Why does ice form and float on liquid water as water cools near 4 C?

Why does ice form and float on liquid water as water cools near 4 C?

• A. Ice forms because water expands upon freezing, decreasing density.
• B. Ice forms because water is denser than liquid water.
• C. Water density increases as it is cooled further below 4 C.
• D. Ice forms because surface tension becomes greater than that of liquid water.

Answer: (B)

The key idea is that water reaches its highest density around 4°C, and when it freezes into ice the structure expands, so ice becomes less dense than the surrounding liquid. As water cools toward freezing, the molecules pack more tightly up to about 4°C, but when freezing begins the water molecules form a crystalline lattice that takes up more space. That expanded, less-dense ice floats on the denser liquid water. So the statement that explains this behavior is the one that ice forms because water expands upon freezing, decreasing density. The other ideas don’t describe why ice floats: density of water does not keep increasing below 4°C, surface tension isn’t the driver of freezing, and saying water is denser than liquid water would imply the opposite of what actually happens.

The key idea is that water reaches its highest density around 4°C, and when it freezes into ice the structure expands, so ice becomes less dense than the surrounding liquid. As water cools toward freezing, the molecules pack more tightly up to about 4°C, but when freezing begins the water molecules form a crystalline lattice that takes up more space. That expanded, less-dense ice floats on the denser liquid water.

So the statement that explains this behavior is the one that ice forms because water expands upon freezing, decreasing density. The other ideas don’t describe why ice floats: density of water does not keep increasing below 4°C, surface tension isn’t the driver of freezing, and saying water is denser than liquid water would imply the opposite of what actually happens.