NCERT Question 9 - Chapter 5 Class 7 - Acid, Bases and Salt

Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and the third is a sugar solution. How will you identify them? You have only turmeric indicator

Answer:

A turmeric indicator is a natural indicator which works as follows:

1. It does not change its colour when acidic or neutral substances are tested.
2. Its colour changes to bright red when basic substances are tested. 

To test the substances,

We will put a drop each of hydrochloric acid, sodium hydroxide, and sugar solution on the turmeric indicator.

• The liquid which changes the colour of turmeric indicator to red is basic in nature, that is, sodium hydroxide . 
• Hence sodium hydroxide is identified.

Now, we'll mix a drop of sodium hydroxide with a drop of the other two solutions and then perform the test again.

1.  

• If the resultant turns the turmeric red , it is a basic solution.
• We know that, (Basic solution + Neutral solution) = Basic solution
• (Sodium hydroxide + sugar) = Basic solution
• Hence this mixture contains the sugar solution.

2.

• If the resultant does not change the colour of the turmeric paper at all, 
• It is a neutral solution formed due to the neutralisation reaction between Sodium Hydroxide and Hydrochloric Acid
•   (Sodium hydroxide + hydrochloric acid) = Neutral
• Hence this mixture would contain hydrochloric acid.