Pure substances and mixtures

What "pure" means in science

A pure substance contains only one kind of particle. Every particle in it is identical. Distilled water is pure: every particle is a water molecule. Spring water is not, because it also contains particles of dissolved minerals.

Pure substances have fixed properties. Pure water always freezes at 0 °C and boils at 100 °C (at sea level). Add salt and both change: salty water freezes below 0 °C, which is why salt is spread on icy roads in cold countries. Testing whether something melts or boils at exactly the right temperature is one way chemists check purity.

Elements and compounds

There are two kinds of pure substance:

• An element is made of only one kind of atom. Iron, gold, oxygen and carbon are elements. There are 118 known elements, listed in the periodic table. Some elements exist as single atoms (iron, gold); others as molecules of identical atoms joined together (oxygen gas is pairs of oxygen atoms).
• A compound is made of two or more different kinds of atom chemically joined in a fixed ratio. Water (two hydrogen atoms joined to one oxygen), carbon dioxide and table sugar are compounds. A compound has completely different properties from the elements it's made from: sodium is a soft metal that explodes in water and chlorine is a poisonous gas, but joined together they make the salt on your chips.

Mixtures

A mixture contains two or more different substances that are mixed together but not chemically joined. That gives mixtures three important features:

1. The amounts can vary. Cordial can be weak or strong; water is always two hydrogens to one oxygen.
2. Each substance keeps its own properties. Salt water still tastes salty; iron filings mixed with sand are still magnetic.
3. They can be separated by physical methods, such as filtering, evaporating or using a magnet, which use those properties. Separating a compound needs a chemical reaction.

Mixtures that look the same all the way through, such as air or salt water, are called homogeneous. Mixtures where you can see the different parts, such as sand in water or a fruit salad, are heterogeneous.

Alloys: mixtures of metals

An alloy is a mixture of a metal with other elements, usually other metals. Alloys are often more useful than pure metals because the different-sized atoms make it harder for layers of atoms to slide, so the alloy is harder and stronger.

Gold jewellery shows how the amounts in a mixture can vary. Purity is measured in carats out of 24: 24-carat gold is pure gold, 18-carat is 18 parts gold out of 24 (75%) and 9-carat is only 37.5% gold. Pure gold is too soft for rings that get knocked about, so it's mixed with copper (rose gold), or with silver, nickel or palladium (white gold).