Solids, liquids and gases
The Three States of Matter
Matter is anything that has mass and takes up space. It exists in three main states: solid, liquid, and gas. The state depends on the arrangement, separation, and motion of its particles.
| Property | Gas | Liquid |
|---|---|---|
| Shape | Fills entire container | Fixed shape |
| Volume | No fixed volume (expands) | Fixed volume |
| Compressibility | Highly compressible | Incompressible |
Why? Solids have fixed shape/volume because particles are tightly packed. Gases have no fixed volume because particles move freely and spread out.
Kinetic Particle Theory
Kinetic Particle Theory states that all matter is made up of tiny particles that are in constant random motion. The kinetic energy of these particles determines the state of matter.
- Solid: Particles are closely packed in a regular arrangement. They vibrate about fixed positions.
- Liquid: Particles are close together but have no regular arrangement. They can slide over each other.
- Gas: Particles are far apart and move randomly at high speeds.
Changes of State
Changing state involves adding or removing energy (heat). This changes the particle motion and separation, but not the temperature during the change.
| Process | Particle Change |
|---|---|
| Melting | Solid → Liquid. Particles gain energy, vibrate more, and break fixed positions to slide. |
| Boiling / Evaporation | Liquid → Gas. Particles gain enough energy to overcome attractive forces and move far apart. |
| Freezing | Liquid → Solid. Particles lose energy, slow down, and lock into fixed positions. |
| Condensing | Gas → Liquid. Particles lose energy, slow down, and come closer together. |
Heating vs. Cooling Curves:
- Rising sections: Temperature increases as kinetic energy increases.
- Flat (horizontal) sections: State change occurs. Temperature remains constant because energy is used to break/form bonds, not increase speed.
Interpreting Heating/Cooling Curves
When to use: When asked to interpret a graph showing temperature vs. time during heating or cooling.
Why examiners accept this: Examiners look for specific identification of physical processes based on the graph's shape. A flat line indicates a phase change where potential energy changes, not kinetic energy.
Correct Usage Example:
'Between points X and Y, the temperature is constant. This indicates melting (or boiling/freezing/condensing depending on context). The energy supplied is used to overcome intermolecular forces, not to increase particle speed.'
'Between points X and Y, the temperature is constant. This indicates melting (or boiling/freezing/condensing depending on context). The energy supplied is used to overcome intermolecular forces, not to increase particle speed.'
⚠︎ Temperature During State Change
The Error: Students often write that temperature increases during melting or boiling.
The Correct Understanding: Temperature remains constant during a change of state. The energy added is used to break the bonds between particles (increasing potential energy), not to increase their kinetic energy (temperature).
Gas Laws: Pressure, Volume, and Temperature
The behavior of gases is described by two main relationships. Note that T must be in Kelvin (K) for calculations (K = ^\circ C + 273).
| Law | Formula | Relationship |
|---|---|---|
| Boyle's Law | ||
| Charles's Law |
Boyle's Law: If temperature is constant, increasing pressure decreases volume. Formula: P_1V_1 = P_2V_2.
Charles's Law: If pressure is constant, increasing temperature increases volume. Formula: \frac{V_1}{T_1} = \frac{V_2}{T_2}.
Explaining Gas Pressure Changes
When to use: When asked to explain why pressure or volume changes using kinetic particle theory.
Why examiners accept this: Examiners require the explanation to link macroscopic observations (pressure/volume) to microscopic particle behavior (collisions, speed, frequency).
Correct Usage Example (Pressure Increase at Constant Temp):
'When volume decreases, particles are forced closer together. This increases the frequency of collisions with the container walls, resulting in higher pressure.'
'When volume decreases, particles are forced closer together. This increases the frequency of collisions with the container walls, resulting in higher pressure.'
⚠︎ Confusing Pressure and Volume Effects
The Error: Students often say 'increasing temperature increases pressure' without specifying that volume must be constant, or they confuse the cause and effect.
The Correct Understanding:
- Constant Volume: Increasing temperature increases particle speed → more frequent/forceful collisions → pressure increases.
- Constant Pressure: Increasing temperature increases particle speed → particles push walls out → volume increases.
Past Paper Style Questions
Q:
Describe the arrangement and motion of particles in a liquid. [2]
A:
Arrangement: Irregular / no particular arrangement / close together.
Motion: Slide over each other / move randomly.
Motion: Slide over each other / move randomly.
Q:
Explain why the pressure of a gas increases when it is heated at constant volume. [2]
A:
- Particles move faster (higher kinetic energy).
- More frequent collisions with walls / greater force per collision.
Q:
A gas has a volume of 2.0 , m^3 at 100 , kPa. If the pressure is increased to 250 , kPa at constant temperature, calculate the new volume. [2]
A:
Using Boyle's Law: P_1V_1 = P_2V_2
100 \times 2.0 = 250 \times V_2
V_2 = \frac{200}{250} = 0.8 , m^3
100 \times 2.0 = 250 \times V_2
V_2 = \frac{200}{250} = 0.8 , m^3
Q:
State the name of the process when a gas changes to a liquid. [1]
A:
Condensation (or Condensing).