Answer:
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Explanation: 3 Marks
Normal behavior for a liquid is to expand on heating and contract on cooling. A liquid that contracts is said to become denser.
Water behaves like a normal liquid when it's cooled down to 4 degrees(it contracts). But when cooled further down from 4 degrees to 0 degrees, it behaves anomalously i.e it expands instead of contracting further. So, ice at 0 degrees is less dense than water at 4 degrees.
When a lake starts getting cold, the water at the top hits 4 degrees becomes dense and sinks to the bottom, this keeps happening until all water in the lake has reached 4 deg Celsius. Now, when the temperature outside drops further, and the water at the top falls to 0 degree Celsius, the surface turns to ice. But ice at the top is less dense than the water at the bottom. So, it does not sink. This is why when you see a lake frozen at the top, the water below still remains at an even temperature of 4 degrees, where aquatic life can survive. This is how the anomalous expansion of water plays a major role in the sustenance of aquatic life in cold places.