Particles Are Always Moving

Introduction

Even a rock sitting perfectly still on a table is full of motion: its particles are vibrating billions of times per second. Particle motion never stops, although its type and speed depend on the state of matter and the temperature. Understanding how particles move explains why solids keep their shape, liquids flow and gases exert pressure.

Core explanation

Solids — vibrating. In a solid, particles are held in fixed positions by strong attractions to their neighbours. They cannot move from place to place, but they vibrate back and forth about their fixed positions. Heating makes the vibrations larger and faster.

Liquids — moving around each other. In a liquid, particles are still close together, but the attractions are not strong enough to hold them in fixed positions. Particles move around and past each other , constantly changing neighbours. This is why liquids flow and take the shape of their container.

Gases — moving freely and fast. In a gas, particles are far apart and the attractions between them are very weak. They move quickly in straight lines in all directions until they collide with each other or with the walls of the container. Air molecules at room temperature move at an average speed of roughly 500 metres per second — faster than a passenger jet.

Kinetic energy and temperature. The energy of movement is kinetic energy . Temperature is a measure of the average kinetic energy of the particles. Heating a substance increases its particles' average kinetic energy; cooling decreases it. At absolute zero (−273 °C, or 0 kelvin) particle motion is at its lowest possible level. It is impossible to cool anything below this temperature.

Gas pressure. Every time a gas particle hits the wall of its container, it pushes on the wall. Countless collisions every second add up to a steady force spread over the wall's area — gas pressure . Pressure increases if there are more particles in the same volume (more collisions) or if the particles move faster because the gas is heated (harder and more frequent collisions). This is why car tyre pressure rises after a long drive.

Particles do not slow down on their own. In a sealed, insulated container, gas particles keep moving indefinitely. Their collisions are like perfectly bouncy balls: energy is passed between particles, but no energy is lost overall.

Step-by-step reasoning

To explain why a sealed can of gas can burst if heated:

1. Heating increases the kinetic energy of the gas particles. 2. The particles move faster. 3. They hit the walls of the can more often and with greater force. 4. The pressure inside rises. 5. If the pressure exceeds what the can can withstand, it bursts — which is why aerosol cans carry warnings not to heat them.

Visual explanation

In the particle simulation, a slider changes the state. In the solid, particles jiggle in a grid; in the liquid, they slide past one another in a clustered mass at the bottom; in the gas, they zoom across the box and bounce off the walls. A pressure gauge on the gas box rises as the temperature slider is increased.

Real-world analogy

Particles in a solid are like people in a packed audience who can only fidget in their seats. In a liquid they are like people moving through a busy corridor, squeezing past each other. In a gas they are like players running freely around an empty sports field, occasionally bumping into each other or the fence.

Real-world example

Weather balloons are only partly filled with helium at launch. As they rise, the air pressure outside decreases, so the gas inside — whose particles are still pushing outwards — makes the balloon expand to many times its launch size. Eventually the rubber bursts at high altitude, and the instruments return by parachute.

Why?

Why does a gas exert pressure equally in all directions? Gas particles move randomly in every direction, so on average just as many hit each part of the container wall with the same force. The pressure is therefore the same on the top, bottom and sides of a container of gas (ignoring the tiny effect of gravity on the gas itself).

Common misconception

"Particles in a solid do not move." They do not move from place to place, but they are always vibrating. If they were completely still, the solid would be at absolute zero, which cannot actually be reached.

Worked example

Question: A syringe of air is sealed and the plunger is pushed in to half the volume, keeping the temperature constant. What happens to the pressure? Explain using particles.

Reasoning: The same number of particles is squeezed into half the space. They hit the walls twice as often, with the same speed.

Answer: The pressure roughly doubles, because the particles collide with the walls twice as frequently.

Quick check

1. Describe the motion of particles in a liquid. Answer: They move around and past each other, staying close together.

Exam focus

Use the right verbs: solids vibrate about fixed positions, liquids move around each other , gases move quickly and randomly in all directions . Explain pressure as particles colliding with the container walls, and explain changes in pressure using the frequency and force of collisions.

Advanced insight

The Kelvin temperature scale starts at absolute zero: 0 K = −273.15 °C, so a temperature in kelvin = temperature in °C + 273. The average kinetic energy of gas particles is directly proportional to the kelvin temperature, which is why gas-law calculations always use kelvin.

Summary

Particles are always moving. In solids they vibrate about fixed positions; in liquids they move around each other; in gases they move quickly and randomly. Temperature measures their average kinetic energy, with absolute zero (−273 °C) as the lowest possible temperature. Gas pressure is caused by particles colliding with container walls and increases when there are more particles or when they move faster.

Practice questions

1. Explain why tyre pressure increases after a long, fast drive. Answer: The tyres and air warm up, so the air particles move faster and hit the tyre walls more often and harder, increasing the pressure. 2. What is the temperature of absolute zero in °C? Answer: −273 °C (more precisely −273.15 °C). 3. Why can a liquid flow but a solid cannot? Answer: In a liquid the particles can move past each other; in a solid they are held in fixed positions and can only vibrate. 4. What happens to the average speed of particles when a substance is cooled? Answer: It decreases, because their average kinetic energy decreases.