CHEMISTRY · Stage 7 · Chapter 1: The Particle Model and States of Matter

Chemistry 1.4 Changes of state — Evaporating, Boiling, and Condensing

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1.4 Changes of state — Evaporating, Boiling, and Condensing — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define and compare evaporation, boiling and condensation and explain them using the particle model.Six changes — melting (solid to liquid), freezing (liqui…

Candle wax
Particle explanation
Higher temperature
Larger surface area
Moving air
Low humidity

Section 1 of 7Changes of state

Six changes
- melting, freezing (liquid to solid), evaporating and boiling (liquid to gas), condensing (gas to liquid) and the reverse of evaporating for gas to liquid.
  - (solid to liquid)
- Most substances change state, not only water.
Candle wax
- it shows several changes, since near a burning wick solid wax melts to liquid wax and some liquid wax evaporates to wax in the gas state,
- which is the part that burns.
Particles
- when a substance changes state its particles stay the same and do not get bigger or smaller.
- What changes is how the particles are arranged, how they move and how far apart they are.

Section 2 of 7Evaporation

Evaporation = the change of state from liquid to gas that can happen at any temperature below the boiling point.
Where
- it happens only at the surface of the liquid.
Particle explanation
- in a liquid the particles touch and slide over each other, and some move faster than others.
- The faster particles near the surface have enough kinetic energy to overcome the attraction of their neighbours and leave the surface.
- They separate from each other and form gas.
Cooling
- the fastest particles leave, so the average energy of those left falls and the liquid cools, which is why sweating cools the body.

Section 3 of 7Factors that speed up evaporation

Higher temperature
- more particles have enough energy to escape.
Larger surface area
- more particles are at the surface, so wet washing spread out dries faster than washing in a heap.
Moving air
- wind carries escaped particles away so they cannot return to the liquid.
Low humidity
- dry air can accept more vapour, so fewer particles return.

Section 4 of 7Boiling

1Boiling

= the change of state from liquid to gas at the boiling point, which happens only if the liquid is hot enough.

2Bubbles3 points

bubbles of the substance in the gas state form everywhere in the liquid, rise to the surface and escape.
In the liquid the particles touch, while inside the bubbles they are spread far apart.
The bubbles are water vapour, which is an invisible gas, and visible steam is condensed droplets.

3Boiling point2 points

the temperature at which a substance boils, different for each substance.
It is high if the particles hold together strongly and low if they hold together weakly.

4Constant temperature2 points

energy from a heater does not raise the temperature while boiling, because it is used to break the forces between particles.
Pure water stays at 100 °C at normal pressure until all the liquid has become gas.

Feature	Evaporation	Boiling
Temperature	Any temperature below boiling point	Only at the boiling point
Where it happens	Surface only	Throughout the liquid
Speed	Slow	Fast, with bubbles

Section 5 of 7Making conclusions from boiling point data

Gas at room temperature
- a substance with a boiling point below 20 °C, such as nitrogen at −196 °C, is a gas at room temperature.
Strong attraction
- a substance with a very high boiling point, such as copper or diamond, has particles held together very strongly.
Comparing
- ethanol boils at 78 °C, water at 100 °C and mercury at 357 °C.

Section 6 of 7Condensation

Condensing = the change of state from gas to liquid, which happens at any temperature below the boiling point and is the reverse of evaporation.
Particle explanation
- when a gas is cooled its particles lose kinetic energy and slow down.
- They get closer until they touch, because the attractive forces can now hold them together.
- Energy is released to the surroundings.
Everyday example
- warm, moist air touching a cold surface such as a bathroom mirror or cold glass of drink leaves droplets of water.
- The water condensed from vapour in the air and did not pass through the glass.
Condenser
- a tube cooled by water cools vapour from a boiling liquid so it drips out as a liquid, which is the basis of distillation.

Section 7 of 7Physical change

Reversible
- a change of state is a physical change that can be reversed, because only arrangement, spacing and movement change.

Last updated: 14 October 2026