Solution, solute and solvent in chemistry: definitions with examples, the difference from a suspension and a colloid, the types of solution, and saturated, unsaturated and supersaturated explained for Class 9 and Year 9

"Define a solution" is the first question in CBSE Class 9 Is Matter Around Us Pure, in GCSE and IGCSE chemistry and in MYP 2 and 3 science, and most children answer it with an example ("salt in water") instead of a definition, which gets zero. This page gives the definitions the way the mark scheme wants them, the particle picture that explains them, the examples, the types, and the three distinctions that are actually tested: solution against suspension against colloid, solute against solvent, and saturated against unsaturated against supersaturated.

The definitions

Solution: a homogeneous mixture of two or more substances. Homogeneous means uniform: the same composition in every part of the sample. The particles of the dissolved substance are of molecular or ionic size (less than 1 nm), so they cannot be seen, do not scatter a beam of light, do not settle, and pass through filter paper.

Solute: the substance that is dissolved; the one present in the smaller amount.

Solvent: the substance that dissolves the solute; the one present in the larger amount, and the one whose state the solution takes. When water is the solvent the solution is called an aqueous solution and is written (aq) in equations.

Solubility: the maximum mass of solute that dissolves in 100 g of solvent at a given temperature, to give a saturated solution.

Fifteen examples, with the solute and the solvent named

The exam trick in this table: a solution does not have to be a liquid. Air is a gaseous solution; brass and steel are solid solutions (alloys). When a child writes "a solution is a liquid in which a solid is dissolved" the definition is wrong on both counts.

The nine types of solution, by state

Solution, suspension or colloid: the three-way question

This is the question that separates a 3-mark answer from a 1-mark one. All three are mixtures; they differ in particle size, and everything else follows from that.

The Tyndall effect is the test to describe: shine a torch through the beaker in a dark room. A solution shows no path; a colloid and a suspension show the beam. Then filter: the suspension is caught, the colloid goes through. Two observations, three mixtures told apart.

Saturated, unsaturated and supersaturated

• Unsaturated: more solute can still dissolve at this temperature. Stir in a spoon of sugar and it disappears.
• Saturated: no more dissolves at this temperature; extra solute lies at the bottom. The amount dissolved per 100 g of solvent is the solubility.
• Supersaturated: holds more than the saturated amount, made by dissolving at a higher temperature and cooling slowly without disturbance. Unstable: a seed crystal makes the excess crystallise out.

Solubility of most solids in water rises with temperature (potassium nitrate: 32 g per 100 g at 20 °C, 246 g at 100 °C), which is why a hot saturated solution deposits crystals on cooling, and how crystallisation is used to purify a solid. Solubility of gases falls with temperature and rises with pressure, which is why a warm bottle of cola goes flat faster and why a fizzy drink is bottled under pressure.

Concentration: the two forms a Class 9 or Year 9 paper uses

Mass by mass percentage = mass of solute ÷ mass of solution × 100. A solution with 20 g of salt in 80 g of water has 20 ÷ 100 × 100 = 20 % salt (the solution weighs 100 g, not 80 g: the most common error).

Mass by volume percentage = mass of solute ÷ volume of solution × 100, used for solutions where the solvent is measured in mL. A 5 % (m/v) glucose drip has 5 g of glucose in every 100 mL.

Molarity (moles of solute per litre of solution) comes in Class 11, A-level and AP; the definition is the same idea with moles in place of grams.

Separating a solution back into its parts

Because a solution is a mixture, physical methods recover the parts: evaporation to get the solid solute back (salt from sea water), simple distillation to get the solvent back pure (drinking water from sea water), fractional distillation when the two liquids have close boiling points (ethanol and water; the gases of air), crystallisation to get a pure solid from an impure one, and chromatography to separate the dyes in a solution of ink. Filtration does not work on a true solution, and that fact is itself a mark.

The questions that come up, and the answer that scores

How we teach the Class 9 and Year 9 chemistry chapters

A 1-on-1 online class where the child writes the definitions in their own words first, then we fix them against the mark scheme, then the particle picture, then the numericals. The suspension-colloid-solution table above is done as a kitchen experiment with a torch in the first class on this chapter. The first 30-minute class is free: send the chapter and the last test on WhatsApp or the contact page. How to draw the particles of a solution and of the three states is in particle diagrams; the subject page is online chemistry tutoring.

Questions parents ask

See how we teach this, 1-on-1 online →