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.
These are some examples of common substances and their pH values.
The pH scale
|strong acidic||0||battery acid|
|10||milk of magnesia|
|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|
|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|
|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
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
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
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.