C1.3 Mixtures · KS3 Science

Mixtures and pure substances

A mixture contains two or more substances that are not chemically joined. Each keeps its own properties, and they can be separated by physical methods. Air, sea water and soil are mixtures.
In science, a pure substance contains only one element or compound.
A pure substance melts and boils at fixed temperatures: pure water melts at 0 °C and boils at 100 °C. Impurities lower the melting point, raise the boiling point, and make the substance melt over a range of temperatures.

A solute (such as salt) dissolves in a solvent (such as water) to make a solution.
Soluble substances dissolve; insoluble substances (such as sand) don't.
A saturated solution can't dissolve any more solute. For most solids, solubility increases as the temperature rises.
Mass is conserved: mass of solution = mass of solute + mass of solvent.

Filtration separates an insoluble solid from a liquid (sand from water). The solid left in the filter paper is the residue; the liquid that passes through is the filtrate.
Evaporation or crystallisation gets a soluble solid from a solution (salt from salt water). For crystals, heat to evaporate some water, then leave the solution to cool.
Simple distillation gets the solvent from a solution (pure water from sea water): the water evaporates, then condenses in a cooled condenser.
Fractional distillation separates liquids with different boiling points, such as ethanol and water, or the parts of crude oil.

Chromatography separates the dyes in inks or food colourings.
Put a spot of ink on a pencil line near the bottom of the paper and stand the paper in a solvent below the line.
The solvent moves up the paper. Dyes that are more soluble travel further, so the dyes separate.
Draw the line in pencil because pencil doesn't dissolve in the solvent.

Mixture: Two or more substances that are not chemically joined.
Pure substance: A single element or compound with nothing else mixed in.
Solute: The substance that dissolves.
Solvent: The liquid that the solute dissolves in.
Solution: A mixture of a solute dissolved in a solvent.
Soluble: Able to dissolve.
Insoluble: Not able to dissolve.
Saturated solution: A solution that can't dissolve any more solute.
Filtration: Separating an insoluble solid from a liquid with filter paper.
Crystallisation: Getting crystals of a solid by evaporating some solvent and letting the solution cool.
Distillation: Separating a liquid from a mixture by evaporating it and condensing it again.
Chromatography: Separating dissolved substances, such as dyes, as a solvent moves up paper.