Acids, Bases and Salts

From the previous activities one observes that all acids generate hydrogen gas on reacting with metals. Hydrogen seems to be common to all acids. In fact all acids contain hydrogen ions (H⁺).

A base is a substance, which on dissolving in water yields hydroxyl ions (OH^-) as the only negative ions. A base may be an oxide or a hydroxide of a metal. If a base is soluble in water, it dissociates to form a metal ion and the only negative hydroxyl ion (OH^-).

What Happens to an Acid or a Base in a Water Solution?

Acids

Since all acids contain hydrogen ions, the more hydrogen ions they contain, the stronger the acids are. A good definition of an acid is a compound that produces H⁺ ions when it dissolved in water. Hydrogen ions cannot exist alone, but they exist after combining with water molecules. H⁺ ions in association with a water molecule form H₃O⁺ ions or hydronium ion.

H⁺ + H₂O ® H₃O⁺

For example, when hydrogen chloride gas is dissolved in water the hydrogen chloride molecules immediately disassociate or split into hydrogen ions and chloride ions. The solution becomes a very strong acid solution called hydrochloric acid.

The separation of H⁺ ion from HCl molecules cannot occur in the absence of water. Thus hydrogen ions must always be shown as H⁺(aq) or (H₃O⁺).

Bases

Bases are substances that produce hydroxide ions (OH^-) when dissolved in water. The characteristic property of a base is due to the presence of these negative hydroxyl ions. A base that is soluble in water is an alkali. For example, when sodium hydroxide is dissolved in water it readily disassociates to produce a lot of hydroxide ions.

All alkalis are bases that dissociate in water to yield hydroxyl ion (OH^-) as the only negative ions. Sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide are the common alkalis.

NaOH(aq) Na⁺(aq) + OH^-(aq)

KOH(aq) K⁺(aq) + OH^- (aq)

Ca(OH)₂(aq) Ca²⁺(aq) + 2OH^-(aq)

NH₄OH(aq) NH₄⁺(aq) + OH^-(aq)

Formation of Ammonium Ion

When ammonia is dissolved in water it forms ammonium ions (NH) and hydroxyl (OH^-) ions by the formation of a co-ordinate covalent bond. This can be shown as follows.

Bases and acids neutralize each other, therefore another way to define a base is 'a compound which reacts with an acid to give salt and water only'. Like acids, alkalis can be strong or weak. The more hydroxide ions they produce, the stronger the alkali.

The acidic property of an acid is due to the presence of hydrogen ions (H⁺) while that of a base or alkali, is due to the presence of hydroxyl (OH^-) ions in them. When an acid and base (alkali) combine, the positively charged hydrogen ion of the acid combines with the negatively charged hydroxyl ion of the base to form a molecule of water. Hence, the water molecule formed does not have any charge because the positive and negative charges of the hydrogen ions and hydroxyl ions get neutralized.

Neutralization can be viewed as a reaction in which an acid combines with a base, neutralizing the positively charged hydrogen ion and the negatively charged hydroxyl ion, to form a molecule of water and the respective salt.

Remember

Addition of Acids or Bases to Water The process of dissolving an acid or a base in water is a highly exothermic one. As this reaction generates lot of heat care must be taken while mixing concentrated acids with water, specially nitric acid or sulphuric acid with water. As a rule: Always add acid to water and never the other way! The acid must be added slowly to water with constant stirring. If one mixes the other way by adding water to a concentrated acid, the heat generated causes the mixture to splash out and cause burns. The glass container may also break due to excessive local heating and cause damages! Mixing an acid or base with water results in dilution. It decreases the concentration of ions (H3O+/OH-) per unit volume thereby dissipating the heat effect easily.