The kinetic particle model
Everything is made of particles
- Look closely enough and all matter is tiny particles that never stop moving.
- This one idea — the kinetic particle model — explains solids, liquids, gases, heat and pressure.
- Let's see what the particles are doing.
The three states of matter
- Solid — particles packed in a regular pattern, only vibrating. Fixed shape and volume.
- Liquid — particles still close but jumbled; they slide past each other. Fixed volume, takes the container's shape.
- Gas — particles far apart, fast and random. Fills the whole container.
In which state are the particles far apart, fast, and moving in random directions?
Gas particles are far apart and move quickly in random directions, filling the container. Solids only vibrate; liquids slide past each other.
Temperature and particle energy
- Heat a substance and its particles move faster — they gain kinetic energy.
- Temperature is a measure of the average kinetic energy of the particles.
- The lowest possible temperature is absolute zero, $-273\ {}^{\circ}\text{C}$ — particles have the least energy.
Temperature is a measure of:
Hotter means the particles move faster, so temperature measures their average kinetic energy.
The kelvin scale
- Scientists often measure temperature in kelvin (K), starting from absolute zero.
- To convert:
- So $0\ {}^{\circ}\text{C} = 273\ \text{K}$ and $27\ {}^{\circ}\text{C} = 300\ \text{K}$.
Convert $27\ {}^{\circ}\text{C}$ to kelvin.
$T = \theta + 273 = 27 + 273 = 300\ \text{K}$.
Gas pressure
- Gas particles constantly hit the walls of their container. Each hit is a tiny push.
- The pressure is the total force of these hits on each unit of area.
- Heat the gas (fixed volume) → faster, harder, more frequent hits → higher pressure.
- Squeeze it smaller (fixed temperature) → more hits per second on each area → higher pressure.
- For a fixed mass of gas at constant temperature, $pV = \text{constant}$ (halve the volume → double the pressure).
A gas has a volume of $200\ \text{cm}^3$ at $100\ \text{kPa}$. It is squeezed to $100\ \text{cm}^3$ at constant temperature. What is the new pressure, in kPa?
$pV$ is constant: $100 \times 200 = p \times 100$, so $p = 200\ \text{kPa}$. Halving the volume doubles the pressure.
Heating a gas in a sealed, fixed-volume can increases its pressure.
Hotter particles move faster and hit the walls harder and more often, so the pressure rises.
Brownian motion
- Under a microscope, smoke specks in air jiggle along jerky, random paths.
- This is Brownian motion: the big specks are knocked about by fast, invisible air particles.
- It is strong evidence that matter really is made of moving particles.
Smoke specks seen under a microscope jiggle randomly. This is because:
Brownian motion: tiny fast air particles knock the larger smoke specks about, evidence for moving particles.
You've got it
- solid (fixed, vibrating) · liquid (close, sliding) · gas (far apart, fast)
- temperature = average kinetic energy of particles; absolute zero $= -273\ {}^{\circ}\text{C}$
- $T\,(\text{K}) = \theta\,({}^{\circ}\text{C}) + 273$
- gas pressure = particle hits per unit area; at constant temperature $pV = \text{constant}$