When water is heated from 0°C to 4°C, its density

Generally, the density of a substance decreases upon heating as particles move apart and volume increases [1]. However, water exhibits a unique phenomenon known as anomalous expansion between 0°C and 4°C. When water is heated from 0°C to 4°C, it contracts instead of expanding, a behavior termed negative thermal expansion. Because density is the ratio of mass to volume, this contraction (decrease in volume) results in an increase in density [1]. Water reaches its maximum density at approximately 4°C. Beyond 4°C, water behaves normally, expanding upon further heating, which causes its density to decrease. This anomalous behavior is critical for aquatic life, as it ensures that the densest water (at 4°C) sinks to the bottom of lakes, while ice (which is less dense) forms at the surface, providing insulation.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 9: The Amazing World of Solutes, Solvents, and Solutions > 9.5.2 Effect of temperature on density > p. 147