Matter is said to exist in three different physical sates, solid, liquid and gas. The same substance may exist in all three states under varying conditions such as temperature and pressure etc.

For example water may exist as ice (solid), water (liquid) and steam (gas).

The difference between three states of the same substance can be explained in terms of the Molecular Kinetic Theory. The basic concepts of this theory are as follows

Gases

· Molecules of the gas are in continuous chaotic motion

· The molecules move at great speeds, colliding with each other and the container walls.

· The molecules are negligible in size compared with the space between them

· There is no attraction or repulsion between the molecules

· The average speed of the molecules (their kinetic energy) is directly proportional to the absolute temperature of the gas.

Liquids

· The molecules in a liquid are moving much slower than the corresponding gas molecules

· Molecules in a liquid are in chaotic motion.

· Liquid molecules, due top their reduced speed, are much closer together, thus liquids are more dense than gases.

· There is weak intermolecular forces between molecules in a liquid due to their reduced speed and denser packing, thus liquids have a fixed volume

Gases

· The molecules in a solid are moving much more slowly than the corresponding liquid molecules.

· Molecules in a solid are in continuous vibrational motion about a fixed point in the solid lattice.

· Molecules in a solid are close packed.

· There are relatively strong intermolecular forces holding the solid molecules into the solid lattice, thus solids have a fixed volume and shape

Explain phase changes in terms of the Kinetic Molecular Theory indicating motion of particles and energy changes.

Phase changes are changes in the physical state of a substance ie.

Melting Solid Liquid

Boiling Liquid Gas

Condensation Gas Liquid

Freezing Liquid Solid

Sublimation Solid Gas

Phase changes can be explained in terms of the Kinetic Molecular Theory as follows

an increase in temperature causes an increase in the kinetic energy of the molecules with in the solid lattice and the vibrations eventually overcome the intermolecular forces and the solid lattice breaks down allowing the particles more mobility

Liquid – Gas

an increase in temperature leads to an increase in the kinetic energy level of the particles in the liquid. Those particles with sufficient energy break through the surface of the liquid, overcoming surface tension) and move into the gas phase. As the temperature increases, eventually large numbers of particles have sufficient energy to form gas bubbles within the body of the liquid, and the liquid is said to be boiling. Boiling occurs when vapour pressure equals external pressure, ie. Air pressure)

Solid – Gas

by the time the particles in the solid have sufficient energy to break the intermolecular lattice bonds they also have sufficient energy to form the gaseous phase of the substance and particles break free into the gaseous state.

The Effect of Changes in Temperature and Pressure on Matter

Melting Point

the melting point of a solid will not be greatly affected by changes in pressure and providing the substance is pure, is relatively constant. Some slight effects may be experienced where the solid state is less dense than the liquid state, e.g. water / ice where an increase in pressure lowers the melting point.

Boiling Point

at normal atmospheric pressure (~ 1 atm) the boiling points of liquids are relatively constant. Variations in pressure however cause variations in the boiling points of liquids. Liquids are said to boil when the internal pressure is equal to the pressure of the surface of the liquid. So increasing the external pressure increases the boiling point and decreasing the external pressure decreases the boiling point.

It can be seen from the above graph that the vapour pressure of liquids increases with an increase in temperature. The average Kinetic Energy of all particles increases with an increase in temperature and so at higher temperatures more particles will “break free” and the vapour pressure will increase.

Solids also possess a vapour pressure as some particles can break free of the solid lattice and move into the gaseous state. Vapour pressures of solids vary greatly from solid to solid. Substances such as napthalene (moth balls), camphor and iodine have relatively high vapour pressures compared with iron and glass. The vapour of solids increases with increase in temperature. The fact that solids possess a vapour pressure is of great interest in many fields such as freeze drying of biological samples, blood treatments in blood banks and domestic deodorants etc.

Solve problems involving the relationship between temperature, pressure, volume and moles for gases.

There are four variables used to describe the behaviour of gases

Pressure (p)

Volume (V)

Absolute Temperature (T)

Number of moles (n)

Boyles Law

The volume of a given mass of gas is inversely proportional to the pressure (for a constant temperature)

p µ -- (n and T are constant)

pV = k (constant)

Charles Law

The volume of a given mass of gas is directly proportional to the absolute temperature (for a given pressure)

i.e. V µ T (n and p are constant)

(constant)

Absolute Zero of Temperature

Extrapolation of Charles Law to low temperature suggests that there is an absolute lower limit of temperature. This is supported by the Kinetic Molecular Theory, which indicates that at – 273° C, molecules would have zero kinetic energy and there for could not be cooled further. The absolute scale of temperature takes –273°C as its zero and the magnitude of a degree is the same as on the celcius scale.

So °K = °C + 273

The General Gas Equation

Combining Boyles and Charles laws yields

for a given mass of gas , ie a constant number of moles)

And this is known as the General Gas Equation

. k is constant for any given gas at a given mass

The gas equation can be rewritten

. pV = nRT p = atm V = m³ T = °K n = no. of moles

R is called the Gas Constant and has the value 8.314 JK^-1mole^-1

Molar Volume

The molar volume of a gas is the volume occupied by one mole of that gas, which at STP is 22.4dm³ or 22400 cm³. If a volume of gas V has a molar volume V_m at the same temperature then

Amount of Substance (no. of moles) =

For gases, equal numbers of gas molecules occupy equal volumes at the same temperature and pressure

Dalton's Law of Partial Pressures

In a mixture of gases the total pressure of the gases is equal to the sum of the partial pressures of each gas in the mixture, providing the gases do not react chemically

.. so P_T = P₁ + P₂ + P₃ + ……….+P_n

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