A test tube is filled with some water .A small piece of wax is fixed at the bottom of the test tube and small piece of wax is made to float on the water what happens to the wax at the top and bottom of the test tube if the test tube is heated at the top middle and bottom?

Explanation

By heating the test tube at its top, the wax present at top surface will only melt but the wax present at the bottom of the test tube in water does not melt as there is no transfer of heat in the downward direction by convection. So, the temperature of water at the bottom of the test tube remains constant and there is no change in the state of wax present at the bottom.
By heating the test tube at the middle, the wax at the top surface will melt but the wax present at the bot tom surface remains same because heat convection takes place in upward direction only. By heating the test tube at bottom, both the wax present at top and bottom melt because heat gets transferred from bottom to top due to convection.