this is not a proper test.

What helps: Link each test to what it shows: pop for hydrogen burning, relight for oxygen helping burning, cloudy for carbon dioxide forming calcium carbonate. A table with gas, test and positive result is worth rewriting by hand.

How to help at home

Count atoms with building bricks or beads. Use one colour per element and build the reactants of CH₄ + 2O₂ → CO₂ + 2H₂O, then rearrange the same pieces into the products. Seeing that nothing is left over or missing makes “balanced” concrete, and it helps your child spot when a number in front is wrong.
Weigh a fizzing reaction on kitchen scales. Stand a cup containing vinegar on kitchen scales, note the mass, then stir in two or three teaspoons of bicarbonate of soda and note it again once the fizzing stops. The reading falls, often by a few grams, because carbon dioxide escaped into the air, which is why chemists say “in a closed system, mass is conserved”. Do it on a tray, and do not taste anything.
Hunt for formulae in the kitchen and bathroom. Packets and labels mention substances such as bicarbonate of soda (sodium hydrogencarbonate), calcium carbonate and salt (sodium chloride). Ask your child to write each formula and say what elements it contains. Fizzy drinks are a good reminder that CO₂ is a gas dissolved in water, so it would carry the state symbol (aq).
Let them teach you the gas tests. Ask your child to explain the four tests as if you were a new Year 10 student, including what each positive result looks like and why. Explaining is stronger revision than rereading, and a pause or a vague answer shows exactly which test to look at again.

Builds on

Key Stage 3: Chemical reactions and equations
Key Stage 3: Acids, alkalis and neutralisation