Description: 1 Mark
Use: 1 Mark
Examples: 1 Mark
Substances that exhibit a change of colour or odour when added to acidic, basic and neutral solutions are called indicators.
Visual indicators show different colours in presence of an acidic and a basic solution. Similarly, olfactory indicators exhibit different odour when added to acids or bases. Thus, they help us in deciding whether a given solution is acidic or basic in nature.
Some of the naturally occurring indicators are china rose petals, litmus, turmeric, etc.

Each part: 1 Mark
Bases turn red litmus blue, while acids turn blue litmus red.

a) Since calcium hydroxide is a base, the red litmus will turn blue.

b) No color change will take place as sodium chloride is a neutral salt.

c) Since lemon juice contains citric acid, the blue litmus paper will become red.

d) No color change will take place as distilled water is neutral by nature.

e) Turmeric powder is yellow in color and it turns pink when treated with a basic solution. Hence here the turmeric will turn pink as sodium hydroxide is a base.

Explanation: 2 Marks
Reaction Diagram: 1 Mark
Changes: 1 Mark
Reason: 1 Mark
The reaction between an acid and a base is known as neutralization. Salt and water are produced in this process with the evolution of heat.
Acid + Base → Salt +    Water + (Heat is evolved)
As for example, when sodium hydroxide (NaOH) is treated with sufficient amount of hydrochloric acid (HCl), a neutralization reaction takes place. Both NaOH and HCl neutralize their effects and form a salt sodium chloride (NaCl) and water (H₂O) with the evolution of heat energy.
 NaOH + HCl → NaCl + H₂O + (Heat is evolved)
When sulphuric acid is added to lime water, the reaction mixture will become hot because it is a type of neutralization reaction. An acid is mixed with a base.
So, heat will be evolved in this reaction.

Soil Treatment: 2 Marks
Too acidic: 1.5 Marks
Too basic: 1.5 Marks
With the increase in demand of crops, farmers have started using excessive fertilizers which is affecting the acid-base balance in the soil. Plants do not grow well when the soil is either too acidic or too basic. This process of restoring the acid-base balance in the soil is called soil treatment.
When the soil is too acidic, it is treated with bases like quicklime (calcium oxide) or slaked lime.
If the soil is too basic, organic matter is added to it, which neutralizes the basic nature of the soil by releasing acids.
 

Nitric acid turns blue litmus red as it is acidic in nature. Acids turn blue litmus solution to red color but do not have any effect on red litmus solution.

Acids turn blue litmus red and are sour in taste.

Ammonia is the common ingredient in most household cleaners. A solution of ammonium hydroxide (base) is used to clean various surfaces.  If vinegar (acetic acid) is added instead of water it would neutralise the ammonia solution and reduce the cleaning action.