Question

What is meant by anomalous expansion of water?

Answer 1

Hint
Most of the liquid expands when the temperature increases. But water shows some anomalous property at a certain temperature range.

Complete step by step answer
We know that when the temperature increases, most of the liquids will expand and they gain kinetic energy to move. The density will decrease. Also when the temperature is decreased, the liquid will freeze. And the density will gradually increase. But this normal behavior of water is maintained up to temperature of ${4^\circ }\;{\text{C}}$ .
When the temperature is increased up to ${4^\circ }\;{\text{C}}$, the density of the water will increase. But when the temperature again decreases to a range of $0^\circ \;{\text{C}}$, the water will behave differently. Water tends to expand and density of water will decrease. Therefore the maximum density will be at ${4^\circ }\;{\text{C}}$ and between ${4^\circ }\;{\text{C}}$ to $0^\circ \;{\text{C}}$, the density will be decreasing. Thus the coldest water will always remain at the top. The heaviest water will remain on the surface. And beneath the denser water, the lightest water will settle down.
This anomalous behavior of water will help the aquatic animals to survive in winter seasons. The water on the top of the water resources will freeze quickly. And it will insulate the water beneath it from freezing. Thus the aquatic animals are safe in the cold.

Note
While freezing the movement is not possible for the water molecules. The bonding between oxygen atoms would be stronger. Even though the bonding between hydrogen and oxygen will be not much stronger which contributes to the anomalous behavior of water.