A block of wood is so loaded that it just floats in water at room temperature. What change will occur in the state of floatation, if

(a) some salt is added to water,

(b) water is heated?

A balloon filled with helium gas floats in a big closed jar which is connected to an evacuating pump. What will be your observation, if air from jar is pumped out? Explain your answer.

An iron nail sinks in water while an iron ship floats on water. Explain the reason.

A man first swims in sea water and then in river water.

(i) Compare the weights of sea water and river water displaced by him.

(ii) Where does he find it easier to swim and why?