Last reviewed: 25 October 2026
Mixtures, elements and compounds form one of the foundations of junior school Integrated Science. The topic introduces how scientists classify matter and how substances can be separated, ideas that come back in chemistry for years afterwards. This guide explains the key terms, the separation methods and the exam questions that test them. Mixtures, elements and compounds is the first strand of KICD’s Grade 9 Integrated Science curriculum design.
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Elements
An element is a pure substance made of only one kind of atom. It cannot be broken down into simpler substances by chemical means. Examples include oxygen, hydrogen, carbon, iron, copper, gold and sulphur. Each element has a chemical symbol of one or two letters, such as O for oxygen, C for carbon, Fe for iron and Cu for copper. Elements are listed in the periodic table, and they can be grouped broadly into metals (shiny, good conductors of heat and electricity) and non-metals (often dull and poor conductors).
Compounds
A compound forms when two or more elements combine chemically in fixed proportions. Water is a compound of hydrogen and oxygen; table salt (sodium chloride) is a compound of sodium and chlorine; carbon dioxide is carbon and oxygen. A compound’s properties are usually very different from those of its elements. Sodium is a reactive metal and chlorine is a poisonous gas, yet together they make the salt we eat. Separating a compound back into its elements needs a chemical reaction, not a physical method.
Mixtures
A mixture contains two or more substances that are physically mixed but not chemically combined. The parts keep their own properties and can be present in any proportion. Examples include air, soil, salt water, muddy water, sand and iron filings, and milk. Because nothing has bonded, mixtures can be separated by physical methods.
Elements, Compounds and Mixtures Compared
- Composition: an element has one kind of atom; a compound has fixed proportions; a mixture can vary.
- Properties: a compound’s properties differ from its elements; a mixture keeps the properties of its parts.
- Separation: compounds need chemical methods; mixtures can be separated physically.
Methods of Separating Mixtures
- Filtration: separates an insoluble solid from a liquid, such as sand from water. The liquid that passes through is the filtrate; the solid left behind is the residue.
- Evaporation: recovers a dissolved solid from a solution by heating off the liquid, such as getting salt from salt water. Salt is made this way from sea water at the coast.
- Crystallisation: like evaporation, but the solution is only partly evaporated and then left to cool so that crystals form.
- Distillation: separates a liquid from a solution by boiling and condensing the vapour, giving pure water from salt water.
- Decantation: gently pouring off a liquid after a heavy solid has settled at the bottom.
- Magnetism: separates magnetic materials, such as iron filings, from non-magnetic ones like sand.
- Sieving: separates solids of different sizes, like stones from flour or sand.
- Chromatography: separates dyes in ink or food colouring as a solvent moves up paper.
- Winnowing: uses wind to blow chaff away from heavier grains, a method farmers use after threshing.
Choosing the Right Method
Exam questions often describe a mixture and ask for the best method. Ask: Is the solid dissolved or not? If not, use filtration or decantation. If dissolved and you want the solid, evaporate or crystallise. If you want the liquid, distil. Is one part magnetic? Use a magnet. Are the particles different sizes? Sieve.
Pure and Impure Substances
A pure substance has a fixed melting and boiling point. Pure water boils at 100°C at sea level. Impurities change these points; salt water boils above 100°C. This is how scientists check purity.
Revision Tips
Make a table of separation methods with the apparatus, an example mixture and what is collected. Draw and label the filtration and distillation set-ups, as diagrams are commonly tested. Then practise with the Grade 7 Integrated Science papers and the Grade 9 Integrated Science page. The exam countdown gives free daily questions and the What to Revise report flags weak topics. Full papers with answers are unlocked with the Monthly Pass (KSh 500 by M-Pesa).