AN ALKALI

Alkali is pronounced like alcohol, with ‘lie’ at the end instead of ‘hol’. An alkali is any substance which produces OH^– ions in water. OH^– ions are called hydroxide ions. If there are excess of (OH)^– ions when a compound is dissolved in water, the solution is called a base or an alkaline solution. A base is generally a metal hydroxide solution.

Table below lists some of the common alkalis available in our everyday lives. Name of alkali Chemical Formula Dissociation in water Sodium Hydroxide NaOH Na⁺ + (OH)^– Potassium Hydroxide KOH H⁺ + NO3^– Ammonium Hydroxide NH₄OH NH4⁺ + (OH)^– A substance which will neutralize an acid, but does not dissolve in water, is called a base. For example, copper (II) oxide, iron (II) oxide and zinc carbonate are bases. They do not dissolve in water.

Any base which dissolves in water is called an alkali.

The outer circle encloses all bases, while the inner circle selects those which are alkalis or soluble bases.

Examples of Alkalis.

The three common alkalis you will find in the laboratory are

1) Sodium Hydroxide solution – NaOH_(aq).

2) Calcium Hydroxide solution – Ca(OH)³_(aq), (lime water)

(Lime water is used in the test for carbon dioxide).

3) Ammonia solution – NH³_(aq).

1 and 2 are strong alkalis, 3 is a weak alkali – see pH.

They all ionise in water to form hydroxide ions (OH- ions).

1) NaOH_{(aq) Na}⁺_{(aq) + OH-(aq)}

2) Ca(OH)2_{(aq) Ca2+(aq) + 2OH-(aq)}

3) NH³_{(aq) + H}²_{O(l) NH4}⁺_{(aq) + OH}^–_{(aq) NH}⁴⁺ is an ammonium ion.

Any metal oxide or hydroxide is a base.

If the base dissolves in water it is called an alkali.

Alkalis are found in:

1. oven cleaner (sodium hydroxide)
2. soap
3. cleaning fluid e.g. spray-and-wipe (ammonia) Notice the connection between these substances? Alkalis are often found in substances for cleaning.

Strong alkali substances are just as dangerous as strong acidic substances, causing very serious burns if they come into contact with your skin.

Properties of Alkalis.

1. They have a pH greater than 7, see pH.
2. They will turn red litmus paper blue.
3. They will react with acids to form a salt and water, see neutralisation.

Uses of Alkalis.

1. Sodium hydroxide is used in the manufacture of paper, soap and ceramics.
2. Calcium hydroxide (called ‘slaked lime’, or just ‘lime’), is added to soils or lakes to make them less acidic (see acid rain).
3. Indigestion may be caused by too much hydrochloric acid in the stomach. Indigestion tablets contain a base such as magnesium oxide, or calcium carbonate to neutralize the acid.
4. A bee sting is acidic. It may be neutralized by a weak alkali such as ammonia solution.

Weak, strong acids and alkalis

Water.

In a sample of water,a very small number of water molecules will form ions. Water hydrogen ion + hydroxide ion. H_{2O(l) H}⁺_{(aq) + OH}^–_(aq)

This ionisation is reversible (shown by the arrow).

The hydrogen ion is acidic. The hydroxide ion is alkaline. Water forms equal amounts of both ions, and so water is neutral. Compare this reaction with neutralisation.