Describe the relationship between the motion of particles and temperature, including the idea that there is a lowest possible temperature (-273°C), known as absolute zero, where the particles have least kinetic energy
2.1.2 Particle Model
Objective
Describe how the motion of particles determines temperature, and explain why a lowest possible temperature – absolute zero (‑273 °C or 0 K) – exists, at which particles have the minimum kinetic energy.
1. Particle arrangement in the three states of matter
| State | Particle spacing | Typical motion | Diagram |
|---|---|---|---|
| Solid | Particles are tightly packed; only a very small space between neighbours. Reason: strong intermolecular forces hold them close. | Vibrate about fixed positions; no translational movement. |
|
| Liquid | Particles are close together but not as tightly as in a solid; intermolecular forces are weaker. | Vibrate and slide past one another; the shape can change. |
|
| Gas | Particles are far apart; large empty spaces between them because intermolecular forces are negligible. | Move freely in all directions; frequent collisions with each other and the container walls. |
|
2. Motion of particles and temperature
- All matter consists of particles that are always in motion.
- The average kinetic energy of these particles determines the temperature of the substance.
- When temperature rises, particles move faster (greater kinetic energy); when temperature falls, they move slower.
Key point: Higher T → larger ⟨Ek⟩ → faster particle motion.
(For an ideal gas, ⟨Ek⟩ = 3⁄2 kB T.)
2.1 Qualitative link to temperature
For any state of matter, an increase in temperature means an increase in the average speed of the particles. This explains why:
- Heated solids expand (greater vibration → larger average separation).
- Liquids become less viscous (particles slide more easily).
- Gases increase in pressure if the volume is kept constant (more energetic collisions).
2.2 Quantitative insight (optional)
Average translational kinetic energy (ideal gas)
\$\langle E{\text{k}} \rangle = \frac{3}{2}\,k{\mathrm{B}}\,T\$
where kB = 1.38 × 10⁻²³ J K⁻¹ and T is the absolute temperature (K). This formula is useful for deeper study but is not required for the IGCSE exam.
3. Temperature, pressure and particle collisions (gases)
- Pressure is the result of particles colliding with the walls of their container.
- Each collision exerts a tiny force; the sum of all forces per unit area gives the pressure
.
- From the kinetic picture we obtain the familiar gas‑law relationship:
\$pV = nRT\$
where V is volume, n the amount of gas (mol), R the gas constant and T the absolute temperature. Thus, at constant volume, raising the temperature increases the average speed of the particles, leading to more frequent and more energetic collisions and therefore a higher pressure.
4. Brownian motion – evidence for the particle model
Microscopic particles suspended in a fluid jiggle randomly because they are constantly bombarded by invisible molecules. This observable motion is called Brownian motion. Examples:
- Pollen grains in water (classic laboratory demonstration).
- Dust particles floating in still air or the swirling of milk particles in a cup of coffee.
Both illustrate that particles of matter are never at rest.
5. Temperature scales
| Scale | Symbol | Conversion to Kelvin | Freezing point of water | Boiling point of water |
|---|---|---|---|---|
| Celsius | °C | K = °C + 273.15 | 0 °C | 100 °C |
| Kelvin | K | Reference scale | 273.15 K | 373.15 K |
| Fahrenheit | °F | K = (5/9)(°F – 32) + 273.15 | 32 °F | 212 °F |
6. What happens at absolute zero?
- Particle motion slows to the minimum allowed by quantum mechanics (zero‑point energy remains).
- Translational kinetic energy approaches zero, so no macroscopic thermal motion remains.
- No heat can be extracted from a system at 0 K; it is the theoretical lower limit of temperature.
- In practice absolute zero cannot be reached, but laboratory techniques have produced temperatures within a few nanokelvin of 0 K.
7. Everyday implications of particle motion
- Thermal expansion: Heating a solid makes its particles vibrate more strongly, increasing the average separation and causing the solid to expand.
- Gas‑pressure change: Cooling a sealed gas reduces particle speed, lowering the pressure (Gay‑Lussac’s law).
- Phase changes: Adding kinetic energy allows particles to overcome intermolecular forces, turning a solid into a liquid (melting) or a liquid into a gas (evaporation).
8. Quick revision checklist
- Define absolute zero and give its value in both Celsius and Kelvin.
- State the qualitative relationship between temperature and particle speed.
- Explain how particle collisions produce gas pressure and link this to the gas law
- Sketch simple particle diagrams for a solid, liquid and gas, indicating spacing and type of motion (use the captions as a guide).
- Give one real‑world example of Brownian motion (e.g., pollen in water, dust in air, or milk in coffee).
- (Optional) Write the equation ⟨Ek⟩ = 3⁄2 kB T and explain what each symbol represents.