How can we explain observations from the adjoining figure?

Explanation

(a)Initially, both thermometers show the temperature.
(b) The temperature of water in the middle of cylinder lowers due to freezing mixture in the outer bowl. Since the temperature of water there decreases, its volume decreases, and its density increases.
( c) The water with higher density moves downwards. Therefore, the lower thermometer `T_1` shows rapid fall in temperature and this continues till the temperature of water becomes equal to `4^@C`.
( d) When the temperature of water starts decreasing below `4^@C`, its volume increases, and density decreases. It, therefore, moves in the upward direction.
( e) The temperature of water in upper part `(T_2 )`, therefore, decreases rapidly to `0^@C`.
(f) The temperature of water in the lower part (`T_1` ), however, remains at `4^@C` for some time and then decreases slowly to `0^@C`