Phase Changes

A phase change is when matter changes from one state (or phase) to another, without changing its chemical identity. Ice melting into water is a phase change. Water boiling into steam is a phase change. Steam condensing back to water is too. The substance is still H2O in every case: the only thing changing is how the molecules are arranged and how much energy they have. Phase changes happen all around you, every day, and they always involve energy being absorbed or released.

  • What it isChange of stateNo chemistry change
  • Six main changesMelt, freeze, boil, condense, sublime, depositPlus rare ionisation
  • Energy neededLatent heatAbsorbed or released
  • TriggersTemperature or pressureOr both
  • At a phase changeTemperature stays putEnergy goes into rearranging
  • Water exampleIce -> water at 0 CWater -> steam at 100 C

The six main phase changes

Between the three common states (solid, liquid and gas) there are six possible changes:

  • Melting: solid -> liquid (ice -> water)
  • Freezing: liquid -> solid (water -> ice)
  • Boiling or evaporating: liquid -> gas (water -> steam)
  • Condensing: gas -> liquid (steam -> water)
  • Subliming: solid -> gas (skipping liquid; dry ice -> CO2)
  • Depositing: gas -> solid (skipping liquid; water vapour -> frost on grass)

Energy in phase changes

Every phase change involves energy. When a substance changes from a more ordered state to a less ordered state, it absorbs energy. When it changes from less ordered to more ordered, it releases energy.

  • Melting, evaporating, subliming: absorb energy (endothermic).
  • Freezing, condensing, depositing: release energy (exothermic).

The energy absorbed or released during a phase change is called latent heat. The word "latent" means "hidden", because the energy goes into rearranging the particles rather than changing the temperature.

Temperature stays put

One of the most surprising things about phase changes is that the temperature does not rise during the change itself. Heat ice on a stove and the temperature climbs from -10 to 0 degrees Celsius, then sits at 0 while the ice melts (even though you are still adding heat), then climbs again from 0 to 100 once all the ice has become water, then sits at 100 while the water boils, then continues to climb if all the water has turned to steam.

During each plateau, the energy you are adding is being used to rearrange the molecules rather than to warm them up.

Fact Melting 1 gram of ice into 1 gram of water (still at 0 degrees Celsius) takes about 334 joules of energy. Boiling 1 gram of water into 1 gram of steam (still at 100 degrees Celsius) takes about 2,260 joules. Boiling takes far more energy than melting because separating molecules into a gas needs to overcome much stronger attractive forces.

Phase changes around you

  • Ice cubes melting in a drink: cool the drink as the ice absorbs heat to melt.
  • Sweat evaporating: cools your skin as the water absorbs body heat to turn to vapour.
  • Steam from a kettle: water boils into steam, then condenses back into the visible mist.
  • Dew on grass: water vapour in cool morning air condenses onto cool grass.
  • Frost on a window: water vapour deposits directly as ice crystals on a very cold surface.
  • Dry ice "fog": solid CO2 sublimes straight into gas, dragging water vapour from the air with it as a thick white fog.
  • Wax in a candle: melts into a liquid near the wick, then evaporates and burns as a gas in the flame.
  • Clouds: water vapour in the air condenses into tiny droplets, becoming visible.
  • Snow: water vapour deposits directly into ice crystals in cold clouds.
Did you know? Cooks use phase changes all the time. A pressure cooker increases the pressure on water, raising its boiling point from 100 C to around 121 C. Food cooks much faster at the higher temperature. Up a mountain, the opposite is true: lower air pressure means water boils at a lower temperature (around 70 C on top of Mount Everest), and pasta takes much longer to cook properly.

Phase changes in weather and climate

The Earths weather depends on phase changes of water:

  • Evaporation: heat from the Sun turns liquid water in oceans, lakes and soil into water vapour, lifting it into the air.
  • Condensation: as the moist air cools at altitude, water vapour condenses into tiny droplets, forming clouds.
  • Precipitation: droplets grow until they fall as rain, or freeze on the way down to fall as snow or hail.
  • Melting and freezing: ice in glaciers and at the poles melts and freezes each season, helping regulate sea level and climate.

This continuous cycle, the water cycle, is powered by the latent heat absorbed and released during these phase changes. The energy carried by water vapour to high altitudes is one of the main drivers of weather, including dramatic storms.

Phase changes in industry

  • Refrigeration: fridges and freezers work by repeatedly evaporating and condensing a refrigerant fluid. Evaporation absorbs heat (cooling the inside); condensation releases it (to the air outside the fridge).
  • Distillation: a mixture is boiled to evaporate one ingredient, which is then condensed and collected separately. Used to purify water, make spirits and refine crude oil.
  • Casting metals: molten metal is poured into a mould, where it cools and freezes into the desired shape.
  • Steam turbines: most of the worlds electricity is generated by boiling water into steam, which spins turbines connected to generators.
  • Freeze drying: frozen food is placed in a vacuum, so the ice sublimes directly into water vapour, leaving dry food that lasts for years.
Try this Boil a kettle and look closely. Right above the spout is a clear gap of invisible steam (water vapour, a gas). A few centimetres higher you see a misty white cloud: that is steam that has condensed back into tiny water droplets. The phase change from gas to liquid happens visibly right in front of you. Hold a cold spoon above the cloud and water will collect on the spoon, demonstrating condensation again.
Deeper dive: phase diagrams and triple points

Whether a substance is solid, liquid or gas depends on two things: temperature and pressure. Scientists draw these on graphs called phase diagrams.

A phase diagram of water has temperature along the bottom and pressure up the side. There are three regions: solid (ice) at low temperature, liquid (water) in the middle, gas (steam) at high temperature. The lines between the regions show the conditions where two phases coexist. The boiling line shows the temperatures at which water boils at different pressures.

At one special spot on the diagram, called the triple point, all three phases coexist in equilibrium. For water this is at about 0.01 degrees Celsius and 0.006 atmospheres (very low pressure). At the triple point, ice, water and water vapour all exist together at once. The triple point of water is so reliable that it was used to define the kelvin temperature scale.

Beyond a certain "critical point" (374 C and 218 atmospheres for water), the difference between liquid and gas disappears. The substance becomes a supercritical fluid with properties of both. Supercritical CO2 and supercritical water are used in industry as powerful, environmentally friendly solvents.

Other substances have their own unique phase diagrams. CO2 is unusual because at normal atmospheric pressure, solid CO2 cannot exist as a liquid: it sublimes straight from solid to gas (dry ice). To get liquid CO2 you need pressures of at least 5 atmospheres. This is why pressurised CO2 fire extinguishers have liquid inside, but when you release it the pressure drops and you get cold CO2 gas and a flurry of dry ice crystals.

For more, see solid, liquid and gas.