Water is most dense at which temperature?

• A. 0°C
• B. 4°C
• C. 10°C
• D. 20°C

Answer: (B)

Water reaches its greatest density around 4°C. At this temperature the balance between thermal motion and hydrogen bonding allows molecules to pack most tightly, giving the highest mass per volume. As the water cools further below 4°C, the hydrogen-bond network starts to organize into a more open, ice-like structure, which expands the volume and lowers density. Above 4°C, warming causes expansion too, so density decreases with increasing temperature. Hence, the densest state of pure water at standard pressure is near 4°C (about 1.00 g/cm³). The other temperatures—0°C, 10°C, and 20°C—are less dense because they lie on the cooler side with the onset of ice-like structure or on the warmer side with greater molecular spacing.