Bases are substances that can react with acids and neutralise them. Alkalies   are bases that are soluble in water. The pH scale measures how acidic or alkaline a substance is. Substances with a pH lower than 7 are acidic, those with a pH of 7 are neutral and those with a pH greater than 7 are alkaline.

solutions with a pH less than 7 are acidic

  • solutions with a pH of 7 are neutral
  • solutions with a pH greater than 7 are alkaline.

If universal indicator is added to a solution it changes to a colour that shows the pH of the solution.

Diagram of the PH scale

Diagram of pH scale and universal indicator colours

These are some examples of common substances and their pH values.

The pH scale

pH example substance
strong acidic 0 battery acid
1 stomach acid
2 lemon juice
3 vinegar
4 acid rain
5 black coffee
6 saliva
neutral 7 pure water
8 sea water
9 baking powder
10 milk of magnesia
11 ammonia
12 soapy water
13 bleach
strongly alkaline 14 drain cleaner

Common acids and alkalies

Name Formula Acid or alkali Information
citric acid COH(CH2)2(COOH)3 acid solid, found in citrus fruit
tartaric acid (CHOHCOOH)2 acid solid, found in tea
sulfuric acid H2SO4 acid liquid, used in car batteries
nitric acid HNO3 acid liquid
ethanoic acid CH3COOH acid liquid, used in vinegar
hydrogen chloride HCl acid gas, dissolves in water to make hydrochloric acid
sodium hydroxide NaOH alkali solid, used in oven cleaners
potassium hydroxide KOH alkali solid
calcium hydroxide Ca(OH)2 alkali solid, also called ‘lime’

Reactions of acids

Acids react with metals, metal oxides, metal hydroxides and metal carbonates. During each of these reactions a salt is made.


Acids will react with reactive metals, such as magnesium and zinc, to make a salt and hydrogen.

acid + metal → salt + hydrogen

For example:

magnesium + sulfuric acid → magnesium sulfate + hydrogen

Mg + H2SO4 → MgSO4 + H2

The hydrogen causes bubbling during the reaction. It can be detected using a lighted splint, which causes the gas to burn with a squeaky pop.

Oxides and hydroxides

Acids react with metal oxides and hydroxides to produce a salt and water.

acid + metal oxide → salt + water

acid + metal hydroxide → salt + water

For example:

hydrochloric acid + copper(II) oxide → copper(II) chloride + water

2HCl + CuO → CuCl2 + H2O

nitric acid + calcium hydroxide → calcium nitrate + water

2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O


When acids react with carbonates, a salt, water and carbon dioxide are made.

acid + carbonate → salt + water + carbon dioxide

For example:

sodium carbonate + hydrochloric acid → sodium chloride + carbon dioxide + water

Na2CO3 + 2HCl → 2NaCl + CO2 + H2O

The carbon dioxide causes bubbling during the reaction. It can be detected using lime-water, which turns a milky white when carbon dioxide is bubbled through it.

Neutralisation – Higher

When an acidic compound dissolves in water it produces hydrogen ions, H+. These ions are responsible for the acidity of the solution.

When an alkaline compound dissolves in water it produces hydroxide ions, OH. These ions are responsible for the alkalinity of the solution.

Acids react with alkalis to form salts. These are called neutralisation reactions. In each reaction, water is also formed:

acid + alkali → salt + water


hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl + NaOH → NaCl + H2O

Hydrochloric acid contains hydrogen ions and chloride ions dissolved in water.

Sodium hydroxide solution contains sodium ions and hydroxide ions dissolved in water.

The salt sodium chloride is formed when the acid and alkali are mixed together. This salt is produced as sodium ions and chloride ions in solution. There is no change to the sodium ions and chloride ions during the reaction to make sodium chloride. They were dissolved in water in the acid and in the alkali, and they are still dissolved in water as the salt.

Hydrogen and hydroxide ions

The only change that does produce something different during the neutralisation is the reaction between hydrogen ions and hydroxide ions, which produces water molecules.

H+ + OH → H2O

This is the ionic equation that represents the neutralisation reaction between any acid and any alkali.

The name of the salt produced in the neutralisation depends on which acid reacts with which alkali. But the ionic equation for the neutralisation is always the same.