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Introduction to one of the four states of matter that has no fixed volume or shape.

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What Is a Gas?




With enough heat, a liquid can transform into gas.

As a liquid is heated, the molecules in the liquid gain kinetic energy and move more quickly. The more heat that is added, the faster the molecules move. Eventually, the molecules will have enough energy to overcome the intermolecular forces holding them together. At this point, the liquid will boil, turning into a gas.

Gases have enough kinetic energy to move randomly.

If water in a pot boils, what happens to the water vapor? Does it move to another container? Does it rest on the kitchen counter in the shape of a pot?

Water vapor is a gas that is made up of freely moving atoms or molecules. Compared to solids and liquids, gases tend to have the most kinetic energy and the weakest intermolecular forces. Gases have no fixed shape or volume. They can expand and contract to fit any container. The molecules in a gas tend to be very spread out. The intermolecular forces that affect solids and liquids do not affect gases.

Pressure of Gases





Pressure in the Atmosphere




Gas Molecules spread out when heated.

What makes a hot air ballon float through the air? And why must the air be hot for the balloon to stay afloat?

When the air inside a balloon is heated, the gas molecules spread out. This spreading out of gas molecules makes the gas molecules in the balloon less dense than the air molecules on the outside of the balloon, which allows the balloon to float away. Gases are strongly affected by changes in temperature and pressure, which is why a little bit of heat makes the same amount of gas molecules take up much more space.




  • pressure: Result of force acting on a given area.



  1. How does a balloon change as a child blows more air into it?
  2. How does air cause this change in the balloon?


  1. What is a gas?
  3. Explain why a gas exerts force
  4. What does pressure measure?
  5. Air pressure affects how high a cake rises when it bakes. Directions for cake mixes often have special high altitude instructions, like those on the label below. Explain why.

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