Question

Riya performs two sets of experiments to study the length of foam formed, which follows:
Set 1: She takes 10ml of distilled water in test tube A and adds 5-6 drops of liquid soap in it and shakes the test tube vigorously.
Set 2: She takes 10ml of distilled water in test tube A and adds 5-6 drops of liquid soap with half spoonful of $CaS{{O}_{4}}$ in it and shakes the test tube vigorously.
Write your Observations and reasons.

Answer 1

Water soluble salts of Magnesium and Calcium make the water hard. After using the soap to wash hands with hard water, it feels that there is a film of residue left on your hands. In hard water, soap reacts with the calcium (which is relatively high in hard water) to form "soap scum".

Complete Solution :
Water can be classified into following two categories on the basis of quantity of salts and ions it contains:
Soft water: Soft water is surface water that contains low concentrations of ions and in particular is low in ions of calcium and magnesium.
Hard water: Hard water is high in dissolved minerals, largely calcium and magnesium.

- In set 1 the sample she had was pure water without any salt in it and as this is not hard water, liquid soap in it will give us a very good length of foam.
- In set 2 the sample contains water soluble salt of Ca that will make this water hard water and we know that in hard water, a great amount of soap gets waste into dealing with these water soluble salts and that’s why in this case we will be having poor length of foam.
So set 1 will give us a longer length of foam than set 2.

Additional Information:
General guidelines for classification of waters are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft, 61 to 120 mg/L as moderately hard, 121 to 180 mg/L as hard and more than 180 mg/L as very hard.

Note: Two types of hardness of water are:
- Temporary hardness: It is a type of water hardness caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). They precipitate out on boiling the water.
- Permanent Hardness: it is a type of water hardness caused by the presence of calcium sulfate and/or magnesium sulfates in the water, which do not precipitate out on boiling.