What does "pure" mean?
On a milk packet, "pure" means nothing has been added. To a scientist, a pure substance is made of only one kind of particle, with the same composition throughout. Milk, sea water and air are actually mixtures.
Pure substances are either elements or compounds.
An element cannot be broken down into simpler substances by chemical reactions. About 118 are known. They are grouped as metals (lustrous, conduct heat and electricity, malleable, ductile; e.g. iron, copper), non-metals (dull, poor conductors; e.g. carbon, oxygen, sulphur) and metalloids, with intermediate properties (boron, silicon).
A compound is formed when two or more elements combine chemically in a fixed proportion. Water is always hydrogen and oxygen in a mass ratio of 1 : 8.
Mixtures and compounds
Mixture | Compound | |
|---|---|---|
Composition | variable | fixed |
Properties | shows the properties of its components | new properties, different from its elements |
Formation | no new substance; usually no energy change | chemical reaction; energy is usually given out or taken in |
Separation | by physical methods | only by chemical or electrochemical reactions |
Example. Heat iron filings and sulphur powder together and you get iron sulphide, a compound. Before heating, a magnet pulls the iron out of the mixture; after heating, it cannot.
Kinds of mixtures
Homogeneous mixtures have a uniform composition throughout: salt in water, air, an alloy such as brass.
Heterogeneous mixtures have visibly different parts: sand and water, oil and water, a fruit salad.
Solutions
A solution is a homogeneous mixture of two or more substances. The component present in the larger amount is the solvent, and the one dissolved in it is the solute. In sugar solution, water is the solvent and sugar the solute.
Solutions need not be liquids: air is a solution of gases, and brass is a solid solution of zinc in copper.
Properties of a solution: its particles are smaller than 1 nm, invisible even under a microscope; they don't settle; they can't be filtered out; and they don't scatter a beam of light.
Concentration tells how much solute is present.
Mass by mass percentage = (mass of solute ÷ mass of solution) × 100.
Mass of solution = mass of solute + mass of solvent.
Example. 40 g of salt in 320 g of water. Mass of solution = 360 g, so the concentration = 40/360 × 100 ≈ 11.1%.
A saturated solution holds as much solute as it can at that temperature. Solubility is the amount of solute in a saturated solution, and it usually increases with temperature for solids.
Suspensions and colloids
Solution | Colloid | Suspension | |
|---|---|---|---|
Particle size | < 1 nm | 1 to 1000 nm | > 1000 nm |
Appearance | homogeneous | looks homogeneous, but is heterogeneous | heterogeneous |
Settles on standing? | no | no | yes |
Separated by filtration? | no | no | yes |
Scatters light (Tyndall effect)? | no | yes | yes (particles visible) |
Examples: chalk in water and muddy water are suspensions; milk, fog, smoke, jelly, shaving foam and blood are colloids.
The Tyndall effect is the scattering of a beam of light by colloidal particles, making its path visible. You see it when sunlight streams through a canopy of trees in mist, or through a gap into a dusty room.
Separating mixtures
Method | Used to separate | Example |
|---|---|---|
Evaporation | a dissolved solid from a liquid, keeping the solid | salt from sea water; dye from ink |
Centrifugation | very fine suspended particles, by spinning fast | cream from milk; blood tests |
Separating funnel | two immiscible liquids | oil and water; kerosene and water |
Sublimation | a sublimable solid from a non-sublimable one | ammonium chloride or camphor from salt |
Chromatography | coloured components of a dye | colours in ink; drugs from blood |
Distillation | two miscible liquids that boil far apart (> 25 °C) | acetone and water |
Fractional distillation | miscible liquids with close boiling points | gases from air; petroleum fractions |
Crystallisation | a pure solid from a solution | pure copper sulphate; sugar refining; alum |
Crystallisation gives purer solids than evaporation, because impurities stay dissolved in the solution, and some solids decompose or char when evaporated to dryness.
Physical and chemical changes
Melting, boiling and dissolving are physical changes: no new substance forms. Burning, rusting and cooking are chemical changes: new substances form.
Summary
Pure substances are elements or compounds; mixtures have variable composition.
Solutions, colloids and suspensions differ in particle size; colloids show the Tyndall effect.
Mass % = solute mass ÷ solution mass × 100.
Mixtures are separated by physical methods suited to the properties of their parts.