Absolute Zero and the Conquest of Cold  




Topic 3: States of Matter

Most people have heard of the three different states of matter: solid, liquid, and gas. Just about everything on earth exists in one of these states, and every material -- depending on outside effects like temperature or pressure -- can be in any of them

Take water, for example. At room temperature, water is a liquid. Heat it to boiling and it turns into steam, which is a gas. Freeze water and it becomes ice, which is a solid. In every case, it's still made of the same material; the water just changed its state when the temperature changed. Pressure is another thing that can be used to change a gas into a liquid or a liquid into a solid. Different materials will change into different states under different circumstances -- unlike water, oxygen for example is obviously a gas at room temperature -- but in the right circumstances, everything can be forced into any of the states.

States of Matter

What's really going on? The states are different because the atoms behave differently. Atoms are always moving -- even in something like a solid that looks like it's not moving. In a solid, the atoms stick very close together, but they still jiggle in their place -- as if everyone in a classroom sat at their desk but wiggled constantly in their seat. In a liquid, the atoms move more freely, they're not bound tightly in one spot, but they still stick close to each other. It might be as if everyone in the classroom stood up in a tight bunch, and then walked around each other. In a gas, on the other hand, the atoms are free to move completely on their own. There is no order whatever, they expand to fill as much space as possible, and they don't stay near each other at all. In this case, it would be as if the whole class went into the gym and then ran around in every direction. (For an image of how the atoms move when in different states, click here)

The properties of solids, liquids, and gases are different. Solids always have the same shape and volume. Liquids always have the same volume, but they can change shape – like when you pour water from a pitcher into a glass. Gases always expand to fill the surrounding area, so they can change both shape and volume. (Imagine how odd a balloon would look if you blew it up, and all the gas just sat at the bottom instead of expanding to fill the whole thing.) When matter changes from one state to another, it also changes volume. An ice cube is larger than the amount of water that was used to make it. (Though that’s an oddity. Most solids take up less space than the equivalent amount of liquid.) Heat water to make steam and the volume will increase yet again. You can see this in the kitchen when boiling water -- the steam rushing to escape the confines of a teakettle is what makes it whistle.

States of matter play a crucial role in achieving cold temperatures, since when a material changes states it uses energy -- and that can translate to losing heat. For example, if you cool oxygen while putting it under extreme pressure it will eventually turn into a liquid -- and because it has lost energy in the process, it will drop in temperature to -298 F or -183 C. For a more detailed description of how to make liquid oxygen , see here)

Links to other explanations of states of matter:
More hands-on experiments on states of matter
Additional demonstrations and activities:


More Topics

  • Topic 1: Measuring the Cold - Thermometers
  • Topic 2: Understanding Heat and Energy
  • Topic 3: States of Matter
  • Topic 4: Refrigeration
  • Topic 5: Cryogenics
  • Topic 6: The Quest for Absolute Zero
  • Topic 7: How Animals Survive the Cold
  • Topic 8: Superconductivity
  • Topic 9: Astronomy
  • Topic 10: Spaceflight
  • Topic 11: Agriculture
  • Topic 12: Cold Medicine
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