Question

If the surface of water in a lake is just going to freeze, then the temperature of water at the bottom is

• A. $0C$
• B. $4C$
• C. $3C$
• D. None of these

Answer 1

The correct option is B $4C$

$Answer:-$ B

As the temperature of a liquid/solid increases, its volume increases. Most liquids behave this way. But, water has a unique quality, above (almost) 4°C water behaves normally i.e its volume increases as the temperature increases. However, between 0°C and almost 4°C its volume decreases with rise in temperature( or increases with temperature drop, thus making it less dense).
Say the water at the surface is at 12°C and its cooling down. Now, as the temperature decreases, its volume decreases and density increases. As the surface water reaches about 4°C, it becomes more dense and "sinks" to the bottom allowing the less dense water to surface up. As temperature falls below 4°C, surface water becomes lighter( less dense) compared to the water below and floats until it freezes. Once frozen, the ice insulates the water at the bottom from cooling so its temperature is maintained between 0 and almost 4°C, keeping it from freezing.