Particles and Temperature

Introduction

We use thermometers every day, but what is a thermometer really measuring? The particle theory gives a clear answer: temperature tells us how fast, on average, the particles of a substance are moving. This link between temperature and particle motion explains expansion, changes of state, faster reactions in warm conditions and why heat always flows from hot to cold.

Core explanation

Temperature and kinetic energy. Particles have kinetic energy because they move. In any sample, some particles move faster than others, but the average kinetic energy is what temperature measures. The higher the temperature, the faster the particles move on average.

Heat versus temperature. Heat (thermal energy transferred) and temperature are different: - Temperature is an average per particle and does not depend on the amount of substance (intensive). - The total thermal energy of an object depends on how many particles it has (extensive).

A swimming pool at 25 °C contains far more thermal energy than a cup of tea at 80 °C, because it contains vastly more particles, even though each particle in the tea is moving faster on average.

Heat flows from hot to cold. When a hot object touches a cold one, the faster particles of the hot object collide with the slower particles of the cold object and pass on energy. The hot object's particles slow down and the cold object's particles speed up until the average kinetic energies are equal — both objects are then at the same temperature, in thermal equilibrium .

The kelvin scale. Scientists often use the kelvin scale, which starts at absolute zero, the lowest possible temperature (−273 °C). One kelvin is the same size as one degree Celsius, so:

temperature in K = temperature in °C + 273

Water freezes at 273 K and boils at 373 K; room temperature is about 298 K. Doubling a kelvin temperature doubles the average kinetic energy of gas particles — a simple relationship that does not hold for Celsius.

Effects of raising temperature. Faster particles cause expansion (page 47), faster diffusion (page 69), higher gas pressure (page 71), faster dissolving (page 68) and — in later units — faster chemical reactions, because particles collide more often and with more energy.

Step-by-step reasoning

To explain what happens when a hot metal spoon is placed in cold water:

1. The spoon's particles vibrate with more kinetic energy than the water particles. 2. Where they meet, collisions transfer energy from the spoon's particles to the water's particles. 3. The spoon's particles slow down; the spoon cools. 4. The water particles speed up; the water warms. 5. Transfer continues until both are at the same temperature.

Visual explanation

In the particle simulation, a temperature slider changes the speed of the particles; a live histogram shows the spread of particle speeds, with the peak shifting to higher speeds and the spread widening as temperature rises. A second panel shows two blocks touching, with arrows showing energy flowing from the "fast" block to the "slow" block until their colours match.

Real-world analogy

Temperature is like the average speed of cars on a motorway, while total thermal energy is like the total fuel being burned by all the cars. A quiet motorway with a few fast cars can have a high average speed but low total fuel use; a jammed motorway full of slow cars has a low average speed but burns much more fuel in total.

Real-world example

Food is refrigerated because bacteria and the chemical reactions that spoil food slow down at lower temperatures, when particles move more slowly and collide less often and less energetically. Keeping food at about 4 °C greatly extends its shelf life compared with room temperature.

Why?

Why can nothing be colder than absolute zero? Temperature measures particle motion. At absolute zero, particle motion has reached its lowest possible level; there is no slower state to reach. Scientists have cooled samples to within billionths of a kelvin of absolute zero, but it can never be reached exactly.

Common misconception

"Cold flows into things" (for example, "close the door to keep the cold out"). Physically, it is heat that flows, always from hotter to colder. Closing the door keeps heat in . Cold is simply the absence of thermal energy.

Worked example

Question: Convert (a) 25 °C to kelvin and (b) 77 K (the boiling point of nitrogen) to degrees Celsius.

Reasoning: (a) K = °C + 273 = 25 + 273. (b) °C = K − 273 = 77 − 273.

Answer: (a) 298 K; (b) −196 °C.

Quick check

1. What does temperature measure in terms of particles? Answer: The average kinetic energy (average speed) of the particles.

Exam focus

Distinguish clearly between heat (energy transferred) and temperature (average kinetic energy). Learn the kelvin conversion and use kelvin in any gas calculation. Explain heat transfer between objects using collisions between faster and slower particles.

Advanced insight

At any temperature, particle speeds follow the Maxwell–Boltzmann distribution: most particles have middling speeds, a few are very slow and a few are very fast. Raising the temperature shifts the whole distribution to higher speeds and greatly increases the fraction of very fast particles — the key to understanding why reactions speed up so dramatically when warmed.

Summary

Temperature measures the average kinetic energy of particles; heat is energy transferred because of a temperature difference. Total thermal energy depends on the amount of substance, but temperature does not. Heat flows from hot to cold through particle collisions until thermal equilibrium is reached. Kelvin = °C + 273, with absolute zero at 0 K.

Practice questions

1. Convert 100 °C into kelvin. Answer: 373 K. 2. Which contains more thermal energy: a bath of water at 40 °C or a kettle of water at 100 °C? Explain. Answer: The bath, because it contains far more water particles, even though their average kinetic energy is lower. 3. Explain why a cold drink warms up when left on a table. Answer: Faster-moving air and table particles collide with the drink's slower particles and transfer energy until the temperatures are equal. 4. Why are foods kept in a refrigerator? Answer: Lower temperatures slow particle motion, slowing the growth of bacteria and the reactions that spoil food.