Acids

Acids

When you were answering the assignment above, you could have classified lime juice, vinegar and vitamin C in that list under the acids. You have used various acids in the laboratory experiments also. Hydrochloric acid (HCl), nitric acid (HNO3 ) and sulphuric acid (H2 SO4 ) are some acids that are often used in the laboratory.

When considering the formulae of the above acids it is clear that hydrogen (H) is a component element in all those acids.

What is an acid?

An acid is a compound that releases hydrogen ions (H+ ) in an aqueous medium. Hydrochloric acid ionises as follows in the aqueous medium and releases H+ ions.

Based on the strength of releasing H+ ions in the aqueous medium, acids are classified as strong acids and weak acids.

Strong acids

The acids that release H+ ions by complete ionisation in aqueous medium are strong acids. It means that all such acid molecules are dissociated into H+ ions and the corresponding negative ions in water. For example, in a solution of hydrochloric acid which is a strong acid, there are only H+ ions and Cl- ions but no free HCl molecules.

Given below are a few examples for some strong acids and how they ionise in the aqueous medium.

Weak acids

The acids which release H+ ions in aqueous medium by incomplete or partial ionisation are called weak acids. This means that in aqueous medium, only a fraction of such acid molecules are dissociated into H+ ions and relevant negative ions. The unionised molecules remain as molecules themselves in aqueous solution.

Examples for weak acids:

Acetic acid (CH3 COOH)
Carbonic acid (H2 CO3 )
Phosphoric acid (H3 PO4 )

Most of the acids in laboratory stores are concentrated acids. Dilute acids of required concentration can be prepared by mixing such concentrated acids with water. Acids of low concentration are known as dilute acids.

Properties of acids

Pay your attention to the warning symbol in Figure seen in the label of the bottles containing concentrated acids. This is a warning about the corrosive nature of the relevant chemical. That is, when they come into contact with substances like wood, metals or cloth they corrode them and if spilled on the skin, they cause severe burns. This shows that acids have corrosive properties.

Recall the taste of lime juice. It is sour. A common feature of acids is that they have a characteristic sour taste.

Dilute acids react with metals above hydrogen in the reactivity series forming the salt of the metal and hydrogen gas.

Think back on the experiment carried out to prepare carbon dioxide gas in the laboratory. Carbon dioxide was prepared by adding diluted hydrochloric acid to calcium carbonate.

Production of carbon dioxide by reacting with carbonates/bicarbonates is a characteristic feature of acids.

Acids react with bases to form salts and water. The salt sodium sulphate (Na2 SO4 ) and water are formed as the products of the following acid - base reaction.

Acids turn the colour of blue litmus red. This is a simple test used to identify acids.

Uses of some acids

Hydrochloric acid

Removal of rust in steel objects
Making gelatin from bony materials in food technology
Making aqua regia ( aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid mixed in the proportion of 1 : 3. Aqua regia is used to dissolve metals like gold and platinum)

Sulphuric acid

Production of fertilizers such as ammonium sulphate and triple superphosphate
Making battery acid ( Battery acid is diluted sulphuric acid)
Production of paints, plastics and detergents
Concentrated sulphuric acid is used as a dehydrating agent
Drying gases (For drying a gas, the relevant gas is bubbled through concentrated sulphuric acid)

Acetic acid