Revision guides: Chemical changes and structures

The speed of a chemical reaction is affected by temperature, concentration, particle size and the presence of a catalyst. It can be calculated by measuring changes in reactants/products.

Atoms are made from protons, neutrons and electrons. In this study guide, you can revise how the periodic table arranges elements according to their atomic size and other properties, atomic theory and atomic numbers, you'll also learn about isotopes and the properties of the main groups of elements.

Only the noble gases exist as individual atoms not bonded to other atoms. In all other substances atoms are held together by chemical bonds, either sharing or gaining/losing electrons.

The chemical formula for a substance shows how many atoms of each element are present in a molecule, or the proportion of atoms of each element. The formula can be worked out using the valency.

Chemical equations must be balanced so that the quantities of reactants and products match. For an equation to be balanced there must be an equal numbers of atoms on each side.

The gram formula mass of a substance is known as the mass of one mole. The mass, number of moles, concentration or volume of a substance can be calculated easily if you learn two formula triangles.

The pH scale measures the acidity or alkalinity of a solution. A pH less than 7 is acidic. Alkalis dissolve in water to give a pH greater than 7. A pH equal to 7 indicates a neutral solution.