PRACTICE QUESTIONS (COMPETITION WING)

Concentrated ${\mathrm{HNO}}_3$ and concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ can't be used for the preparation of ${\mathrm{H}}_2\mathrm{S}$. Give a reason. 

Rekha and Lekha took two fresh red roses and kept them in two containers A and B into which anhydride of sulphurous acid and a greenish yellow coloured gas are passed, respectively. After a few minutes, these two roses lose their colour but on being exposed to the atmosphere, it was observed that Rekha's rose regained its colour but Lekha's rose did not. How do you account for the above changes? 

During the summer vacation, a chemistry laboratory assistant stored the chemicals in their respective bottles. After a few months it was observed that the lids of the bottles in which sodium hydroxide was stored was struck tight. He went to a lecturer for finding the solution for this problem. What solution could be suggested by the lecturer? 

Bleaching powder in cold water is an excellent bleaching agent. But it cannot act as a good bleaching agent when dissolved in hot water. Why? 

Hydrofluoric acid cannot be stored in glass bottles like all other acids. Give a reason. 

Certain phosphates are helpful in making hard water soft. Which phosphates are used and how do they help in making water soft? 

Both ${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{HNO}}_3$ cause damage to the skin when the skin comes in contact with them. Compare and contrast the reasons. 

Nitric acid attacks all the metals except noble metals but the nature of products of the same metal along with corresponding nitrates depends upon the concentration of ${\mathrm{HNO}}_3$. Explain.