Question

Why is water denser at four $4°\mathrm{C}$?

Answer 1

Density:

• The density of a liquid is a measurement of its mass in proportion to the volume being measured.
• The heavier liquid is denser when two liquids are weighed in the same amounts or volumes.

Water density:

• Molecules come closer to one other as the temperature of the water drops.
• When water is at $0°\mathrm{C}$, it takes a cage-like structure, and thus the volume increases which results in the density decrease.
• 
• The temperature of the water increases and gets to $4°\mathrm{C}$, here, the density is maximum.
• The reason behind this is that the cage-like structure starts to break and come closer. As a result, volume decreases.
• And we know that when volume decreases, density increases.

Hence, water is denser at $4°\mathrm{C}$ because the volume is minimum here.