GCSE SCIENCE CHEMISTRY HIGH SCHOOL - Strong - Weak - Acid - Alkali

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Strong and Weak Acids - Strength and Concentration.

Acids and alkalis can be described as strong or weak.
This does not mean the same as concentrated or dilute.

The strength of an acid or alkali depends on how ionised it is in water.

A strong acid or alkali is completely (100%) ionised. For hydrochloric acid

hydrogen chloride (in water) hydrogen ion + chloride ion
HCl_(aq) H⁺_(aq) + Cl^-_(aq)

All of the hydrogen chloride molecules
become hydrogen ions and chloride ions in water
(see examples for other strong acids).

For sodium hydroxide

sodium hydroxide (in water) sodium ion + hydroxide ion
NaOH_(aq) Na⁺_(aq) + OH^-_(aq)

Sodium hydroxide exists as ions both in water and in the solid.
(see examples for other strong alkalis).

A weak acid or alkali is only partly (less than 100%) ionised. For ethanoic acid

ethanoic acid (in water) hydrogen ion + ethanoic ion
CH₃CO₂H_(aq) H⁺_(aq) + CH₃CO₂^-_(aq)

Some of the ethanoic acid molecules become ions in water
but most of them stay as molecules.
The reaction is reversible (shown by the arrow).

For ammonia

ammonia + water ammonium ion + hydroxide ion
NH_3(g) + H₂O_(l) NH₄⁺_(aq) + OH^-_(aq)

Some of the ammonia molecules become ions in water
but most of them stay as molecules.

See also Concentration and Differences between Strong and Weak Acids.