Atoms, Elements & Compounds
Atom — the smallest part of an element that can exist. The word comes from the Greek atomos, meaning indivisible. All atoms have a tiny central nucleus surrounded by electrons.
Element — a substance made from only one type of atom. Each element has a unique chemical symbol (e.g. Na, Fe, Mg, Cl). Elements are arranged on the Periodic Table.
Compound — a substance made from atoms of two or more different elements chemically bonded together. A compound can only be separated by a chemical reaction.
Mixture — two or more elements or compounds that are not chemically bonded. The chemical properties of each substance are unchanged, and they can be separated by physical means.
Pure substance — in chemistry, a pure substance is a single element or compound, not mixed with anything else. This is different from the everyday meaning of "pure" (e.g. pure orange juice).
Dark circles and orange circles represent two different types of atom. In a compound, the two types are chemically bonded; in a mixture, they coexist but are not bonded and retain their individual chemical properties.
One practical way to check whether a substance is pure is to measure its melting point and compare it to the known value:
- A pure substance melts sharply at a single, fixed temperature that is close to the expected value.
- A mixture begins to melt below the expected melting point and melts over a range of temperatures rather than at one sharp point.
Pure substances and this melting-point test are covered in full in C8 Chemical Analysis — here it is just a handy way to picture the difference between a pure substance and a mixture.
In everyday language, "pure" means natural or uncontaminated. In AQA Chemistry, "pure" means a single element or compound — nothing else mixed in. Pure water is a compound (H₂O only). "Pure orange juice" is a mixture and would not be considered pure in a chemistry exam.
Chemical Symbols and Formulae
Every element has a unique symbol — one or two letters, where the first letter is always a capital and any second letter is always lowercase.
A chemical formula shows how many atoms of each element are in one unit of a substance. Subscript numbers (written below the line) indicate more than one atom of that element:
| Formula | Meaning |
|---|---|
| H₂O | 2 hydrogen atoms bonded to 1 oxygen atom |
| CO₂ | 1 carbon atom bonded to 2 oxygen atoms |
| NaCl | 1 sodium atom bonded to 1 chlorine atom |
| CaCO₃ | 1 calcium, 1 carbon, and 3 oxygen atoms bonded together |
| NH₃ | 1 nitrogen atom bonded to 3 hydrogen atoms |
Symbols must have the first letter as a capital and the second as lower case. NaCl is correct — NACl and nacl are wrong. Be careful: Co is cobalt, not CO (carbon monoxide).
Classify each substance below. Click a button for immediate feedback and an explanation.
🧪 Exam-style questions
Which statement correctly describes an element? Tick (✓) one box.
The formula of magnesium chloride is MgCl2. How many atoms are shown in this formula? Tick (✓) one box.
Sea water is a mixture, but sodium chloride is a compound. Explain the difference between a mixture and a compound.
Show answer
- In a compound the different elements are chemically bonded / joined together 1 mark
- In a mixture the substances are not chemically joined, so they can be separated by physical methods 1 mark
Allow: a compound can only be separated by a chemical reaction; the parts of a mixture keep their own properties.
Chemical Equations & Conservation of Mass
Word Equations
A word equation describes a chemical reaction using the names of reactants (left) and products (right), separated by an arrow:
Magnesium + Oxygen → Magnesium oxide
Naming rule: when a metal reacts with a non-metal, the metal's name is unchanged and the non-metal's name changes to end in -ide.
Balanced Symbol Equations
Atoms are never created or destroyed in a chemical reaction — they are rearranged. This means the number of atoms of each element must be the same on both sides of the equation. We balance equations by adding coefficients (large numbers in front of formulae) — never by changing the subscripts inside a formula.
Step 1 — Write the unbalanced equation: Al + O₂ → Al₂O₃
Step 2 — Count atoms: Al: 1 left, 2 right | O: 2 left, 3 right. Not balanced.
Step 3 — Add coefficients: 4Al + 3O₂ → 2Al₂O₃
Step 4 — Check: Al: 4 left, 4 right ✔ O: 6 left, 6 right ✔
🧱 Interactive — build the balanced equation
Balance lithium reacting with water
Group 1 metals react with water to give a metal hydroxide and hydrogen gas. Change the big number in front of each substance — that many particles appear in the picture, and the atom tally updates. Keep adjusting until lithium, hydrogen and oxygen each have the same number of atoms on both sides. (Never change a small subscript — that would change the substance.)
| Element | Reactant side | Product side | |
|---|---|---|---|
| Li | 1 | 1 | |
| H | 2 | 3 | |
| O | 1 | 1 |
Change the numbers so each element matches on both sides.
🧪 Exam-style questions
Balance the equation for hydrogen reacting with chlorine.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check. Any correct set of numbers is accepted.
Show answer
H₂ + Cl₂ → 2HCl
- Only HCl needs a balancing number: a 2 in front gives the ratio 1 : 1 : 2. 1 mark
- Check: H 2 = 2 · Cl 2 = 2 ✓ (any equal multiple, e.g. 2 : 2 : 4, also balances — but never change the formulae themselves)
Magnesium burns in oxygen to form magnesium oxide. Balance the equation.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check. Any correct set of numbers is accepted.
Show answer
2Mg + O₂ → 2MgO
- O2 supplies 2 oxygen atoms, so 2 MgO are needed on the right. 1 mark
- That gives 2 Mg on the right, so a 2 is needed in front of Mg — ratio 2 : 1 : 2. 1 mark
Check: Mg 2 = 2 · O 2 = 2 ✓ (any equal multiple, e.g. 4 : 2 : 4, also balances).
A balanced symbol equation must obey the law of conservation of mass. Describe what conservation of mass means in terms of atoms.
Show answer
- No atoms are made or lost / destroyed in a chemical reaction 1 mark
- The atoms are only rearranged, so there is the same number of each type of atom on both sides (the same total mass) 1 mark
Allow: the total mass of the products equals the total mass of the reactants.
Conservation of Mass
Because atoms are neither created nor destroyed, the total mass of the products always equals the total mass of the reactants. This is the Law of Conservation of Mass.
Half Equations and Ionic Equations H
At Higher Tier you may also need to write half equations and ionic equations.
A half equation shows either the oxidation or reduction step in a reaction, including electrons (e⁻). Example — sodium losing an electron:
e.g. Na → Na⁺ + e⁻
An ionic equation shows only the ions and species that actually change — spectator ions (present but unchanged) are cancelled out. Example — the ionic equation for neutralisation:
e.g. H⁺(aq) + OH⁻(aq) → H₂O
You will meet half equations and ionic equations throughout the course wherever they are relevant (e.g. electrolysis, displacement reactions, neutralisation).
Separating Mixtures
A mixture contains two or more substances that are not chemically joined, so each keeps its own properties. That means we can pull them apart using physical processes — by particle size, solubility, or boiling point — with no chemical reaction and no new substances made. You need to be able to describe each method below, and choose the right one for a given mixture.
Several of these methods depend on whether a substance dissolves, so it is worth being clear on a few closely-related words before we start.
The solute doesn’t vanish: 10 g of salt in 100 g of water makes 110 g of solution — and evaporating the water gives the salt back.
Solvent — the liquid that does the dissolving. Water is by far the most common solvent.
Solute — the substance (often a solid) that dissolves into the solvent.
Solution — what forms when a solute dissolves in a solvent: the solute particles spread evenly all the way through. A solution looks clear (you can see through it), even when it is coloured.
Soluble — describes a substance that will dissolve in a given solvent (e.g. salt and sugar are soluble in water).
Insoluble — describes a substance that will not dissolve (e.g. sand is insoluble in water).
Filtration
Filtration separates an insoluble solid from a liquid — for example sand from water. The mixture is poured into a filter funnel lined with filter paper. The liquid passes through the tiny holes in the paper and is collected as the filtrate, while the solid particles are too big to pass through and stay on the paper as the residue.
Crystallisation
Crystallisation separates a soluble solid that is dissolved in a solution — for example salt from salt water. The solution is gently heated in an evaporating basin — usually resting on a beaker of hot water as a water bath, which gives slow, even heat so the crystals do not spit or break down — and the water evaporates away. The dissolved solid cannot evaporate, so as the solution becomes more concentrated it is left behind, forming crystals in the basin.
Simple distillation
Simple distillation separates the liquid (solvent) from a solution — for example pure water from salt water. The solution is heated until the liquid boils. Its vapour travels into a water-cooled condenser, where it cools back into a liquid and drips out as the pure distillate; the dissolved solid is left behind in the flask. It only works cleanly when the parts of the mixture have very different boiling points.
Fractional distillation
Fractional distillation separates a mixture of two or more liquids that mix completely but have different boiling points — for example water from ethanol.
Chromatography
Chromatography separates substances that are dissolved in a solvent, such as the coloured dyes in an ink.
For each mixture, click the method you would use. Pick the technique that does the key separation — some mixtures take more than one step, which the feedback will explain.
🧪 Exam-style questions
Which technique would you use to obtain pure water from sea water? Tick (✓) one box.
Which technique separates a mixture of coloured dyes in an ink? Tick (✓) one box.
A student has a mixture of sand and salt. Describe how they could separate the mixture to recover both the dry sand and the dry salt.
Show answer
- Add water and stir, so the salt dissolves (the sand does not) 1 mark
- Filter the mixture: the sand is the residue (left in the filter paper) and is rinsed and dried 1 mark
- Evaporate / crystallise the salt solution (the filtrate) to leave dry salt crystals 1 mark
Allow: heat the salt solution to evaporate the water; do not accept distillation as the route to recover the salt.