Chapter 4 · Carbon and its Compounds

NCERT Class 10 Science Chapter 4 Intext Questions (Page 76)

All 2 questions of NCERT Class 10 Science Chapter 4 Intext Questions (Page 76) — Carbon and its Compounds — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Would ​you ​be ​able ​to check if water is hard by using a ​detergent?
   Show answer

   No: a detergent lathers equally well in hard and soft water, so only a soap solution can show hardness.

   No. Hardness of water is detected by the behaviour of soap: in hard water soap reacts with the dissolved calcium and magnesium salts to form an insoluble white curdy precipitate (scum) and gives very little lather. A detergent does not form insoluble precipitates with calcium and magnesium ions — it lathers equally well in hard and in soft water and forms no scum — so with a detergent there is no visible difference between hard and soft water, and it cannot be used to check whether water is hard. A soap solution must be used for this test.

   Step-by-step solution →
2. Q2
   People use a variety of methods to wash clothes. Usually after adding the ​soap, they ‘beat’ the clothes on a ​stone, ​or beat it with a ​paddle, scrub with a brush or ​the mixture ​is agitated in a washing ​machine. Why is agitation necessary to get clean ​clothes?
   Show answer

   Soap only loosens the dirt; agitation pulls the dirt-laden micelles off the fibres and disperses them in water so rinsing can remove them.

   Most dirt on clothes is oily and is held on the fibres of the cloth. Soap molecules attach their hydrocarbon tails to this oily dirt and form micelles around it with their ionic ends facing the water, so that the dirt can be carried away in water as an emulsion. But soap by itself only loosens the dirt — the micelles containing the dirt are still attached to the fibres. Agitation (beating, scrubbing or the tumbling of a washing machine) supplies the mechanical force needed to break the oily dirt into small droplets that the micelles can surround, to pull the dirt-laden micelles away from the fibres, and to disperse them in the water. The suspended dirt is then removed by rinsing. Without agitation most of the dirt would stay on the cloth even after soap had been added.

   Step-by-step solution →