Give reasons

1.  Lakes freeze from top towards bottom.

2.  Ice floats on water.

(i) Density of ice is less than that of liquid water. During winter, the temperature of lake water keeps on decreasing. Since, cold water is heavier, therefore, it moves towards bottom of the lake and warm water from the bottom moves towards surface. This process continues. The density is maximum at 277 K.
Therefore, any further decrease in temperature of the surface water will decrease in density. The temperature of surface water keeps on decreasing and ultimately it freezes.
Thus, the ice layer at lower temperature floats over the water below it. Due to this, freezing of water into ice takes place continuously from top towards bottom.
(ii) Density of ice is less than that of liquid water, so it floats over water.