Chapter 2 Solutions — Acids, Bases and Salts

Red litmus paper turns blue in a basic solution and shows no change in acidic or neutral solutions.

Step 1: Dip the red litmus paper in each of the three test tubes one by one.

Step 2: The solution that turns red litmus blue is the basic solution.

Step 3: Now take a drop of the identified basic solution and put it on a fresh red litmus paper to turn it blue.

Step 4: Dip this blue litmus paper (made from step 3) in the remaining two solutions.

Step 5: The solution that turns the blue litmus paper back to red is the acidic solution.

Step 6: The solution that does not change either litmus paper is distilled water.

The process of dissolving an acid in water is highly exothermic — it releases a large amount of heat.

If water is added to a concentrated acid:

→ The small amount of water may instantly boil due to the intense heat generated.

→ This causes the acid to splash out of the container, which can cause severe acid burns.

If acid is added to water:

→ The large volume of water absorbs the heat generated.

→ The heat gets distributed throughout the water, and the temperature rise is gradual and controlled.

Remember: "Add Acid to Water" (A to W, alphabetical order).

The reaction is: H₂SO₄ + H₂O → H₂SO₄(aq) + Heat

pH plays a crucial role in many everyday processes:

1. Our body works within a narrow pH range (7.0 to 7.8):

→ The stomach produces HCl (pH ~ 1.2) for digestion.

→ Excess acid causes acidity/indigestion.

→ Antacids (mild bases like Mg(OH)₂) are used to neutralise excess acid.

2. pH of soil affects plant growth:

→ Most plants grow best in soil with pH 6 to 7.

→ If soil is too acidic, it is treated with quicklime (CaO) or slaked lime (Ca(OH)₂).

→ If soil is too basic, organic matter or gypsum is added.

3. Tooth decay and pH:

→ Tooth enamel (calcium phosphate, hardest substance in body) starts corroding when mouth pH falls below 5.5.

→ Bacteria in the mouth produce acids by degrading food particles (especially sugar).

→ Using basic toothpaste neutralises the excess acid and prevents decay.