Heating Particles
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Temperature measures how fast a substance’s particles move on average.
- During melting and boiling, added energy changes the state, not the temperature.
- Evaporation removes the fastest particles, so it cools what is left.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Temperature is particle motion
Every substance is made of particles that are always moving. In a solid they are packed tightly and vibrate in place. In a liquid they are still close together but slide past one another. In a gas they are far apart and fly about, bumping into each other and the walls of their container. Temperature measures how fast the particles move on average. Heating a pan of water does not add "hotness"; it gives the molecules energy so they move faster. Cooling slows them down. At absolute zero, about −273 °C, their motion would be at its lowest. Faster particles push each other further apart, so things expand when heated, which is why bridges have expansion gaps.
The flat parts of the heating curve
Heat a block of ice from −10 °C at a steady rate and record its temperature every minute. The graph is not a straight line. The temperature climbs to 0 °C, then stops rising while the ice melts, even though energy is still going in. Only when all the ice has become water does it climb again, until it stalls a second time at 100 °C while the water boils. During melting, the energy breaks particles free of their fixed places in the solid. During boiling, it pulls them apart into a gas. So a change of state takes in energy without changing the temperature, and the reverse gives it back: steam condensing releases that energy, which is why a steam burn is worse than a burn from boiling water.
Why evaporation cools
Particles in a liquid do not all move at the same speed. Some are slow, most are in between, and a few are fast enough to break away from the surface and escape into the air, even well below boiling point. That is evaporation. Because it is the fastest particles that leave, the ones left behind are slower on average, and slower means cooler. So evaporation cools whatever the liquid is on. Your body uses this: sweat evaporating from your skin carries energy away. That is why a breeze feels so good on a hot day, and why humid air, which slows evaporation, feels uncomfortable. Rubbing alcohol feels cold on your hand because it evaporates quickly.
Curriculum links: NGSS MS-PS1-4