Concepts of Physics

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

Density of Cold Water


If you cool water from room temperature, it shrinks like any other liquid and hence the density increases. The density of water is maximum at 40C. As water is cooled below 40 degree Celsius, unlike any other liquid water expands and the density again decreases. This helps aquatic species to live under water in cold places.


A vessel fitted with two thermometers, Lid, cold water, ice-pieces.


  1. Prepare cold water in a separate vessel.
  2. Put the water in the experimental vessel up to level where the upper thermometer is dipped in water.
  3. Put some ice pieces in water and close the vessel with its lid.
  4. Note the temperature of the two thermometers at regular internal of say 2 minutes


As ice floats on water, it cools the water from top. As the temperature goes down, its density increases and the colder water goes down. This gives rise to convection current and whole of water gradually cools down.

However when the temperature reaches around 4 degree celsius to 5 degree celsius, the density is largest and it sinks at the bottom and convection current does not affect this layer. As a result, the upper layer can so much lower but the lower layer remains at 40 degree celsius.

If the temperature above water is in negative Celsius range, the top layer may even freeze but the lower layer remains liquid. That saves aquatic species.

Structure of the Atom: there are several experiments of the why the density is maximum at \(4\,{}^o C\). one simple model is that at \(4\,{}^o C\), the thermal motion comes to such a low level that bonding tendency to form solid ice crystal can start here and there. Such tiny ice crystals will make and break maintain a statistical equilibrium. As ice has lower density than water, the net average density will decrease. As you lower the temperature from \(4\,{}^o C\), the fraction of these tiny ice crystal increases and hence density keeps decreasing at 0 degree celsius, thermal energy is too small and ice start forming at a much larger scale.


Figure and Table to be made