NCERT Question 11 - Chapter 5 Class 7 - Acid, Bases and Salt

Consider the following statements:

1. Both acids and bases change colour of all indicators.
2. If an indicator gives a colour change with an acid, it does not give a change with a base.
3. If an indicator changes colour with a base, it does not change colour with an acid.
4. Change of colour in an acid and a base depends on the type of the indicator. 

Which of these statements are correct?

(i) All four (ii) a and d (iii) b and c (iv) only d

Answer:

Let us go through all the statements:

1. Both acids and bases change colour of all indicators.

This is not true . 

• There are several natural as well as man-made indicators that c hange colour when introduced to either acid or base , and are unable to distinguish from neutral solutions .
• For Example - Turneric , phenolphthalein 

2. If an indicator gives a colour change with an acid, it does not give a change with a base.

• This is also not true.
• There are several indicators that change to different colours when introduced to acids and bases.
• For Example - Litmus Paper , China Rose , etc

3. If an indicator changes colour with a base, it does not change colour with an acid.

• This statement is also not true.
• Some indicators can change colours with both bases as well as acids . 
• For Example - Litmus Paper , China Rose , etc

4. Change of colour in an acid and a base depends on the type of the indicator.

• The statement is true . 

Hence, the correct answer is (d)