P6: Molecules and matter
6.1: Density
The density of a substance is defined as its mass per unit volume
It can be calculated using the equation: Density (kg/m cubed) = mass (kg) / volume (m cubed)
Objects with a density that is less than the density of a given liquid will float in that liquid (objects with a density less than 1000kg/m cubed will float in water)
6.2: States of matter
The 3 states of matter are solid, liquid and gas
Substances can change from 1 state to another when they're heated or cooled
Changes of state are physical changes because no new substances are produced
When a substance changes state, the number of particles in the substance stays unchanged, so the mass of the substance is the same before and after the change of state
In a solid, the particles vibrate about fixed positions so the solid has a fixed shape
In a liquid, the particles are in contact with each other but can move around at random - hence, a liquid doesn't have a fixed shape, and it can flow
In a gas, the particles are usually far apart and move about at random, much faster than particles in a liquid - so the density of gas is much less than that of a solid or liquid
The particles in a substance in its solid, liquid, or gas state have different amounts of energy depending on the state
For a given amount of a substance, its particles have the most energy in the gas state and the least energy in the solid state
6.3: Changes of state
The temperature at which a solid changes to a liquid is called the melting point - this is the same temperature at which the liquid changes to a solid, when it is called the freezing point
The temperature at which a liquid changes to a gas is called the boiling point - this is the same temperature at which a gas changes to a liquid (condenses)
Boiling takes place throughout a liquid at its boiling point - evaporation takes place from the surface of a liquid when its temperature is below its boiling point
Impurities change the melting and boiling points of a substance
For a substance to change state, energy must be transferred to or from the substance - this energy is called latent heat
Water freezes at 0 degrees C and boils at 100 degrees C
6.4: Internal energy
The internal energy of a substance is the total energy in the kinetic and potential energy stores of the particles in the substance
The particles in a substance have kinetic energy due to their individual motions relative to each other
The particles in a substance have potential energy due to their individual positions relative to each other
When a substance is heated the energy of its particles increases, so its internal energy increases
If the total kinetic energy of the particles increases, the temperature of the substance increases
If the substance changes state, the potential energy of its particles increases
For a solid
There are strong forces of attraction between the particles and each particle vibrates about a fixed position
When a solid is heated, the particles' energy stores increase and they vibrate more
For a liquid
There are weaker forces of attraction between the particles - the forces stop the particles moving completely away from each other, but they're not strong enough to hold the particles in a fixed structure
When a liquid is heated, some particles gain enough energy to break away from the other particles and are in a gas state
For a gas
The forces of attraction between the particles are negligible, so the particles of a gas are completely separate from each other
When a gas is heated, its particles gain kinetic energy and move faster
The particles in a gas collide with each other and with the walls of their container - during these collisions the particles exert a force, and hence a pressure, on the walls of the container
6.5: Specific latent heat
The latent heat of fusion is the energy required by a substance to change from a solid to a liquid, with no change in temperature
If the substance is cooled, and changes from a liquid to a solid, the latent heat is transferred to the surroundings
The specific latent heat of fusion is the energy needed to change 1 kg of the substance from a solid to a liquid, with no change in temperature
It's calculated using the equation: Specific latent heat of fusion (J/kg) = Energy needed (J) / mass (kg)
Latent heat of vaporisation is the energy required by a substance to change from a liquid to a vapour, with no change in temperature - if the substance is cooled, and changes from a vapour to a liquid, the latent heat is transferred to the surroundings
The specific latent heat of vaporisation is the energy needed to change 1 kg of the substance from a liquid to a vapour, with no change in temperature
The specific latent heat of vaporisation can be calculated using the same equation for specific latent heat of fusion
Simple experiments can be used to determine the specific latent heat of fusion and the specific latent heat of vaporisation of water - a low voltage heater is used to melt ice or boil water - the energy supplied can be measured using a joulemeter
Remember that 'specific' in this context indicates that the energy transferred 'per kg'
6.6: Gas pressure and temperature
The particles in a gas move at high speeds in random directions
The particles collide with each other and with the walls of their container, rebounding after each collision
Each collision exerts a tiny force on the surface of the container and there are millions of collisions every second
Pressure is force per unit area, so the total force from all the collisions produces a steady pressure on the walls of the container
If the temperature of the gas in a sealed container increases
The particles in the gas move faster and collide with the container walls with more force, causing the pressure to increase
There are more collisions each second with the walls of the container, also causing the pressure to increase
You can see how particles in a gas move about randomly by using a microscope to observe smoke particles in a smoke cell - the air particles collide with the smoke particles and cause them to move along unpredictable paths - this random motion is called Brownian motion
6.7: Gas pressure and volume
For a fixed mass of gas
The number of gas molecules is constant
If the temperature of the gas is constant, then the average speed of the gas molecules is constant
If the volume of a fixed gas is reduced
The molecules have less space to move in so they don't travel so far between collisions
So, the number of collisions per second increases
And so the total force per unit area (that is, the pressure) of the gas increases
The relationship between pressure and volume is: Pressure (Pa) x volume (m cubed)