Condensation

Introduction

Take a cold can of drink out of the fridge on a warm day and within seconds its surface is covered in tiny droplets. The can is not leaking. Water vapour, invisible in the air around it, has turned into liquid water on the cold metal. This process is condensation , the reverse of boiling and evaporation. It shapes everything from misty bathroom mirrors to clouds and the dew on morning grass.

Core explanation

Definition. Condensation is the change of state from gas to liquid . The temperature at which a gas condenses is the same as the boiling point of the liquid: pure water vapour condenses at 100 °C at standard pressure, although water vapour mixed with air can condense on any surface cold enough.

The particle picture. In a gas, particles are far apart and move quickly in all directions. The attractive forces between them have almost no effect because the particles are rarely close together and move too fast to stick. When a gas is cooled:

- the particles lose energy and move more slowly; - when slow particles collide or meet a cold surface, the attractions between them can hold them together; - particles clump together into small groups, then droplets; - the particles end up close together but still able to move past one another — a liquid.

Condensation gives out energy. When particles come together and attractions form, energy is released to the surroundings. Condensation is therefore exothermic . This is why a scald from steam is more severe than one from boiling water at the same temperature: the steam releases extra energy as it condenses on the skin.

Cold surfaces help. Water vapour in air most often condenses on a cold surface, such as a window, a mirror or a cold can. The surface takes energy from the particles that touch it, slowing them enough for attractions to take over.

Visible "steam" is actually condensed water. Pure water vapour is invisible. The white cloud above a kettle is made of tiny liquid droplets formed as the vapour meets cooler air.

Step-by-step reasoning

To explain droplets on a cold window:

1. Air in the room contains invisible water vapour. 2. Water particles hit the cold glass and transfer energy to it. 3. The particles slow down. 4. Attractive forces pull slow particles together. 5. They form liquid droplets on the glass.

Visual explanation

Draw two boxes. In the first, place a few circles widely spaced with long arrows to show fast movement: a gas. In the second, draw the circles touching each other in a random arrangement at the bottom, with short arrows: a liquid. An arrow labelled "cooling — energy given out" joins the first box to the second.

Real-world analogy

Imagine people rushing around a large hall. When they are all running fast they bump into one another and bounce off. As they tire and slow down, they start stopping to chat and link arms, forming small groups. The slowing and grouping of people is like gas particles condensing into liquid droplets.

Real-world example

Dew forms on grass on clear nights. The ground and grass lose energy to the sky and become colder than the air. Water vapour in the air touching the cold grass condenses into droplets. In deserts, some plants and animals collect dew or fog droplets as an important source of water.

Why?

Why does condensation release energy when boiling takes it in? Boiling needs energy to pull particles apart against their attractions. Condensation is the exact reverse: particles move together and attractions form, so the same amount of energy is given back out.

Common misconception

"The water on a cold can has leaked through the metal" or "the cold made the water appear from nowhere". The water comes from water vapour already present in the air, which condenses when it touches the cold surface.

Worked example

Question: After a hot shower, the bathroom mirror is covered in mist but the tiled wall next to the radiator is dry. Explain the difference.

Reasoning: The shower adds lots of water vapour to the air. The mirror is cool, so water particles hitting it lose energy, slow down and condense. The wall near the radiator is warm, so particles hitting it do not lose enough energy to condense.

Answer: Vapour condenses on the cool mirror but not on the warm wall.

Quick check

1. What change of state happens during condensation? Answer: Gas changes into liquid.

Exam focus

Say that particles lose energy, slow down and move closer together so that the forces of attraction hold them as a liquid. State that condensation is exothermic. A common exam question asks why droplets form on cold surfaces; answer in terms of water vapour from the air losing energy to the surface.

Advanced insight

Clouds form when rising air expands and cools. Water vapour condenses onto tiny particles of dust, salt or smoke called condensation nuclei, forming droplets about one hundredth of a millimetre across. Without these nuclei, air can hold more water vapour than it normally would without condensing. Scientists have tried "cloud seeding", adding particles to clouds to encourage droplets and rain.

Summary

Condensation is the change from gas to liquid. As a gas cools, its particles lose energy, slow down and are pulled together by attractive forces into liquid droplets. Condensation is exothermic and happens at the same temperature as boiling. Water vapour in air often condenses on cold surfaces, forming mist, dew and droplets.

Practice questions

1. Name the change of state that is the reverse of boiling. Answer: Condensation. 2. Explain, using particles, why water droplets form on a glass of iced water. Answer: Water vapour particles in the air hit the cold glass, lose energy and slow down, so attractions pull them together into liquid droplets. 3. Is condensation exothermic or endothermic? Explain. Answer: Exothermic, because energy is released as particles come together and attractions form between them. 4. Why is the white "steam" seen above a kettle not really a gas? Answer: It is made of tiny liquid water droplets formed when the invisible water vapour condenses in the cooler air.