Core Physics Topic 2 - Kinetic Theory

Kinetic theory is about how molecules move about and change state from solid to liquid to gas and back.  Kinetic comes from the Greek word "kinein" which means "to move".

Water, H20, is one of the most common substances in the universe.  On planets that are close to their stars, it exists as steam, a gas.  On distant planets it is a solid, ice

NASA, Wikimedia Commons

For example, Saturn's Moon, Titan, is thought to consist of water ice, with a dense atmosphere of nitrogen, with rivers and lakes of liquid methane (CH4).  At the temperatures found, the water ice is as hard as rock.  Volcanic activity occurs, with liquid water as the lava.  The liquid water will rapidly freeze, to form a hard rock.  The picture shows the surface of the moon.

Andrey Pivovarov, Wikimedia Commons

On Titan the temperature is -170 oC.  Although it is cold, there is weather.  It rains methane.

Fortunately for us on Earth, water is in a state that allows life to exist.  It can exist as a solid, a liquid, or a gas.  These states are called phases.  Liquid water is essential for life.

The three states of matter can be summed up in these diagrams:

A solid has long range order.  The molecules cannot move about.  However the bonds in a molecule can vibrate.  They vibrate more if the temperature is higher.  If the molecules vibrate enough, the bonds between molecules break, and the long-range order breaks down.  We say that the substance is melting.

The material changes phase and becomes a liquid.  It melts.

Small groups of molecules can move past each other.  A liquid adopts the shape of its container.  If there is no container, it will spread out into a thin layer.  In the liquid phase, some molecules can escape.  The liquid evaporates.  In very volatile liquids, lots of molecules escape.  The liquid evaporates quickly.  If you spill some ethanol on the bench, it disappears quickly.

If we continue to put energy into the liquid, the molecules move about faster.  Also intermolecular bonds break more, and single molecules are given off as a gas.  All liquids have a boiling point at which the liquid becomes a gas.

The molecules are now on their own.  The space between molecules is large, and the molecules are free to move about.  They collide against the walls of the container.  The force from these collisions is called the pressure.  Since there is a lot of space between molecules, the gas can be squashed together, or compressed.  If we compress the gas enough, we can actually make it liquid again.  Under extreme conditions, hydrogen, H2, can exist as a solid metal.

Some materials, for example carbon dioxide, can change state from solid to gas without passing a liquid phase.  This is called sublimation.

If we measure the energy put in as a liquid changes state, we see a graph like this:

Notice that the temperature does not change at all while the substance is changing state.  The energy used up in changing state is called latent heat, and it's the heat required to turn 1 kg substance from solid to liquid, or liquid to gas.  You are not expected to know anything more than that; you will study it in detail at A2 level.

When a gas turns back into a liquid, it condenses.  When a liquid turns to a solid, it freezes.  Both changes of state give out energy, as does a gas, or liquid as it cools.  Gaseous water (steam) can hold a lot of energy, which makes it useful for a steam engine.

Water freezes at 0 oC.  At that temperature, it can exist in all three states, solid, liquid, or gas.  Hence 0 oC is called the triple point of water.

The boiling point of water depends not just on the temperature, but also on the pressure.  At normal atmospheric pressure (1 atmosphere) it boils at 100 oC.

 In the steam engine above, would you expect the boiling point of water to be higher or lower than 100 oC?  Explain your answer.

When you go up a mountain or in an aeroplane, the atmospheric pressure falls.  That is why your ears "pop" as you climb.

 A pilot boils water to make coffee for drinking during his flight.  He puts it in a flask and screws the lid tight.  When he is at a height of 3000 m, he decides to have a cup of coffee.  He has difficulty unscrewing the lid of the flask, and when he opens it, coffee sprays over him.  (The flask remains upright.)  Badly hurt, he manages to land his aeroplane at the nearest aerodrome.   Explain, in terms of what you have learned in this topic, how the accident happened.

The pressure in a gas is the result of movement of gas molecules.  Gas molecules move about randomly.  They collide with each other, and the walls of the container.  They move at a range of speeds, about 300 to 500 m/s.

The movement of molecules in a substance results in the material having an internal energy. The higher the internal energy, the higher the temperature.