Acids, Bases and Neutralisation

This lesson covers: 

  1. The definitions of acids and bases
  2. The difference between strong and weak acids and bases
  3. Neutralisation reactions between acids and bases
  4. Reactions of acids with metals, metal compounds and ammonia

Acids are proton donors, bases are proton acceptors

An acid is defined as a substance that donates protons (H+) when dissolved in water. These protons combine with water molecules to form hydronium ions (H3O+), indicating the acidic nature of the solution.

Examples of common acids include:

  • Hydrochloric acid (HCl)
  • Sulfuric acid (H2SO4)
  • Nitric acid (HNO3)
  • Ethanoic acid (CH3COOH)


A base, in contrast, is a substance that accepts protons. When a base dissolves in water, it can produce hydroxide ions (OH-), making the solution alkaline. Bases that dissolve in water are specifically called alkalis.

Examples of common alkalis include:

  • Sodium hydroxide (NaOH)
  • Potassium hydroxide (KOH)
  • Aqueous ammonia (NH3 in H2O)


Aqueous ammonia is a special case; it doesn’t produce hydroxide ions directly but accepts protons from water to form ammonium (NH4+) and hydroxide ions (OH-):

NH3 + H2O ➔ NH4+ + OH-

Strength depends on extent of dissociation

The strength of an acid or a base is determined by its ability to dissociate in water. This dissociation process can be reversible, as shown in the equations below:

  • Acid dissociation: HA + H2O ⇌ H3O+ + A-
  • Base dissociation: B + H2O ⇌ BH+ + OH-


Strong acids and bases:

Strong acids, such as hydrochloric acid (HCl), and strong bases, like sodium hydroxide (NaOH), dissociate completely in water. This results in a significant release of H3O+ and OH- ions, respectively, with the forward reaction being predominantly favoured:

  • HCl(aq) ➔ H+(aq) + Cl-(aq)
  • NaOH(aq) ➔ Na+(aq) + OH-(aq)


Weak acids and bases:

Conversely, weak acids and bases, like ethanoic acid (CH3COOH) and ammonia (NH3), only partially dissociate in water, releasing fewer H3O+ and OH- ions. In these cases, the reverse reaction is favoured:

  • CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
  • NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

Neutralisation produces salt and water

Neutralisation is the reaction between an acid and an alkali, resulting in the formation of water and a salt. This reaction specifically involves the combination of H+ ions from the acid with OH- ions from the alkali to form water (H2O).

The general ionic equation for this process is:

H+(aq) + OH-(aq) ➔ H2O(l)


A salt is an ionic compound formed when the H+ ions in an acid are replaced by metal ions or other positive ions, such as ammonium ions (NH4+). In a neutralisation reaction, the salt is comprised of the cation from the base, which can be a metal or ammonium ion, and the anion from the acid.


The type of salt produced depends on the specific acid used in the reaction:

AcidAnion in salt
Sulfuric acid (H2SO4)Sulfate (SO42-)
Hydrochloric acid (HCl)Chloride (Cl-)
Nitric acid (HNO3)Nitrate (NO3-)

For instance, the reaction between hydrochloric acid and sodium hydroxide yields sodium chloride and water:

HCl(aq) + NaOH(aq) ➔ NaCl(aq) + H2O(l)

Acids react with metals and metal compounds

  1. Acids and reactive metals - Acids react with certain metals, producing a salt and hydrogen gas:

Metal + acid ➔ salt + hydrogen

For example, calcium reacting with sulfuric acid: Ca(s) + H2SO4(aq) ➔ CaSO4(aq) + H2(g)


  1. Acids and metal oxides - These reactions yield a salt and water:

Metal oxide + acid ➔ salt + water

An example is zinc oxide with hydrochloric acid: ZnO(s) + 2HCl(aq) ➔ ZnCl2(aq) + H2O(l)


  1. Acids and metal hydroxides - Similar to oxides, these reactions produce a salt and water:

Metal hydroxide + acid ➔ salt + water

For example, potassium hydroxide reacting with nitric acid: KOH(aq) + HNO3(aq) ➔ KNO3(aq) + H2O(l)


  1. Acids and metal carbonates - These reactions produce a salt, water, and carbon dioxide:

Metal carbonate + acid ➔ salt + water + carbon dioxide

An example is sodium carbonate with hydrochloric acid: Na2CO3(s) + 2HCl(aq) ➔ 2NaCl(aq) + H2O(l) + CO2(g)

Ammonia reacts with acids to form ammonium salts

Ammonia interacts with acids to form ammonium salts, exemplified by a proton transfer from the acid to the ammonia:

Ammonia + acid ➔ ammonium salt


For instance, ammonia reacting with sulfuric acid produces ammonium sulfate:

2NH3(aq) + H2SO4(aq) ➔ (NH4)2SO4(aq)


The ionic equation highlights the proton transfer process:

NH3(aq) + H+(aq) ➔ NH4+(aq)