Explain the formation of scum when hard water is treated with soap.
When hard water is treated with soap, scum is formed as an insoluble substance. Hard water contains calcium and magnesium salts which cause the hardness of water.
When soap is added to hard water, it reacts with these calcium and magnesium salts present in the water. This reaction produces an insoluble precipitate which appears as a white curdy solid called scum.
The formation of scum creates difficulty in forming foam and reduces the cleansing effectiveness of soap. As a result, a larger amount of soap is needed for washing with hard water.
Explanation
The textbook clearly explains that scum formation occurs due to the reaction between soap and the calcium and magnesium salts present in hard water. These salts cause water hardness, and when they react with soap, they form an insoluble substance (scum) which appears as a white curdy precipitate. This is why foam forms with difficulty when using soap in hard water, and more soap is required for effective cleaning.