IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Water Floating over Water

## Objective

To study the density of the hot and the cold water.

## Apparatus

four empty water bottles, coloring material

## Procedure

Fill the bottles A and B with cold water and the bottles C and D with hot water. Mix coloring material (red ink etc) with the water of A and D. Close the mouths of the bottles B and C with your thumms. Bring the bottles over the mouths of bottle D and A and remove your thumbs from the mouths of bottles. After sometime, the water in bottle B is getting colored. But there is no change in the color of water in bottle C.

## Discussion

Density of hot water is less than that of the cold water. So the cold water of bottle B comes down to bottle D and the hot water of bottle D moves up to bottle B by the process of convection. Thus the colorless water in bottle B gets colored and color of water in bottle D becomes lighter. But water in bottle C is hot and that in bottle A is cold. Hence the cold water in bottle A, being more dense, remains at the bottom and warm water of bottle C tries to remain at the top. Hence water in these bottle do not get mixed.

When the temperature of bottle A and C become equal due to radiation, exchange of water takes place between the bottles through the process of diffusion.