Particle Model of Matter (AQA)
1. States of matter
Solid
A solid has a fixed shape and volume because its particles are tightly packed together in an orderly structure.
These particles vibrate in place but do not move freely, which makes solids rigid and resistant to changes in shape.
Because of this strong molecular arrangement, solids do not easily compress and maintain their form unless a force is applied to break or deform them.
Examples of solids include ice, metal, and wood.
Liquid
A liquid has a definite volume but takes the shape of its container because its particles are not as tightly packed as in a solid.
The molecules in a liquid can move past one another, allowing the substance to flow and change shape while maintaining a constant volume.
Liquids cannot be easily compressed and will settle at the lowest point in a container due to gravity.
They also exhibit surface tension, which allows small objects to float on their surface.
Examples of liquids include water, oil, and milk.
Gas
A gas has neither a fixed shape nor a fixed volume because its particles are spread far apart and move freely in all directions.
Unlike solids and liquids, gases expand to fill any container they are placed in, and they can be compressed easily because of the large spaces between their molecules.
The movement of gas particles is random and fast, which allows gases to mix quickly with other gases.
Air, helium, and steam are examples of gases that exist in everyday life.
2. Changes of state
Melting
Melting occurs when a solid changes into a liquid due to an increase in temperature.
As heat is absorbed, the particles in the solid vibrate more rapidly until they break free from their fixed positions.
This allows them to move more fluidly, transforming the solid into a liquid.
The temperature at which this happens is called the melting point.
Example, ice melts into water at 0°C.
Freezing
Freezing is the process in which a liquid turns into a solid when its temperature decreases.
As the liquid loses heat, its particles move more slowly and come closer together, eventually arranging themselves into a rigid structure.
This occurs at the freezing point, which is the same as the melting point for a given substance.
Example: water freezes into ice at 0°C.
Evaporation
Evaporation and boiling are processes that turn a liquid into a gas.
Evaporation happens at the surface of a liquid at any temperature, but boiling occurs throughout the liquid when it reaches its boiling point.
As heat is added, the particles gain energy and move faster until they overcome the forces holding them together.
Example: Water boils at 100°C, turning into steam.
Condensation
Condensation is the opposite of evaporation and occurs when a gas cools down and turns into a liquid.
As the temperature drops, the particles lose energy and move closer together, forming a liquid.
This process is commonly seen when water vapor in the air condenses on a cold surface, such as a glass of ice water, forming droplets.
Sublimation
Sublimation happens when a solid changes directly into a gas without becoming a liquid first.
This occurs when the particles in the solid gain enough energy to break free completely.
Dry ice, which is frozen carbon dioxide, sublimates at room temperature, turning directly into carbon dioxide gas.
Deposition
Deposition is the reverse of sublimation, where a gas changes directly into a solid without becoming a liquid.
This happens when gas particles lose a large amount of energy quickly and arrange themselves into a solid structure.
Example is frost forming on a cold surface when water vapor in the air freezes directly into ice.
3. Density of materials
Definition: Density is a specific property of matter, widely used in Chemistry, which determines the amount of mass present in a given volume.
Formula:
Density = mass / volume → (unit: kg/m³)
Factors that affect density:
Pressure: Changes in pressure have a major influence on gaseous systems.
Gases are completely expandable, which means that they will always take the shape and volume of the container in which they are inserted.
An increase in pressure results in a more compressed system (the volume of the gas decreases).
A system with lower pressure results in a more expanded system (the volume of the gas increases).
Temperature: Different temperatures generate greater proximity or distance between the particles of the substance.
An increase in temperature causes the particles to have more kinetic energy, moving faster and further away from each other.
This causes an increase in volume, thus decreasing density.
The opposite happens with a decrease in temperature.
4. Temperature changes and energy
Internal energy
Definition: The internal energy is the total amount of kinetic energy and chemical potential energy of all the particles in the system.
When energy is given to raise the temperature, particles speed up and gain kinetic energy.
Energy and Temperature Difference:
Internal energy is a measure of the total energy of all the particles in the object or substance.
This includes the kinetic energy of the particles and chemical potential energy of the bonds between them.
Temperature is a measure of the average speed of the particles.
This is based on the kinetic energy of individual particles
Heating water causes the water molecules to gain kinetic energy and speed up.
It takes more energy to raise the temperature of a large amount of water because more molecules need to have their speed changed.
In the diagrams above the two beakers have been heated by the same Bunsen burner for the same amount of time, so both have been given the same amount of energy.
However, the smaller beaker has had a bigger temperature rise because the same energy has been given to a smaller number of particles so each particle is moving faster than those in the other beaker.
Specific heat capacity
Definition: The specific heat capacity of a material is the energy required to raise one kilogram (kg) of the material by one degree Celsius (°C).
Formula:
C = Q / (m × ΔT)
C = specific heat capacity
Q = energy added (heat)
m = mass
ΔT = change in temperature
Units: The units for specific heat capacity are J/kg K or J/kg C. This means that it measures the heat or energy required to change the temperature of a substance of unit mass by 1 °C or 1 °K.
Specific latent heat
Definition: The amount of energy required to change the state of 1 kilogram (kg) of a material without changing its temperature.
Formula:
L = Q/M
L: specific latent heat
Q: energy added (heat)
M: mass
Units:
The units for specific latent heat are kilojoules per kilogram (kJ/kg) or joules per gram (J/g)
Multiple changes
Definition: Specific heat capacity relates only to the energy required for a change in temperature. Specific latent heat relates only to the energy required for a change in state.
If a change in internal energy of a material will cause it to change temperature and change state, both equations can be used.
Example:
What happens when 1 kilogram (kg) of water at 60 degrees Celsius (°C) is heated with 3 megajoules (MJ) (3,000,000 J)?
When 1 kg of water at 60°C is heated with 3 MJ (3,000,000 J), some energy raises its temperature to 100°C. With a specific heat capacity of 4,180 J/kg°C, the energy required is:
Q = 1 × 4180 × 40 = 167,200 J
Next, 2,260,000 J is used to turn the water into steam (latent heat of vaporization). The remaining energy, 572,800 J, heats the steam. With a specific heat capacity of 1,859 J/kg°C:
ΔT = 572,800 ÷ 1859 = 308°C
The steam, initially at 100°C, reaches 408°C.
5. Particles in gases
Particle motion
Definition: The particles in a gas are moving very quickly in random directions. The speeds of the particles vary but, on average, they move quicker than they do in liquids and solids.
Since the particles in a gas are moving fast and randomly, collisions occur frequently. These collisions may be between two particles, between a particle and the wall of the container, or between a particle and something else in the container.
The force acting on the container due to these collisions is at right angles to the container.
Gas pressure
Definition: Gas pressure is the force that gas particles exert when they collide with the walls of their container. These collisions occur as the particles move rapidly in all directions. The pressure increases if the number of particles or the temperature of the gas rises, as more frequent and forceful collisions happen.
Formula:
P = F/A
P is the pressure,
F is the force applied,
A is the area over which the force is distributed.
Gas temperature
The temperature of a gas is a measure of the average kinetic energy of its particles - the higher the temperature, the higher the average kinetic energy.
As a result, the gas particles will be travelling faster and will collide with the walls of the container more frequently, and with more force.
Gas volume
Definition: In gases, temperature and pressure are directly related. As the temperature of a gas increases, its particles move faster, leading to more frequent and forceful collisions with the container walls. This results in an increase in pressure, as long as the volume of the gas remains constant.
Ideal gas law
PV = nRT
P is the pressure,
V is the volume,
n is the number of moles of gas,
R is the ideal gas constant (8.314 J/mol·K),
T is the temperature in Kelvin.
6. Practicals
Measuring density
There are different methods to measure density, depending on the type of material.
For a regular solid, such as a cube or cylinder, you measure its mass using a balance and calculate its volume using a ruler and geometric formulas, then apply density = mass ÷ volume.
For an irregular solid, the water displacement method is used: the object is placed in a measuring cylinder with water, and the volume change gives the object’s volume.
For liquids, the mass is found by weighing an empty container, then weighing it again with the liquid inside. The volume is measured directly with a measuring cylinder, and density is calculated the same way.
Each method ensures an accurate measurement suited to the material being tested.
Measuring specific heat capacity
To measure the specific heat capacity of a metal block, start by measuring its mass using a balance.
A heater is placed in a hole in the block, and a thermometer is inserted into another hole to monitor temperature changes.
The block is connected to a power supply, and an ammeter or voltmeter is used to measure the energy supplied.
The heater is turned on, and the temperature is recorded at regular intervals. Once the temperature increases by a set amount, the energy supplied is noted.
Using the formula (specific heat capacity = energy / (mass x temperature change)), you can calculate the specific heat capacity of the metal.
This experiment shows how much energy is needed to raise the temperature of a material.