Question

Why does ice form on the top of ponds and lakes, and not the bottom? How does this property of water help support life in lakes and ponds?

Answer 1

Hint:The water exists in three states- solid, liquid and the gas. The solid form is ice and the gaseous form is water vapor. The change of state takes place when a certain temperature is attained. During the change of state, the temperature of the system does not change and this unique property plays a very determining role in the existence of life on this planet. Also, freezing of water bodies does not take place during every winter.

Complete answer:
Water has a unique property of freezing from the top to the bottom, when most other substances freeze from the bottom up. This property of water allows the ice to float on the top of the water. As temperature is decreased, volume decreases and as a result density of the substance increases but water shows anomalous behavior.

AS the temperature of water is decreased its density decreases but as soon the temperature falls to ${{4}^{0}}C$ its anomalous behavior starts. From ${{4}^{0}}C$ to ${{0}^{0}}C$ the volume increases and hence density decreases. At ${{4}^{0}}C$ water is in liquid state. In water bodies, surface water cools down, grows denser, and descends. As water approaches its freezing point, it will become less dense than the water around it, and it will rise to the top. This ensures supply of oxygen and heat to the aquatic animals.

Note: If water instead froze from the bottom of a lake or river to the top, there would be profound ecological consequences. Had this anomalous behavior of water been absent, then water bodies would cool down from bottom to top and as such no life could survive inside it. This would have resulted in chaos. Under these circumstances, Earth would look very different; the planet’s polar regions would be nearly devoid of life.