Advanced #63

Water has a high heat of sublimation

The energy required to convert ice directly into vapor is unusually high.

Scientific Explanation

Sublimation is the direct transition from a solid to the gaseous state, bypassing the liquid phase. For water, the heat of sublimation is about 2830 kilojoules per kilogram — the sum of the heat of fusion (334 kJ/kg) and the heat of vaporization (2260 kJ/kg), though at lower temperatures it can be slightly higher than this simple sum suggests.

This value is nearly five times greater than for carbon dioxide (571 kJ/kg) and naphthalene (561 kJ/kg). The reason is that sublimating ice requires simultaneously breaking the ordered crystal lattice and severing all hydrogen bonds. In the ice crystal, each molecule is connected to its neighbors through four bonds arranged in a diamond-like geometry, making the structure especially stable.

The high heat of sublimation also explains why ice at sub-zero temperatures sublimates only extremely slowly, unless the air is very dry or a vacuum is applied.

Heat of Sublimation of Water vs Other Substances Bar chart comparing the heat of sublimation of ice (2830 kJ per kg) with CO2 (571 kJ per kg) and naphthalene (561 kJ per kg). Water's sublimation energy is far higher because it must break hydrogen bonds and the crystal lattice simultaneously. Heat of Sublimation (kJ/kg) 0 1500 3000 2830 Ice (H₂O) 571 CO₂ 561 Naphthalene Heat of Sublimation Comparison
Heat of sublimation of various substances. Ice exceeds the others by a large margin.

Everyday Relevance

Sublimation of ice is something we encounter in winter, when laundry dries on the line despite sub-zero temperatures — the ice in the fabric sublimates directly into the dry winter air. In food technology, freeze-drying (lyophilization) is an important preservation method in which water sublimates from frozen food under vacuum. The high heat of sublimation makes this process energy-intensive but ensures gentle drying at low temperatures.