Intermediate Topic 273 of 1102 States of Matter & Solutions
Phase Diagram of Water
Water's solid-liquid line slopes backward, so ice melts under increased pressure.
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In-depth explanation of Phase Diagram of Water
The phase diagram of water has an unusual solid-liquid boundary that slopes backward, so the melting temperature decreases as pressure increases. This happens because ice is less dense than liquid water, so increasing pressure favours the denser liquid phase. Most substances have the opposite slope because their solids are denser than their liquids. The slope is steep, so the melting point of ice changes only slightly for large pressure changes. The same effect means that pressure can melt ice slightly, which contributes to ice skating, though friction heating also plays a role.
Going deeper: Phase Diagram of Water
Key ideas: The ice I and liquid boundary has a negative slope of about -0.0075 C per atm, so a pressure of 100 atm lowers melting by only 0.75 C. At very high pressures, many other ice forms exist with higher densities. The triple point of water is 0.01 C and 0.006 atm, and the critical point is 374 C and 218 atm.
Common mistakes and connections: Students often claim ice skating works only because of pressure melting, but the effect is too small for typical skaters and surface premelting and friction contribute. Another error is generalizing the negative slope to other substances. Few substances, such as bismuth and gallium, share this behaviour. Links to density anomaly and hydrogen bonding.
Visualization of Phase Diagram of Water
The solid-liquid boundary of water slopes backwards: higher pressure lowers the melting point because ice is less dense than liquid water. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Pressure under a skate blade lowers the melting point of ice slightly, which helps form a thin film of water on the ice.
Frequently asked questions about Phase Diagram of Water
What is Phase Diagram of Water?
Water's solid-liquid line slopes backward, so ice melts under increased pressure.
Is Phase Diagram of Water a beginner, intermediate or advanced topic?
Phase Diagram of Water is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Phase Diagram of Water?
Study these earlier topics in States of Matter & Solutions first: Triple Point, Critical Point, Supercritical Fluids.
What comes after Phase Diagram of Water?
The next topics in States of Matter & Solutions are Crystalline Solids and Amorphous Solids.
Can I practise Phase Diagram of Water with quizzes?
Yes. The interactive lesson includes a self-check question on Phase Diagram of Water, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Phase Diagram of Water fits in States of Matter & Solutions
Phase Diagram of Water is topic 54 of 109 in States of Matter & Solutions.