In a chemical reaction, atoms are rearranged into new substances. This topic covers the main reaction types and the chemistry of acids and alkalis.
A chemical reaction makes one or more new substances with different properties. The atoms are not created or destroyed. The bonds between them are broken and new ones formed.
Because the same atoms are still there, the total mass does not change. Signs of a reaction include a colour change, a gas given off, a temperature change or a precipitate forming.
A word equation lists the reactants on the left and the products on the right with an arrow between: magnesium + oxygen → magnesium oxide.
A symbol equation uses formulae, and must be balanced, with the same number of each kind of atom on both sides. Balance by putting numbers in front of formulae; never change a formula itself, because that would change the substance.
Combustion is burning in oxygen, giving out energy. Thermal decomposition is breaking a compound down by heating it, turning limestone into quicklime and carbon dioxide.
Oxidation can be described at this stage as gaining oxygen, as when a metal tarnishes or rusts. Displacement is a more reactive element taking the place of a less reactive one in a compound, so iron will displace copper from copper sulfate.
Acids and alkalis are opposite kinds of substance. An alkali is a base that dissolves in water. When an acid and a base react they neutralise each other, and if exactly the right amounts are used, so that neither is left in excess, the solution can end up neutral.
That reaction, neutralisation, is why indigestion tablets settle stomach acid and why lime is spread on acidic soil.
The pH scale runs from 0 to 14. Below 7 is acidic, exactly 7 is neutral, above 7 is alkaline. The lower the pH, the more acidic the solution; the higher the pH, the more alkaline it is.
Universal indicator gives a colour for each pH: red at very low pH, through green at pH 7, to purple at very high pH. Litmus is simpler, going red in acid and blue in alkali, and a pH meter gives a number directly.
acid + metal → salt + hydrogen. The hydrogen is seen as bubbles, and it gives a squeaky pop with a lit splint.
With dilute hydrochloric or sulfuric acid, metals above hydrogen in the reactivity series generally react this way, so magnesium fizzes vigorously while copper does not react.
acid + alkali → salt + water. The salt formed depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.
Carbonates react too, giving a salt, water and carbon dioxide, which is the fizzing when an acid is dropped on limestone.
A catalyst speeds up a reaction without being used up in the reaction.
It does not change the amount of product that can ultimately be formed, or the position of an equilibrium. It changes only how quickly the reaction happens. Enzymes are the catalysts of living things.
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