Practical Chemistry — Question 6

(a) Differentiating hard water from soft water —

1. Two unknown samples 'X' and 'Y' containing hard water and soft water are taken separately in a trough or beaker.
2. Ordinary soap is rubbed by the hands inside each sample.

Observation —

1. One sample of water 'X' lathers with soap
2. The sample of water 'Y' does not lather with soap.

Result —

1. The sample 'X' which lathers with soap is soft water.
2. The sample 'Y' which does not lather with soap is hard water.

(b) Distinguish between temporary hard water and permanent hard water —

1. Two unknown samples 'A' and 'B' containing temporary and permanent hard water are taken separately in a trough or beaker.
2. The water is boiled slowly, gases allowed to escape out and then the water is filtered.
3. Ordinary soap is rubbed by the hands inside each filtered sample.

Observation —

1. One sample of water 'A' lathers with soap.
2. The sample of water 'B' does not lather with soap.

Result —

1. The boiled and filtered sample 'A' which lathers is temporary hard water whose hardness is removed by boiling.
2. Sample 'B' is permanent hard water whose hardness cannot be removed by boiling.

(c) Remove temporary hardness from water without using a chemical compound.

1. Temporary hard water is taken in a beaker and heated slowly.
2. After the gases escape out, the water is filtered through a filter paper.
3. Ordinary soap is rubbed inside the filtered solution.

Observation — The boiled and filtered sample of temporary hard water lathers readily with soap.

Result — Temporary hard water can be softened by heating.

$\underset{\text{[in temporary hard water]}}{\text{Ca(HCO}_3)_2} \longrightarrow \underset{\text{[ppt. filtered out]}}{\text{CaCO}_3\downarrow} + \text{CO}_2 + \underset{\text{ [filtered water is soft]}}{{\text{H}_2\text{O}}}$

(d) Remove temporary hardness and permanent hardness from water using a chemical compound.

1. Temporary and permanent hard water are taken in separate beakers and washing soda is added in it. Then the solutions are filtered to remove the precipitate.
2. Ordinary soap is rubbed inside the filtered solution.

Observation — The filtered sample of temporary and permanent hard water lathers readily with ordinary soap.

Result — Temporary hard water and permanent hard water can be softened by using washing soap.

(e) Advantage of detergent over soap

1. A sample of hard water 'X' is taken and ordinary soap is rubbed inside the water.
2. Another sample of hard water 'Y' is taken and detergent is rubbed inside the water. [Detergents are sodium salts of sulphonic acid and do not form scum].

Observation — Lather forms in 'Y' but not in 'X'.

Result — Detergents form lather even with hard water, while ordinary soap is wasted due to formation of scum.

$\underset{\text{[in hard water]}}{\text{Ca(HCO}_3)_2} + \underset{\text{[soap]}}{2\text{NaSt}} \longrightarrow \underset{\text{[scum]}}{\text{Ca(St)}_2} + 2\text{NaHCO}_3$