WJEC GCSE Chemistry

Elements

An element is a substance that cannot be broken down into anything simpler by chemical means, and every substance in the universe is built from the roughly 100 elements. It contains only one kind of atom: a piece of pure copper is copper atoms and nothing else, and a sample of oxygen gas is oxygen atoms, joined in pairs as $\ce{O2}$ molecules.

Each element has a one or two letter symbol, with the first letter a capital and the second, if there is one, in lower case: C, O, Na, Cl. Some symbols come from older names, so sodium is Na and iron is Fe.

Some elements exist as single atoms (helium, He), some as molecules made of a fixed number of atoms joined together ($\ce{H2}$, $\ce{N2}$, $\ce{O2}$, $\ce{Cl2}$, $\ce{Br2}$, $\ce{I2}$, $\ce{P4}$, $\ce{S8}$), and some as giant structures (carbon, silicon, metals). A chemical formula says how many atoms of each element are in one molecule or in the simplest unit of the substance.

Compounds

A compound contains two or more different kinds of atom that are chemically joined, in a fixed proportion. Water is always $\ce{H2O}$, two hydrogen atoms for every oxygen atom, and a compound with a different ratio would be a different compound (hydrogen peroxide, $\ce{H2O2}$, is an example).

The properties of a compound are completely different from those of the elements in it. Sodium is a soft, reactive metal and chlorine is a poisonous green gas, yet sodium chloride is the harmless white solid in a salt cellar. A compound can only be split back into its elements by a chemical reaction, which usually needs energy, for example by heating or by passing electricity through it.

The name of a compound tells you what it is made of. A compound of two elements ends in -ide (sodium chloride, magnesium oxide, hydrogen sulfide). A name ending in -ate or -ite shows that oxygen is also present, in an ion such as sulfate $\ce{SO4^2-}$, nitrate $\ce{NO3-}$ or carbonate $\ce{CO3^2-}$. Sodium sulfate contains sodium, sulfur and oxygen.

Formulae and diagrams of simple molecules

A molecular formula gives the atoms in one molecule. Water is $\ce{H2O}$, ammonia $\ce{NH3}$, methane $\ce{CH4}$, carbon dioxide $\ce{CO2}$ and hydrogen chloride $\ce{HCl}$.

A molecule can also be drawn as a diagram with a key. Each kind of atom is given its own circle, shaded or labelled, and the circles are drawn touching for atoms that are joined. The key matters because the diagram is meaningless without it. A water molecule is one large circle (oxygen) with two small circles (hydrogen) touching it, and a carbon dioxide molecule is one carbon circle with an oxygen circle on each side. A diagram for an element has circles of only one kind, and a diagram for a compound has more than one.

Mixtures

A mixture contains two or more substances that are not chemically joined. Each keeps its own properties, and the proportions can vary. Air is a mixture of nitrogen, oxygen, argon and other gases, sea water is a mixture of water and dissolved salts, and brass is a mixture of copper and zinc.

Because nothing is joined, a mixture can be separated by physical processes, which need no chemical reaction and produce no new substances. The method depends on which property differs between the parts.

Filtration and evaporation

To separate sand from salt water, the mixture is poured through filter paper in a funnel. The sand stays on the paper as the residue and the salt solution passes through as the filtrate. Heating the filtrate in an evaporating basin drives off the water. To get larger, neater crystals, heat only until about half the solvent has gone (the point where a drop of solution placed on a cold glass rod forms crystals), then leave the hot solution to cool slowly and filter the crystals off.

Distillation

In simple distillation the solution is heated in a flask, and the solvent vapour passes into a condenser, where it is cooled by water flowing round the outside and turns back into a liquid that drips into a receiving flask. The water in the condenser enters at the bottom and leaves at the top so that the jacket stays full. Pure water can be obtained from salt water this way, and the salt remains in the flask. Substances with different boiling points can be separated by fractional distillation, where a column above the flask lets the vapour of the substance with the lower boiling point reach the condenser first. Ethanol boils at 78 °C and water at 100 °C, so ethanol distils first.

Chromatography

Paper chromatography separates coloured substances, such as the dyes in an ink or a food colouring.

1. Draw a pencil line about 1 cm from the bottom of a sheet of chromatography paper. Pencil is used because its graphite does not dissolve in the solvent, whereas ink would run and form its own spots.
2. Put a small spot of each sample on the line, with a spot of each known substance beside them for comparison.
3. Stand the paper in a container with a little solvent (water or ethanol) so that the solvent level is below the pencil line, and cover the container.
4. The solvent rises up the paper, carrying the substances with it. Take the paper out when the solvent is close to the top and mark where the solvent front has reached in pencil straight away.

The components separate because each one has a different balance between dissolving in the solvent and sticking to the paper. A substance that dissolves well in the solvent and sticks weakly to the paper travels a long way, and one that sticks strongly to the paper travels a short way. A pure substance gives one spot, and a mixture gives more than one. If a spot from the sample lines up with a spot from a known substance, they may be the same substance, and if the spots from two different samples line up, they contain a common component.

Rf values

The Rf value of a substance is the ratio of the distance it has moved to the distance moved by the solvent, both measured from the baseline:

The distance to a spot is measured to the centre of the spot. Rf has no units and is always less than 1. It is constant for a given substance in a given solvent, so it can be compared with a table of values to identify a substance.

For example, a dye spot is 3.6 cm above the baseline and the solvent front is 8.0 cm above it:

A second dye with an Rf of 0.72 would be 5.76 cm up the same paper, since $0.72 \times 8.0 = 5.76$.