Q.Describe the anomalous expansion of water. How is it helpful in our lives?

Step 1. Almost all liquids simply contract on cooling at every temperature; water is the striking exception -- between 0°C and 4°C it actually CONTRACTS as it is heated (equivalently, keeps expanding as it is cooled further below 4°C), so its volume reaches a minimum, and density a maximum, at exactly 4°C. This is called the anomalous expansion of water.

Step 2. As a lake cools through winter, surface water above 4°C sinks as it cools further (getting denser, as normal for a liquid). But once a surface layer's temperature drops below 4°C, it becomes progressively LESS dense (not more) the closer it gets to freezing, so it stays at the top rather than sinking.

Step 3. This surface layer eventually freezes into ice, which floats (since ice is less dense than liquid water) and insulates the water below from further heat loss. As a result, the water at the bottom of the lake stays close to its densest temperature, 4°C, throughout the winter, rather than the entire lake freezing solid from the bottom up. …

Explain the 0-4°C contraction-on-heating behaviour first, then connect it to why ice …

• Saying water 'always expands on cooling' -- the anomaly is specifically restricted to the narrow 0-4°C range; above 4°C …