Page 81 - NCERT Science Class 10 English Medium

Activity 4.11
Activity
Activity 4.11
Activity 4.114.11
Activity 4.11
n Take about 10 mL of distilled water (or rain water) and 10 mL of
hard water (from a tubewell or hand-pump) in separate test tubes.
n Add a couple of drops of soap solution to both.
n Shake the test tubes vigorously for an equal period of time and
observe the amount of foam formed.
n In which test tube do you get more foam?
n In which test tube do you observe a white curdy precipitate?
Note for the teacher : If hard water is not available in your locality,
prepare some hard water by dissolving hydrogencarbonates/
sulphates/chlorides of calcium or magnesium in water.

Activity 4.12
Activity 4.124.12
Activity 4.12
Activity
Activity 4.12
n Take two test tubes with about 10 mL of hard water in each.
n Add five drops of soap solution to one and five drops of detergent
solution to the other.
n Shake both test tubes for the same period.
n Do both test tubes have the same amount of foam?
n In which test tube is a curdy solid formed?

Have you ever observed while bathing that foam is formed with
difficulty and an insoluble substance (scum) remains after washing with
water? This is caused by the reaction of soap with the calcium and
magnesium salts, which cause the hardness of water. Hence you need
to use a larger amount of soap. This problem is overcome by using
another class of compounds called detergents as cleansing agents.
Detergents are generally sodium salts of sulphonic acids or ammonium
salts with chlorides or bromides ions, etc. Both have long hydrocarbon
chain. The charged ends of these compounds do not form insoluble
precipitates with the calcium and magnesium ions in hard water. Thus,
they remain effective in hard water. Detergents are usually used to make
shampoos and products for cleaning clothes.

Q U E S T I O N S

1. Would you be able to check if water is hard by using a detergent?
2. 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?

76 Science

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