Advanced #57

The specific heat capacity (Cp and Cv) is unusually high

Both at constant pressure and constant volume, the heat capacity of water is unusually large.

Scientific Explanation

Comparing water’s specific heat capacity at constant pressure (4.18 joules per gram per kelvin) to other common liquids reveals the scale of the anomaly. Ethanol reaches 2.44, acetone 2.17, and olive oil about 1.97 joules per gram per kelvin. Water is nearly double most other liquids.

This high heat capacity holds for both Cp (at constant pressure) and Cv (at constant volume), though in liquids the difference between the two is relatively small. The physical reason is the same as in anomaly 56: the network of hydrogen bonds acts as an additional energy reservoir. Each bond binds about 20 kilojoules per mole, and with roughly 3.5 bonds per molecule, there is an enormous pool that can absorb heat without the temperature rising significantly.

Interestingly, water’s high heat capacity barely diminishes with rising temperature. Although fewer bonds are intact at higher temperatures, those that remain are easier to break — the two effects partially compensate.

Specific Heat Capacity of Water Compared to Other Liquids Bar chart comparing the specific heat capacity at constant pressure of water (4.18 J per g per K), ethanol (2.44), acetone (2.17), and olive oil (1.97). Water's value is roughly double that of most common liquids. Cp (J g⁻¹ K⁻¹) 0 2.5 4.5 4.18 Water 2.44 Ethanol 2.17 Acetone 1.97 Olive Oil Specific Heat Capacity Comparison at 25 °C
Specific heat capacity of common liquids. Water far exceeds all others.

Step by Step

H₂OOil⬆Same energy⬆Same energy+1°C+2.1°CH-bonds absorb energyOcean -- thermal bufferH₂O4.18 kJ/(kg·K)Oil2 kJ/(kg·K)Fe0.45 kJ/(kg·K)

The Experiment

Two containers -- one with water, one with oil -- receive the same amount of energy. What happens to their temperatures?

Same Result?

Oil heats up more than twice as fast as water. Water absorbs far more energy per degree -- 4.18 kJ/(kg*K) compared to 2.0 for oil.

Why So Much Energy?

In water, hydrogen bonds form a network. Added energy goes not just into molecular motion, but also into breaking and rearranging these bonds -- they act as energy sinks.

Climate Impact

Thanks to water's high heat capacity, the oceans store vast amounts of energy and act as a thermal buffer. Coastal regions benefit: milder winters, cooler summers. Water: 4.18, oil: 2.0, iron: 0.45 kJ/(kg*K).

Everyday Relevance

Water’s high heat capacity is one of the reasons Earth’s climate remains comparatively stable. The oceans store vast amounts of thermal energy and release it only slowly, counteracting extreme temperature swings. Coastal regions benefit especially: their winters are milder and summers cooler than in the interior — thanks to the thermal inertia of seawater.