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We are surrounded by gases in our atmosphere which support and protect life on this planet. In this chapter, we are going to try to understand more about gases, and learn how to predict how they will behave under different conditions. The kinetic theory of matter was discussed in Grade 10. This theory is very important in understanding how gases behave.

A review of the kinetic theory of matter

The main assumptions of the kinetic theory of matter are as follows:

  • Matter is made up of particles (e.g. atoms or molecules)
  • These particles are constantly moving because they have kinetic energy. The space in which the particles move is the volume of the gas.
  • There are spaces between the particles
  • There are attractive forces between particles and these become stronger as the particles move closer together.
  • All particles have energy . The temperature of a substance is a measure of the average kinetic energy of the particles.
  • A change in phase may occur when the energy of the particles is changed.

The kinetic theory applies to all matter, including gases. In a gas, the particles are far apart and have a high kinetic energy. They move around freely, colliding with each other or with the sides of the container if the gas is enclosed. The pressure of a gas is a measure of the frequency of collisions of the gas particles with each other and with the sides of the container that they are in. If the gas is heated, the average kinetic energy of the gas particles will increase and if the temperature is decreased, so does their energy. If the energy of the particles decreases significantly, the gas liquifies. An ideal gas is one that obeys all the assumptions of the kinetic theory of matter. A real gas behaves like an ideal gas, except at high pressures and low temperatures. This will be discussed in more detail later in this chapter.

Ideal gas

An ideal gas or perfect gas is a hypothetical gas that obeys all the assumptions of the kinetic theory of matter. In other words, an ideal gas would have identical particles of zero volume, with no intermolecular forces between them. The atoms or molecules in an ideal gas would also undergo elastic collisions with the walls of their container.

Real gas

Real gases behave more or less like ideal gases except under certain conditions e.g. high pressures and low temperatures.

There are a number of laws that describe how gases behave. It will be easy to make sense of these laws if you understand the kinetic theory of gases that was discussed above.

Boyle's law: pressure and volume of an enclosed gas

Demonstration : boyle's law

If you have ever tried to force in the plunger of a syringe or a bicycle pump while sealing the opening with a finger, you will have seen Boyle's Law in action! This will now be demonstrated using a 10 ml syringe.


To demonstrate Boyle's law.


You will only need a syringe for this demonstration.


  1. Hold the syringe in one hand, and with the other pull the plunger out towards you so that the syringe is now full of air.
  2. Seal the opening of the syringe with your finger so that no air can escape the syringe.
  3. Slowly push the plunger in, and notice whether it becomes more or less difficult to push the plunger in.

Questions & Answers

The number of HCl molecules in 16 mol
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what is the formula of Newton's first law?
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determine the molecular formula if the molar mass of the compound is 282 g .mol
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What is friction?
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when one object rub against each other they cause friction
what is ohms
Xolile Reply
ohm is an unit to measure the electrical ressistance of a conductor(generally metalic and a few non metallic)
A device that bars the passage of an electric charge between two points.
Mike Reply
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deduce the relationship between the force and the acceleration
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a graph o force vs acceleration
a graph o force vs acceleration
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rate of change of velocity
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Alloys are mixtures of metallic elements or metallic and nonmetallic elements usually fused together or dissolving into each other when molten

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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