Viva Voce: Cleaning Capacity of Soap in Hard and Soft Water | Lab Manuals for Class 10 PDF Download

Q.1. Why is scum formed when hard water is treated with soap?

Hard water contains bicarbonate, chloride or sulphate salts of calcium and magnesium which reacts with soap to form insoluble precipitate called scum.

Q.2. Was there any difference in the length of the foam formed in test tube C having water containing calcium hydrogen carbonate (or calcium sulphate) and test tube B containing well water or underground water?

Yes, test tube B contains larger length of foam while C contains smaller length of foam because test tube C contains calcium hydrogen carbonate dissolved in distilled water. This calcium hydrogen carbonate reduces the foaming capacity of soap.

Q.3. What do you understand by temporary and permanent hardness of water?

Temporary hardness of water is due to the presence of calcium and magnesium bicarbonates in water and can be removed by boiling or by adding Na₂CO₃ to hard water.
Permanent hardness of water is due to the presence of chlorides and sulphates of calcium and magnesium and can be removed by using an ion exchanger.

Q.4. What is hard water?

The water that has high mineral content (like magnesium and calcium ions) and produces scum on addition of soap.

Q.5. Which salts cause hardness in water?

The hydrogen carbonates, chlorides and sulphates of calcium and magnesium cause hardness in water.

Q.6. Why do detergents not produce scum with hard water?

The calcium and magnesium salts of detergents are soluble in water. Hence, detergents can be used effectively even in hard water

Q.7. When soap is added to hard water, a white curdy substance is observed which floats on the surface of hard water. Name this white curdy substance.

When soap is added to hard water, it forms an insoluble white curdy substance called scum which floats on the surface of water.

Q.8. A student observed that when Na₂CO₃ is added to hard water, it produces lather with soap. What could you conclude from this observation?

When Na₂CO₃ (sodium carbonate) is added to hard water, it reacts with calcium and magnesium ions and gets precipitated out as carbonates thereby, increasing the foaming capacity of soap and hence, soap produces lather with this type of water.

Q.9. Do both hard water and soft water produce foam with soap?

No, only soft water produces foam with soap.

Q.10. Why did we add calcium hydrogen carbonate (or calcium sulphate) to beaker C?

Beaker C contains distilled water which is free ofCa⁺² and Mg⁺² ions. When calcium hydrogen carbonate (or calcium sulphate) is added to distilled water, the water becomes temporary hard.
On addition of soap, these ions react with soap thereby reducing lather capacity of soap by forming scum.

Q.11. With their prolong use, white scales get deposited in the interior of boilers and electric kettles. What is the reason for this observation? How can these scales be removed?

On prolong use, scales get deposited in the interior of boilers and electric kettles due to the precipitation of CaCO₃ and CaSO₄. The precipitation of CaCO₃ is also caused due to the thermal decomposition of bicarbonate ions. Descaling agent (like white vinegar contains acetic acid) can be used to remove scaling.

Q.12. What is soft water?

The water which has few or low mineral content like calcium (Ca) or magnesium (Mg) ions and produces lather on mixing soap to it is called soft water.

Q.13. What is the reaction between soap molecules and ions present in hard water?

Q.14.  What can be used instead of soap to wash clothes in hard water?

Synthetic detergents are used to wash clothes in hard water, as they do not produce the insoluble scum.

Q.15. Tanu tried to dissolve the soap in water completely but it does not dissolve and gives no lather also. What conclusion can you drawn from this observation?

The soap does not dissolve in water completely, this suggests that the water taken by Tanu is hard, i.e. it contains salts of calcium and magnesium (generally bicarbonates, chlorides or sulphates).

Q.16. When soap is added to ethanol solution, will micelles formed or not?

Soap gets completely soluble in ethanol and forms a clear solution. Thus, it does not form micelles.