Acids, bases and salts are three important classes of chemical compounds. Their properties, the pH scale and indicators are tested regularly in competitive science papers.
Acids
Acids release H+ ions in water. They taste sour and turn blue litmus red. Examples: hydrochloric acid (HCl), sulphuric acid (H2SO4). Citric acid is in lemons and acetic acid is in vinegar.
Bases
Bases release OH- ions in water. They taste bitter, feel soapy and turn red litmus blue. A base soluble in water is called an alkali. Examples: sodium hydroxide (NaOH), calcium hydroxide (lime water).
The pH Scale
- pH scale ranges from 0 to 14.
- pH less than 7 = acidic.
- pH equal to 7 = neutral (pure water).
- pH greater than 7 = basic.
- pH was introduced by Sorensen.
Indicators
Indicators show whether a substance is acidic or basic. Litmus, methyl orange and phenolphthalein are common indicators. Turmeric and red cabbage are natural indicators.
Neutralisation and Salts
When an acid reacts with a base, it forms salt and water. This is called neutralisation. Common salt is sodium chloride (NaCl). Baking soda is sodium bicarbonate, and washing soda is sodium carbonate.
Quick Revision Points
- Acids release H+; bases release OH-.
- Acid turns blue litmus red.
- pH scale ranges from 0 to 14; 7 is neutral.
- Acid + base gives salt + water (neutralisation).
- Common salt is NaCl.
- Acetic acid is found in vinegar.
- An alkali is a water-soluble base.