AQA GCSE Chemistry

Atoms and elements

All substances are made of atoms, and an atom is the smallest piece of an element that still has that element's properties. An element contains only one type of atom, and there are about 100 different elements, all shown in the periodic table. Each is represented by a chemical symbol of one or two letters, the first a capital: O for oxygen, Na for sodium, Cl for chlorine.

The names and symbols of the first 20 elements, of the elements in Groups 1 and 7, and of the other elements that turn up in the course are worth knowing by heart, because every equation is written with them.

Other symbols used in the course include Fe (iron), Cu (copper), Zn (zinc), Ag (silver), Au (gold), Pb (lead), Sn (tin), Br (bromine), I (iodine), Cr (chromium), Mn (manganese), Co (cobalt) and Ni (nickel).

Compounds

A compound is formed when two or more elements are chemically combined in fixed proportions. Water is always two hydrogen atoms to one oxygen atom, so its formula is $\ce{H2O}$, and carbon dioxide is always one carbon to two oxygen, $\ce{CO2}$. A compound can be broken back into its elements only by a chemical reaction, and its properties are usually very different from those of the elements it contains: sodium is a reactive metal and chlorine a poisonous gas, but sodium chloride is the salt used on food.

A chemical reaction always makes one or more new substances and often has a detectable energy change, such as a temperature rise or fall. A reaction can be described in a word equation or in a symbol equation with formulae:

A symbol equation has to be balanced, with the same number of atoms of each element on both sides. The number in front of a formula multiplies everything in it, and a small number lower down applies only to the atom or bracket before it, so $\ce{2H2O}$ means four hydrogen atoms and two oxygen atoms. State symbols show the physical state: (s) solid, (l) liquid, (g) gas and (aq) for a substance dissolved in water.

Higher tier only: equations can also be written as ionic equations, which leave out the ions that do not change, and as half equations that show electrons being lost or gained. Both are covered where they are used, in the notes on displacement, acids and electrolysis.

Mixtures

A mixture contains two or more elements or compounds that are not chemically combined. Nothing new is made when they are mixed, so each substance keeps its own chemical properties, and the mixture can be separated again by physical processes, which do not involve chemical reactions. Air is a mixture of nitrogen, oxygen and other gases, and sea water is a mixture of water and dissolved salts.

Separating mixtures

The right method depends on what the mixture is made of and on how the substances differ physically.

Filtration

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel, the liquid (the filtrate) passes through and the solid stays on the paper. Sand can be separated from water in this way, but salt dissolved in the water goes through with the filtrate.

Crystallisation

Crystallisation gets a dissolved solid back out of a solution. The solution is heated gently so that some of the water evaporates, until crystals start to form on a drop cooled on a glass rod, or until it is about saturated. The solution is then left to cool, so that crystals grow, and they are filtered off and dried. Heating gently to the point where crystals start to appear, and not until it is dry, means that the crystals are larger and the solid does not decompose.

Simple distillation

Simple distillation separates a liquid from a solution, for example pure water from salt water. The solution is heated in a flask, the water boils and the vapour passes into a condenser, where cooling water on the outside turns it back into a liquid that is collected. The salt is left in the flask because it does not evaporate at that temperature.

Fractional distillation

Fractional distillation separates a mixture of liquids that have different boiling points, such as ethanol and water. The mixture is heated and the vapours rise up a fractionating column. The column is hotter at the bottom and cooler at the top, so vapours condense on the way up. The liquid with the lowest boiling point reaches the top of the column first and passes into the condenser, while liquids with higher boiling points condense lower down and drain back. Crude oil is separated into fractions by the same principle, and it is described in the note on crude oil.

Chromatography

Chromatography separates substances that are dissolved in the same solution by how far they travel through a stationary phase. It is described in the note on purity and chromatography.

Choosing a method

Given information about a mixture, the choice comes from a few questions. If one part is an insoluble solid, filter. If a soluble solid is wanted, crystallise. If the liquid is wanted rather than the solid, distil. If there are several liquids with different boiling points, use fractional distillation. If the substances are coloured and dissolved together, use chromatography. A mixture of sand and salt in water, for example, is filtered first to remove the sand, and the salt solution is then crystallised.