Thermal anomaly of water

Water has a density anomaly: up to a temperature of 4 °C, water has a negative coefficient of expansion , i.e. it shrinks when warmed. After passing through zero at 4 °C, the coefficient of expansion becomes positive. So the density of water reaches a maximum at 4 °C.

In experiment C1.3.3.1, the density maximum of water is proven by measuring its expansion in a vessel with a riser. Starting from room temperature, the complete setup is cooled down in an ice water bath to about 1 °C under constant stirring, or after cooling in a freezer, it is warmed slowly by the ambient temperature. The level h, in a riser with a cross-sectional area A is measured as a function of the water temperature ϑ. Because the change in volume relative to the total volume V₀ is small, the result obtained for density is
.