In cold countries, water pipes sometimes burst, because

The correct option/answer is: Water expands on freezing.

Let's understand the physical reasons behind this phenomenon step-by-step:

1. Anomalous Expansion of Water:
Unlike most liquids that contract and become denser as they cool and freeze, water exhibits a unique property called anomalous expansion. When liquid water is cooled, it contracts and its density increases until it reaches . However, as it cools further from down to (its freezing point), it begins to expand instead of contracting.

2. Behavior During Freezing:
When water transitions from a liquid to a solid (ice) at , its molecules organize into a crystalline, open-cage hexagonal structure held together by hydrogen bonds. This open lattice structure occupies more space than the disorganized molecules in liquid water. As a result, the volume of ice is about 9% greater than the volume of the same mass of liquid water. Therefore, water expands upon freezing.

3. Impact on Water Pipes:
When water is trapped inside a pipe and freezes due to extremely cold weather, it must expand to accommodate this increase in volume. If the pipe is closed or has no room for the water to expand, the expanding ice exerts a massive outward pressure on the pipe walls. When this pressure exceeds the mechanical strength of the pipe material (whether metal or plastic), the pipe bursts.