Question

Riya performs two set of experiments to study the length of the foam formed which
are as follows:
Set I: she takes 10 ml of distilled water in test tube “A” and adds 5-6 drops of liquid
soap in it and shakes the test tube vigorously.
Set II: she takes 10 ml of distilled water in a test tube “A” and adds 5-6 drops of
liquid soap with half spoonful of CaSO4 in it and shakes the test tube. Write your
observation and reason.

Answer 1

in the Caso4 case water does not form foam with soap but a white precipitate as hard water having Ca+2 or Mg +2 or Fe +2 ions tends to precipitate acetate ions of soap. however same does not happens for surf or detergent as they have sulphonate ion which is not precipitated by hard water ions.
while in other case foam is formed.

Answer 2

Set I will have more length of foam because it consist of soft water.
Set II will form less foam because it consist of hard water due to the presence of CaSO4.

Explanation : Soft water has no calcium and magnesium ions and is free from all ions. So the soap has more cleansing action with soft water.
Cleansing action of soap is mainly due to the following.
Clothes become dirty due to the deposition of dirt (dust/smoke along with oily/greasy substances). Such oily/greasy dirt is not wetted by water. When a dirty fabric or a piece of dirty cloth is dipped into soap solution, the hydrocarbon end of the soap anion dissolves in the oil/grease, and the polar end (-COO-) is directed towards water. Thus, a number of hydrocarbon chains get embedded into the dirt particle, and a micelle is formed. These micelles are negatively charged. Being similarly charged, they repel each other. These emulsified dirt particles (micelles) get washed away in water during rinsing.


Regards

Jeane