Elements, compounds and mixtures for iGCSE Chemistry
An element is a substance made of only one type of atom. A compound is two or more elements chemically joined in fixed proportions. A mixture is two or more substances physically combined that are not chemically joined and can be separated again. That three-way distinction is the foundation of iGCSE Chemistry Topic 2 and shows up on Paper 1, Paper 2, and the practical papers.
This guide explains the definitions Cambridge International rewards, the particle diagrams you need to be able to draw, the separation techniques for mixtures, and the patterns examiners look for in 4-mark and 6-mark answers.
Elements: One type of atom
Every element is listed on the Periodic Table. Examples: Hydrogen, oxygen, iron, gold. Around 118 elements are currently named.
Compounds: Chemically joined
Atoms of two or more elements bonded together in fixed ratios. Examples: Water (H2O), carbon dioxide (CO2), sodium chloride (NaCl).
Mixtures: Physically combined
Two or more substances mixed together without bonding. Examples: Air, sea water, brass. Can be separated by physical means.
Defining each term properly
An element is a pure substance made up of only one type of atom. Every element has its own symbol on the Periodic Table, like H for hydrogen or Fe for iron. You cannot break an element down into anything simpler using chemical reactions.
A compound is a pure substance formed when atoms of two or more different elements are chemically bonded in a fixed ratio. Water is always 2 hydrogen atoms to 1 oxygen atom. A compound has different properties from the elements it is made of: Sodium is a reactive metal, chlorine is a toxic gas, but sodium chloride is the table salt on your dinner.
A mixture is two or more substances combined but not chemically bonded. The substances keep their own properties, and the ratio is not fixed. You can separate a mixture using physical methods like filtration, distillation, or chromatography.
Pure versus impure In iGCSE Chemistry, a pure substance is either a single element or a single compound. A mixture is not pure, no matter how clean it looks. Sea water is a mixture, even though it looks like one liquid. "Pure" in chemistry is not the same as "clean" in everyday speech.
Comparing elements, compounds and mixtures
The table below summarises the three. Cambridge International often asks students to classify a substance and justify the answer. Use the table as a checklist when you do.
| Property | Element | Compound | Mixture |
|---|---|---|---|
| Number of substances | One | One (but made of two or more elements) | Two or more |
| Chemically bonded? | Atoms of one type | Yes, in fixed ratios | No |
| Fixed composition? | Yes | Yes | No, ratios can vary |
| Separated by physical means? | No | No | Yes |
| Properties | Unique to that element | Different from constituent elements | Same as constituent substances |
| Examples | Oxygen, copper, gold | Water, methane, NaCl | Air, brass, sea water |
Particle diagrams: What examiners want
Particle diagrams are how iGCSE Chemistry tests whether you really understand the difference. The rules are tight, and a sloppy diagram is the easiest way to drop marks on Paper 1.
For an element, draw circles of one colour or shape, all identical. For a compound, draw circles of two or more colours or shapes touching or joined, in a repeated fixed ratio. For a mixture, draw a jumble of different particles not bonded together, with no fixed ratio. Always label your key so the examiner knows which circle represents which atom.
Particle diagram checklist Different atoms must look different (size, colour, or shading). Compound atoms must be touching to show bonds. Mixture particles must not be touching as bonded pairs. Always include a key. Draw at least 8–10 particles so the examiner can see the pattern.
Separating mixtures
Because mixtures are not chemically bonded, you can separate them using physical methods. iGCSE Chemistry expects you to know five core techniques and when to use each one. The right method depends on the type of mixture: Solid–solid, solid–liquid, liquid–liquid, or dissolved solid in liquid.
| Technique | Used for | How it works |
|---|---|---|
| Filtration | Insoluble solid from a liquid | Pour the mixture through filter paper. Solid stays as residue, liquid passes through as filtrate |
| Evaporation | Soluble solid from a solution | Heat the solution to evaporate the solvent, leaving the solid behind |
| Simple distillation | A solvent from a solution | Boil the solution, condense the vapour back to liquid. Used to collect pure water from sea water |
| Fractional distillation | Two or more miscible liquids | Uses a fractionating column. Liquids with different boiling points separate at different heights |
| Chromatography | Coloured substances in a mixture | Solvent moves up the paper, carrying different substances at different speeds. Used to test inks and food dyes |
Worked example: Classifying salt water
Question: Salt water contains sodium chloride dissolved in water. State whether salt water is an element, a compound, or a mixture, and describe how you would separate the salt from the water.
Answer: Salt water is a mixture. It contains two substances (sodium chloride and water) that are not chemically bonded and can be present in varying ratios.
To separate them, use simple distillation. Heat the salt water in a flask. The water boils and evaporates, leaving the salt behind. The water vapour passes into a condenser, where it cools back to liquid water and is collected in a beaker. This recovers both the pure water and the salt.
Common mistakes that cost easy marks Calling air a compound (it is a mixture of nitrogen, oxygen, and other gases). Saying a compound can be separated by physical means (it cannot, only by chemical reaction). Drawing identical particles for a compound (a compound has two or more types of atom). Forgetting to label the key on a particle diagram. Mixing up filtration (insoluble solid) with evaporation (soluble solid).
Quick test: Element, compound, or mixture?
Try classifying each of these. Air: Mixture (nitrogen, oxygen, argon, carbon dioxide). Diamond: Element (just carbon atoms). Sugar: Compound (carbon, hydrogen, oxygen in fixed ratio). Brass: Mixture (copper and zinc, an alloy). Carbon dioxide: Compound (one carbon, two oxygen). Sea water: Mixture (water, salts, dissolved gases).
If you got six out of six, your classification is solid. If you got tripped up on diamond or brass, look back at the three definitions and check what makes each one different: One type of atom, chemically bonded, or physically combined.
Key facts to memorise for the exam
- Element: Made of one type of atom. Cannot be broken down chemically
- Compound: Two or more elements chemically bonded in fixed ratios. Different properties from the elements
- Mixture: Two or more substances not chemically bonded. Ratios vary. Separable by physical means
- Air, sea water, and brass are mixtures, not compounds
- Water, carbon dioxide, and salt are compounds
- Filtration separates insoluble solids from liquids
- Evaporation separates soluble solids from solutions
- Distillation separates a solvent from a solution; fractional distillation separates miscible liquids
