Acids and Bases

Acids and Bases

Acid – a substance that acts as a donor of hydrogen ions

Base – a substance that acts as an acceptor of hydrogen ions

Alkali – soluble bases

Acid Base
Sour Taste Bitter Taste
pH less than 7 pH greater than 7
In solution, contains hydronium ions (H3O+) In solution, contains hydroxide ions (OH-)
Turns blue litmus red Turns red litmus blue
Turns phenolphthalein colourless Turns colourless phenolphthalein pink
Corrosive Soapy feel
Reacts with metals to produce salt and hydrogen Cannot react with metals
Examples of Acids Examples of Bases
Hydrochloric Acid HCl Sodium Hydroxide NaOH
Nitric Acid HNO3 Potassium Hydroxide KOH
Sulphuric Acid H2SO4 Calcium Hydroxide Ca(OH)2
Ethanoic Acid CH3COOH Ammonia Solution NH3 (aq)


Hydronium Ion – same as a single proton because when a hydrogen atom loses an electron, only a proton remains. H+ is irresistibly attractive to water molecules and therefore it would form H3O+.

Dissociation – breaking apart


Strong Acids – in aqueous solutions, strong acids donate all their protons to water molecules.


Weak Acids – there is only a slight tendency to donate protons to water molecules, therefore an aqueous solution of a weak acid contains mainly undissociated molecules and a low concentration of H3O+.


Strong Acids Weak Acids
Dissociation in Aqueous Solution Completely dissociate Partially dissociate
Equilibrium None (forward only) Equilibrium reaction
Electrolyte Good Poor
Electrical conductivity Good Poor
[H3O+] Higher Lower
pH value Lower Higher
Examples HCl, HNO3, H2SO4 +


Amphiprotic – substances can act as both an acid and a base e.g. H2O, HCO3 , HSO4


Amphoteric – substances will undergo chemical reactions with both acids and bases


Neutralisation – an alkali or base can neutralise an acid by removing the H+ ions and converting them to water. Neutralisation always produces a salt.


Concentration – a measure of the amount of acid per dm3, refers to the proportion or ratio of acid to water in the solution

Concentrated Acids – high proportion of acid to water

Dilute Acids – low proportion of acid to water

Monoprotic – having one transferrable proton

Diprotic – having two transferrable protons

Titration – an indicator shows when the acid properties are just destroyed by the alkali. The salt can then be recovered by evaporating the water away allowing the salt to crystallise. This method is used when the base, acid and salt are all soluble.