Physics · Ch 8 — Heat and Thermodynamics
Anomalous Expansion of Water
Anomalous Expansion of Water
Unlike most liquids, which simply contract on cooling at every temperature, water shows anomalous expansion: between 0°C and 4°C it actually contracts as it is heated (equivalently, it expands as it is cooled further below 4°C), so its volume is at a minimum, and its density at a maximum, at exactly 4°C (Figure 8.5). This has an important consequence for life in cold climates: as a lake cools in winter, the surface water first cools toward 4°C and sinks (being denser), but once the surface drops below 4°C it becomes less dense than the water beneath it and stays at the top, where it can freeze into ice; because ice floats and insulates the water below, the bottom of the lake stays close to 4°C (its densest, warmest-relative-to-freezing state) all winte …
What this figure shows. Two companion graphs plotted over the temperature range 0°C to 10°C. Graph (a) plots the volume of 1 kg of water (in cm³) against temperature: the curve dips down from a higher value at 0°C to a minimum volume at 4°C (around 1000.00 cm³), then rises again as temperature increases further. Graph (b) plots the density of water (in g cm⁻³) against the same temperature range: it rises from a lower value at 0°C to a maximum density (close to 1.0000 g cm⁻³) at 4°C, then falls again above 4°C. Together the two graphs show that water's volume is minimum, and its density is maximum, at exactly 4°C -- the signature of its anomalous expansion behaviour, opposite to the ordinary con …
What this figure shows. A cross-section of a lake in winter, showing a vertical temperature profile through the water column: the surface layer near 0°C (frozen or near-freezing) at the top, warmer layers below it, down to the densest water at 4°C sitting at the very bottom of the lake, next to numbered temperature markers (0°C at top through roughly 8°C style progression at the bottom in a summer/winter comparison). Because ice and near-0°C water are LESS dense than the 4°C water below, the coldest water stays trapped at the surface where it can freeze, while the denser 4°C water stays insulated at the bottom -- which is exactly why fish and other aquatic species can survive a lake freezing over, s …