Concepts of Physics

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

Warm Water is Lighter than Cold Water

Introduction

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.

Apparatus

A glass tumbler, few drops of red ink, coffee heater

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.