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

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A hiker pauses to view the impressive peak of Mount Everest, the tallest mountain in the world. At the top of Mount Everest, the air is very thin. Climbers may need oxygen tanks to get enough oxygen to breathe, even though oxygen is the second most plentiful gas in the atmosphere.

What Is a Gas?

A gas is one of four well-known states of matter. (The other three are solid, liquid, and plasma). The particles of a gas can pull apart from each other and spread out. As a result, a gas does not have a fixed shape or a fixed volume. In fact, a gas always spreads out to take up whatever space is available to it. If a gas is enclosed in a container, it spreads out until it has the same volume as the container.

Q: The sketches in the Figure below represent two identical sealed boxes that contain only air particles (represented by dots). There are more air particles in box B than box A. Which box contains a greater volume of air?

A: This is a trick question! The air inside each box expands to fill the available space, which is identical for both boxes. There are more air particles in box B, but the volume of air is exactly the same in both boxes.

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

Particles of gas are constantly moving in all directions at random. As a result, they are always bumping into each other and other things. This is modeled in the Figure below. The force of the particles against things they bump into creates pressure. Pressure is defined in physics as the amount of force pushing against a given area. How much pressure a gas exerts depends on the number of gas particles in a given space and how fast they are moving. The more gas particles there are and the faster they are moving, the greater the pressure they create. To learn more about gas pressure and how to measure it, go to this URL: http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH5004.

The arrows show that particles of a gas move randomly in all directions.

Q: Look at box A and box B in the previous question. Is air pressure the same in both boxes? Why or why not?

A: Air pressure is greater in box B. That’s because there are more air particles in box B to bump into each other and into the sides of the container. Therefore, the particles in box B exert more force on a given area.

Pressure in the Atmosphere

We live in a “sea” of air called the atmosphere. Can you feel the air in the atmosphere pressing against you? Not usually, but air actually exerts a lot of pressure because there’s so much of it. The atmosphere rises high above Earth’s surface, so it contains a huge number of gas particles. Most of them are concentrated close to Earth’s surface because of gravity and the weight of all the air in the atmosphere above them. As a result, air pressure is greatest at sea level and drops rapidly as you go higher in altitude. The Figure below shows how air pressure falls from sea level to the top of the atmosphere. In the graph, air pressure is measured in a unit called the millibar (mb). The SI unit of pressure is Newtons per square centimeter (N/cm2).

Q: The top of Mount Everest is almost 9 km above sea level. What is the pressure of the atmosphere at this altitude?

A: Air pressure at the top of Mount Everest is about 260 mb. This is only about 25 percent of air pressure at sea level, which is 1013.2 mb. No wonder it’s hard for climbers to breathe when they get close to Mount Everest’s summit!

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.


  • Gas is a state of matter in which particles of matter can pull apart from each other and spread out. As a result, a gas does not have a fixed shape or a fixed volume.
  • Gas particles are constantly moving and bumping into things, and this creates force. The amount of force pushing against a given area is called pressure.
  • The pressure of gases in the atmosphere is greatest at sea level and decreases rapidly as altitude increases.


  • gas: State of matter that has neither a fixed volume nor a fixed shape.
  • pressure: Result of force acting on a given area.


Examine this sequence of photos in the Figure below, and then answer the questions below.

  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?
  2. Why does a gas not have a fixed shape or a fixed volume?
  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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