Question

A layer of ice is formed on the surface of lakes in cold regions because:

• A. Ice is less dense than liquid water
• B. There is no exchange of heat between water below and the cold air above
• C. Cold wind makes the surface water to freeze
• D. Ice is denser than liquid water

Hint:

Water is weird! It reaches its "Maximum Density" at $4^{\circ}$C.
Ice floats because its crystal structure has more "empty space" than liquid water.
If ice were denser than water, lakes would freeze from the bottom up, killing all fish every winter!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the fundamental physical property of water that allows ice to form as a surface layer on lakes rather than sinking to the bottom.
Detailed Explanation:

• Anomalous Expansion of Water:
Unlike most substances that continuously contract as they cool, water has a unique property. It is most dense at $4^{\circ}$C.
As water cools from $4^{\circ}$C to $0^{\circ}$C, it actually expands.

• Density Comparison:
At $0^{\circ}$C, water freezes into ice. Because of the open crystalline structure (hexagonal lattice) formed by hydrogen bonding, the molecules in ice are spaced further apart than in liquid water.
This means the density of ice (approx. $0.917$ g/$cm^3$) is less than that of liquid water (approx. $1.0$ g/$cm^3$).

• Mechanism of Lake Freezing:
As the winter air cools the surface of a lake, the surface water becomes denser and sinks, while warmer water rises. This continues until the entire water body reaches $4^{\circ}$C.
Once the water is at $4^{\circ}$C, further cooling makes the surface water less dense. This colder water ($3^{\circ}$C, $2^{\circ}$C, $1^{\circ}$C) stays on top until it freezes at $0^{\circ}$C.
Since the resulting ice is less dense than the $4^{\circ}$C water below it, it floats.

• Biological Importance:
The floating ice layer acts as an insulator, significantly slowing down further heat loss from the water below to the freezing atmosphere.
This prevents lakes from freezing solid and allows aquatic life to survive in the liquid water at the bottom.

Step 2: Final Answer:
The formation of ice on the surface is a direct result of ice being less dense than liquid water, allowing it to float.