Water Floating over Water
To study the density of the hot and the cold water.
four empty water bottles, coloring material
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.
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.