Is Matter Around Us Pure?

1. What is a pure substance?

A pure substance is made up of only one kind of particle and has a fixed composition throughout. For example, gold, water, salt and sugar are pure substances because every part of the sample is identical. A pure substance cannot be separated into other substances by any physical process such as filtering or evaporating. In everyday language we call milk “pure”, but a scientist would say milk is a mixture because it contains water, fat, proteins and minerals all mixed together.

2. What is a mixture?

A mixture contains two or more pure substances mixed together in any proportion, and they are not chemically combined. Air, sea water, soil, milk and even our blood are mixtures. The substances in a mixture keep their own properties, and they can be separated by physical methods. Mixtures are of two types:

Homogeneous mixtures have a uniform composition throughout — you cannot see the different parts. Examples: salt dissolved in water, sugar in water, air, and alloys like brass. Heterogeneous mixtures have a non-uniform composition — you can usually see the separate parts. Examples: a mixture of sand and salt, oil and water, soil, and a mixture of iron filings and sulphur powder.

3. Solutions, suspensions and colloids

A solution is a homogeneous mixture of two or more substances. The substance present in the larger amount is the solvent; the substance dissolved in it is the solute. In a sugar solution, water is the solvent and sugar is the solute. Solutions can be solid-in-liquid (salt water), gas-in-liquid (soda water), gas-in-gas (air) or solid-in-solid (alloys). Properties of a true solution: it is homogeneous; particles are smaller than 1 nanometre (1 nm) and cannot be seen even with a microscope; it does not scatter a beam of light (no Tyndall effect); particles do not settle down; and they pass through filter paper.

Concentration tells how much solute is dissolved. Mass by mass percentage = (mass of solute ÷ mass of solution) × 100. Mass by volume percentage = (mass of solute ÷ volume of solution) × 100. A saturated solution is one in which no more solute can dissolve at a given temperature; an unsaturated solution can still dissolve more. The maximum amount of solute that dissolves in 100 g of solvent at a particular temperature is called its solubility, which usually increases with temperature for solids.

A suspension is a heterogeneous mixture in which solid particles are spread through a liquid but are large enough to be seen (greater than 100 nm), e.g. chalk powder in water or muddy water. Suspension particles settle on standing, scatter light (show the Tyndall effect) and can be filtered out. A colloid (or colloidal solution) appears homogeneous but is actually heterogeneous; its particles (1–100 nm) are too small to see but big enough to scatter light. Examples: milk, fog, smoke, cheese, ink and shaving foam. Colloid particles do not settle and cannot be separated by ordinary filtration — we use centrifugation.

4. The Tyndall effect

The Tyndall effect is the scattering of a beam of light by the tiny particles of a colloid or suspension, which makes the path of the light visible. You see it when sunlight enters a dusty room through a small hole, or when light from a car’s headlamp passes through fog. A true solution does not show the Tyndall effect because its particles are too small to scatter light. This is one easy way to tell a colloid apart from a true solution.

5. Separating the components of a mixture

Because the parts of a mixture keep their own properties, we can separate them using physical methods that exploit a difference in some property:

Evaporation separates a dissolved solid (non-volatile) from a liquid by boiling off the liquid, e.g. getting salt from sea water, or coloured dye from ink. Centrifugation spins a mixture fast so denser particles sink — used to separate cream from milk and to test blood and urine. Separating funnel separates two immiscible liquids of different densities (oil and water, kerosene and water): the denser liquid is run off from the bottom first.

Sublimation separates a mixture where one component sublimes (turns directly from solid to vapour), such as separating ammonium chloride, camphor or naphthalene from common salt. Chromatography separates substances that dissolve in the same solvent but travel at different speeds — used to separate the dyes in black ink, colours in a pigment, and drugs from blood. Distillation separates two miscible liquids whose boiling points differ by a reasonable amount (more than about 25 K); the liquid with the lower boiling point vaporises first and is condensed back. When boiling points are close, we use fractional distillation with a fractionating column — used to separate the gases of air and the fractions of petroleum.

Obtaining gases from air: Air is first compressed and cooled to form liquid air, which is then warmed in a fractionating column. The gases boil off one by one at their own boiling points — nitrogen first, then argon, then oxygen. Crystallisation is a process that separates a pure solid in the form of its crystals from a solution; it is better than simple evaporation because it removes soluble impurities and gives purer crystals. It is used to purify salt from sea water and to obtain pure copper sulphate crystals.

6. Physical and chemical changes

A physical change alters only the physical state or appearance — no new substance is formed and the change is usually reversible. Melting ice, dissolving sugar, boiling water and making a solution are physical changes. A chemical change (chemical reaction) produces one or more new substances with new properties; it is usually difficult to reverse. Burning of a candle, rusting of iron, cooking food and digestion are chemical changes. Mixing iron and sulphur is a physical change (you can pull the iron out with a magnet), but heating them forms iron sulphide, a new compound — a chemical change.

7. Types of pure substances: elements and compounds

Pure substances are of two kinds. An element is the simplest pure substance that cannot be broken into anything simpler by chemical reactions, e.g. iron, copper, oxygen, gold. Elements are of three types: metals (shiny, conduct heat and electricity, malleable and ductile, e.g. iron, copper; mercury is a liquid metal), non-metals (usually dull, poor conductors, e.g. oxygen, sulphur, carbon) and metalloids which show properties of both (e.g. boron, silicon, germanium).

A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio, e.g. water (H₂O), carbon dioxide (CO₂) and common salt (NaCl). In a compound the elements lose their own properties and a brand-new substance is formed; the parts can be separated only by chemical or electrochemical methods, not by physical means. This is the key difference between a compound and a mixture — a mixture has no fixed ratio and keeps the properties of its parts.