2.1 Kinetic Theory

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2.1.1 States of Matter

A molecule is one or more atoms , joined together by chemical bonds, capable of existing as a unit. We think of matter as existing in three states, solid liquid and gas. The molecular structure and physical properties of each are as follows :

Solid:

Particles are arranged in a regular repeating pattern

Particles are close together with strong bonds ( attractive forces )

Vibrating in fixed positions

Cannot flow

Difficult to separate and as a result solids are hard to break

Particles are already tightly packed and cannot be pushed any closer - solids are incompressible

Liquid:

Particles are arranged randomly

Their particles are close together and vibrating with weak bonds between them

Particles can move past one another and as a result liquids can be poured and can flow

Takes the shape of the bottom of its container

Particles are close with weak bonds

as particles are already close together they cannot be pushed much closer and are also incompressible

Gas:

Particles are arranged randomly

Particles are very far apart with negligible forces between them and are moving randomly in all directions

Easy for particles to move past one another and so gas can flow

Fills entire container

Particles can be pushed closer together easily and so are compressible

Substances that can flow are called fluids, gasses and liquids are therefore fluids.

As temperature of a gas increases, as does the average kinetic energy of its particles, and therefor the average speed of the particles increases.

Pressure due to a gas

If the gas is placed in a sealed container, when the particles hit the sides of the container they rebound. They , for example arrive with a momentum of mv and leaves with a momentum of -mv (direction change). There is therefore a momentum change of 2mv.

The gas molecules exert a force on the walls of the container, the pressure exerted on the side is = to this force/the area of the side.

If the temperature increases, the average speed of the particles and therefore the collisions per second increases. Hence the force exerted on the container is greater and the pressure increases.

Brownian motion

The kinetic theory of matter states that all matter is made of small particles that are in random motion and that have space between them. This means that no matter what phase matter is in, it is made of separate, moving particles.

If smoke is introduced into a glass cell and observed through a microscope, the smoke particles can be seen clearly, they appear to be in random motion. This is because they are being constantly hit by other rapidly moving particles in the air in the glass cell. this is evidence that the particles in air are in constant random motion. it shows that massive particles may be moved by light fast moving particles.

Brownian motion - the motion of small particles suspended in a liquid or gas, caused by molecular bombardment

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2.1.3 Evaporation

Evaporation

Evaporation is the escape of more-energetic molecules from the surface of a liquid. Basically the liquid particles with more kinetic energy may be moving fast enough to overcome the force of attraction between them and the other particles. The fastest moving particles escape from the surface of the liquid, forming a gas. The rate of evaporation can be increased by

Increasing the temperature (put liquid in warmer room)

increasing the surface area (put liquid in bigger container)

increasing the air flow (or draught) over the surface of the liquid (blow on surface)

Objects in contact with an evaporating liquid cool as the liquid evaporates, this is because the particles that are escaping from the liquid have the most energy, and means that those that remain have less energy and the liquid is therefore cooler.

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2.1.4 Pressure Changes

Boyle's Law

When the pressure of the gas is doubled, the volume of the gas halves.Volume is said to be inversely proportional to the pressure applied.

Boyle's Law = For a fixed mass of gas at a constant temperature, the volume is inversely proportional to the applied pressure. pV = constant