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Why Hot Water Floats Over Cold Water?


When two immiscible liquids of unequal densities are kept in a container, the less dense liquid will float on the other. But what if the liquids are miscible? Can water float on water? Yes it can.


Take two bottles, say C (cold) and H (hot). Fill the bottle C with cold water and the bottle H with hot water. Mix a few drops of blue ink in the bottle C and of red ink in the bottle H. Close the mouths of the bottles C and H with your thumbs. Bring the bottle C over the mouth of bottle H and remove your thumbs. After sometime, the water in bottle C is getting colored. But there is no change in the color of water when bottle H is placed over the bottle C.


Density of hot water is less than that of the cold water. So the cold water of bottle C comes down to bottle H and the hot water of bottle H moves up to bottle C leading to mixing of color.

But in another case, water in bottle H is hot and that in bottle C is cold. Hence the cold water in bottle C, being more dense, remains at the bottom and hot water of bottle H tries to remain at the top. Hence water in these bottle do not get mixed.


Procedure: Take water in a glass tumbler filling it nearly up to the brim (say 90% of height). Put a drop of red ink on the surface of water and watch how the red colour spreads in water. Ink is heavier than water and it should sink, but it mixes in water because of Brownian motion.

Now take fresh water in the glass. Put a coffee heater on its rim in a vertical position so that it reaches up to half the depth of the tumbler. Put the heater on for few seconds and well before water boils, put it off and remove it gently.

Don't disturb the glass and put a drop of red ink on the surface of water. Study how the ink spreads. It will quickly mix up in the upper hot water and will not go to the lower cold water. This shows that the hot water floats on cold water and does not get mixed up.


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