Question

a. Water extinguishes most fires, but it does not extinguish petrol fires. Explain.
b. Soft water lathers with soap, but not hard water. Why?

Answer 1

a Water extingushes most fires by lowering down the temperature of the burning material. But in case of petrol fire, since petrol is lighter than water, it floats over water and hence fire spreads instead of extingushing.
b Hard water contains calcium and magnesium salts. these react with soap to form insoluble calcium and lmagnesium salts of fatty acids, i.e., form scum and not lather.
`2RCOONa+Ca^(2+)to(RCOO)_(2)Cadarr+2Na^(o+)`
`2RCOONa+Mg^(2+)to(RCOO)_(2)Mgdarr+2Na^(o+)`