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Phase diagrams

Heat and matter · level 6, lesson 5 of 6

A question to start

Mars's polar caps hold water ice all year, but carbon dioxide ice only in winter. Why does carbon dioxide come and go while water ice stays?

The idea

A phase diagram maps which state a substance is in at each pressure and temperature. Lines mark where two states can exist together; the triple point is where three meet.

solidliquidgas triple pointEarthMarstemperaturep

For water, Earth's surface (101 kPa, 15 °C) sits in the liquid region. Mars's average surface (636 Pa, −59 °C) sits in the solid region, just above the triple point's pressure: ice is stable, and if it warms it soon turns to vapour.

Carbon dioxide has a very different diagram. Its triple point is at 518 kPa and −56.6 °C, over five times Earth's air pressure. So at the surface of either planet, carbon dioxide can never be liquid: it goes straight between solid and gas.

Carbon dioxide turns to ice at about Mars Earth
Pressure 636 Pa 101 kPa
Frost point −125 °C −78.5 °C

On Mars, only the polar winter gets colder than −125 °C. So carbon dioxide frosts out of the air onto the winter cap and sublimes back in spring, while water ice, stable at far warmer temperatures, stays all year.

Worked examples

1. Dry ice on Earth. A block of dry ice in a room never melts into a puddle. It sublimes into gas, because 101 kPa is below carbon dioxide's triple point.

2. Liquid carbon dioxide. At room temperature, squeeze carbon dioxide to about 5.7 MPa, some 57 times Earth's air pressure, and it turns liquid. Carbon dioxide fire extinguishers hold it that way.

3. Reading the diagram. Start in the solid region and lower the pressure at a fixed temperature: you cross into the gas region without passing through liquid. That is sublimation.

Try it

  1. What does a line on a phase diagram show?
  2. Why can carbon dioxide not be liquid on Earth's surface?
  3. In which region of water's diagram is Mars's average surface?
  4. Why does Mars's carbon dioxide ice come and go each year?
Answers
  1. The pressures and temperatures at which two states can exist together.
  2. Earth's air pressure, 101 kPa, is below its triple point pressure of 518 kPa.
  3. The solid region, just above the triple point pressure.
  4. It frosts out only when the polar winter falls below about −125 °C, and sublimes again when spring warms the cap.

Common mistake

"Every substance melts before it boils." Only above its triple point pressure. Below it, the solid turns straight to gas.

Where it leads