Phase Changes

1. Under Initial conditions, set the Water temperature. Below 0 °C the water starts as ice. You can also add Ice cubes and change the Altitude (how high up you are).
2. Set Add/remove heat energy. Slide right to heat the beaker with the burner. Slide left to cool it in an ice bath. You can change the heat while it runs.
3. Press Play (the triangle). Watch the Temp, Ice and Water boxes under the beaker. The words in the corner of the picture tell you what is happening, like Boiling or Freezing.
4. Choose Micro view to see the water molecules. Choose Macro view to see the water the normal way.
5. Open Graph to see the temperature over time. Look for flat parts. Use Zoom out or Zoom in to change the time axis.
6. Press Pause to freeze the clock. Press Reset to start over. You must reset before you can change the initial conditions.

Water can be a solid (ice), a liquid (water) or a gas (water vapor, or steam). Changing from one of these to another is a phase change.

Melting is solid to liquid. Freezing is liquid to solid. Both happen at 0 °C. Boiling is liquid to gas all through the water, at 100 °C at sea level. Evaporation is liquid to gas at the surface, and it can happen at any temperature. Condensation is gas to liquid.

During a phase change, the temperature stays the same even though heat keeps going in or out. That is why the graph has flat parts. The heat is pulling molecules apart, or letting them lock together, instead of changing their speed.

Higher up, there is less air pushing down on the water, so it boils at a lower temperature. Try a high altitude and watch the boiling point drop.

Each red and white shape is one water molecule. The red ball is oxygen and the white balls are hydrogen. The blue pairs above the beaker are molecules of air.

In ice, the molecules are locked in rows. They only wiggle in place. In liquid water, they slide past each other. In water vapor, they fly around far apart.

Temperature measures how fast the molecules move on average. The average molecular speed under the beaker goes up as the water gets hotter and down as it gets colder.

• Heat to warm water: 4.18 J for each gram for each °C. For ice it is 2.09 J.
• Heat to melt ice (or given off when water freezes): 334 J for each gram
• Heat to boil water into vapor: 2,260 J for each gram
• Heat energy added = heat rate × time. For example, 500 J/s × 10 s = 5,000 J
• Ice takes up more space than the same water: 100 g of water is 100 cm³, but 100 g of ice is about 109 cm³.