From 0 °C to 4 °C, water contracts on heating and expands on cooling. Its density is maximum at about 4 °C. Below 4 °C, colder water stays above denser 4 °C water, so lakes freeze from the top and liquid water survives beneath insulating ice.

Key graph: volume minimum and density maximum at 4 °C.

Between 0 °C and 4 °C, ordinary liquid water contracts on warming and expands on cooling. Its density is greatest near 4 °C at normal pressure. Cooling a lake first sends the denser near-4 °C water downward; colder surface water remains above and freezes. Ice is less dense than liquid water and floats, while the surface ice layer slows further heat loss, allowing water below to remain liquid. Above 4 °C water expands normally on warming. The anomaly reflects water’s molecular structure and is not a universal law for every liquid or pressure condition.

Worked example: When water cools from 4 °C to 0 °C at approximately fixed mass, its volume increases and its density falls.

NCERT trap: Water’s maximum density occurs near 4 °C, not at the freezing point.

NEET method: Track mass and volume before predicting density; for lakes explain convection and floating ice separately.

Reading estimate excludes checks and retake.

Thermal Properties of Matter

Check 1 of 3

Check 2 of 3

Check 3 of 3

Topic checkpoint