Question

In cold countries pond freeze only at surface.Why

Answer 1

Since water can change its state from liquid to solid. They do so when the temperature of the atmosphere decreases.

Hope it helps

Answer 2

Answer:

Anomolus expantion of water

Explanation:

In cold weather,when the atmospheric temperature starts below 0°C water at the surface of a pond initially at temperature above 4°C, begins to radiate heat to the atmosphere,so the temperature of water near the surface starts falling.When temperature of water at the surface falls below 4°C,Water contracts and density increases and therefore it sinks to the bottom. This continues till the temperature of entire water reaches to 4°C.Now further cooling of top layers  below 4°C Results in expansion of water and so its density decreases. As a result, water doesn’t Sink further , but it remains on the surface.When the temperature of atmosphere falls below 0°C,Water on the surface loses further heat to the atmosphere and gradually freezes into ice but water below the ice layer remains at 4°C.The water layer just below the ice in contact with it will remain at 0°C.Since ice is a poor conductor of heat so ice now prevents the flow of heat from water of ponds to the atmosphere.Thus,temperature of water in contact with ice is at 0° C,while the temperature of water layers below the ice gradually increases to 4°C. As a result fish and other aquatic creatures remain alive in water of the pond though water on the surface has frozen into ice.