Acids and bases

Sour, bitter and the litmus test

Acids have been known for centuries for their sour taste, like lemon, and for turning blue litmus paper red. Curd, tamarind, lemon and lime all taste sour because they contain acids.

The book splits acids by where they come from. Natural (organic) acids come from living sources: acetic acid in vinegar, citric acid in lemons and oranges, lactic acid in curd, formic acid in an ant sting. Mineral acids are made from minerals such as sodium chloride: hydrochloric, sulphuric and nitric acid.

Bases taste bitter, feel slippery and turn red litmus blue. An alkali is a base that dissolves in water, such as sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), calcium hydroxide (lime water) and aqueous ammonia. Caustic soda and caustic potash can burn your skin.

Metal oxides such as Na₂O, CaO, ZnO and MgO are bases too, because they react with acids to give a salt and water. When an acid and a base are mixed in equal amounts they cancel each other's properties: this is neutralization, and it forms a salt and water.

Every alkali is a base, but not every base is an alkali: only the ones that dissolve in water.

Key terms

Organic acid

Food acids.

Mineral acid

Labs and industry.

Base

Cancelling acids.

Alkali

Soaps and cleaners.

Neutralization

Cancelling an acid.

Short questions with model answers

1. Q1. Name some fruits which contain citric acid. (Exercise, p. 94)

   • Look up Table 7.1
   • Citrus family

   Lemon and orange (and lime).

2. Q2. Why is magnesium oxide called a base even though it is not an alkali?

   • Test for a base
   • Test for an alkali

   MgO neutralizes acids (so it is a base), but it is not a water-soluble base like NaOH.

Common mistakes

• ✗ Every base is an alkali.

  ✓ Only bases that dissolve in water are alkalis (p. 94).

• ✗ Acids turn red litmus blue.

  ✓ Acids turn blue litmus red; bases turn red litmus blue.

• ✗ Hydrochloric acid is an organic acid.

  ✓ HCl is a mineral acid, made from sodium chloride.

MCQs

1. 1. Which of the following contains oxalic acid? (Exercise viii, p. 103)

   1. (a) Tomato
   2. (b) Orange
   3. (c) Tamarind
   4. (d) Sour milk
   Show answer

   (a) Table 7.1: oxalic acid, tomato.

2. 2. Which acid is NOT used as a food or mixed with food? (Exercise i, p. 103)

   1. (a) Tartaric acid
   2. (b) Ascorbic acid
   3. (c) Citric acid
   4. (d) Formic acid
   Show answer

   (d) Formic acid comes from an ant sting (Table 7.1).

3. 3. A solution turns red litmus blue. It is:

   1. (a) An acid
   2. (b) A base
   3. (c) A salt
   4. (d) Pure water
   Show answer

   (b) p. 94.

4. 4. An alkali is:

   1. (a) Any acid
   2. (b) A base that dissolves in water
   3. (c) A salt
   4. (d) Any metal oxide
   Show answer

   (b) p. 94.

5. 5. Neutralization of an acid by a base forms:

   1. (a) Salt and water
   2. (b) Hydrogen gas
   3. (c) Only water
   4. (d) An oxide
   Show answer

   (a) p. 94.

Quick revision

• Acids are sour and turn blue litmus red; bases are bitter, slippery and turn red litmus blue.
• Organic acids come from nature; mineral acids are made from minerals.
• An alkali is a water-soluble base; metal oxides are bases too.
• Acid + base gives salt + water: neutralization.