P.6 - Molecules and Matter

Density

state of matter	arrangement	particle movement	density	intermolecular forces	compressed?	flow?
Solid	regular structure/ lattice	vibrate about fixed positions	high	strong	no	no - keeps shape
Liquid	irregular structure particles are close	free flowing	medium	weak	no	yes - fills container’s shape
Gas	random arrangement far apart	range of speeds in random directions	low	very weak	yes	yes - fills whole container

Impurities can change the melting and boiling point of substances e.g. melting point of water can be reduced by adding salts
Evaporation is different to boiling as it happens below the substance’s boiling point

Internal energy is the total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system
The kinetic energy of all the particles is due to their individual motion relative to each other
The potential energy of the particles is due to their individual position relative to each other
change in thermal energy = mass x specific heat capacity x temperature change
ΔE = mcΔθ

Energy is required to melt or boil a substance. The temperature does not rise during these changes of state
Different substances need different amounts of energy, and the energy required can be calculated using the substances specific latent heat
The amount of energy required depends on:
o The mass of the substances
o It’s specific latent heat (s.l.h)
Specific latent heat is the amount of energy that must be supplied to melt or boil 1kg of a substance without changing temperature
Specific latent heat of fusion - the amount of energy required to change the state of 1kg of solid to liquid
Specific latent heat of vaporisation - the amount of energy required to change the state of 1kg of liquid to gas
change in energy = mass of substance x s.l.h
ΔE = mL
measured in J / kg

The pressure in fluids causes a force to be exerted normal (at right angles) to any surface
Gas pressure arises as a result of the following:
o The particles in the gas move randomly and collide with the inside surface of the container
o Each of these collisions exert a force on the surface of the container
o This results in gas pressure which is the total force exerted per unit area of the surface
Work is the transfer of energy by a force
Doing work on a gas increases the internal energy of the gas and can cause an increase in temperature of the gas
When higher temperature:
o More frequent collisions
o Higher rate of collisions
o More collisions per second

Pressure is inversely proportional to volume because when the volume is decreased, there will be more frequent collisions between the particles and the walls of the container as they don’t have to travel as far between collisions. Therefore, in a given time, the particles will exert a greater total force, and hence the pressure will be greater
This is only relevant at a constant temperature
If a gas is compressed quickly, the work done will increase the internal energy of the gas
If the gas is compressed slowly, the gas loses energy to the surroundings at the same rate that it gains it
The temperature of a gas can increase if it is compressed rapidly. This is because work is done on it, and the energy is not transferred quickly enough to the surroundings
pressure x volume = constant
pV = constant
pressure is measured in pascals (Pa)