Question

In summer the water at the surface of lake is warmer,while the water at the bottom is comparatively cool.justify.

Answer 1

It is because the hot sun's rays fall on the surface of the lake water but it cannot really reach till the bottom. If the sunlight cannot reach the bottom, the water at lower level will be normal or cold.

Answer 2

The surface of water gets heated up. water,being a poor conductor of heat does not conduct heat to the bottom of the pond.
Since cold water denser than warm water, it sinks to the bottom of the lake.