Matter in Our Surroundings • Topic 3 of 3

Evaporation & Factors Affecting It

Hang out wet clothes and they dry, even on a day far cooler than 100°C. A puddle on the road vanishes by afternoon. Both happen because of evaporation — the change of a liquid into vapour at a temperature below its boiling point. This is one of the most useful everyday ideas in chemistry.

Why evaporation happens

The particles in a liquid are always moving with different speeds, so they have different kinetic energies. At the surface, a few of the fastest-moving particles have enough energy to overcome the attraction of their neighbours and escape into the air as vapour. Because only surface particles leave, evaporation is called a surface phenomenon. This is the key difference from boiling, which happens throughout the whole liquid at a fixed temperature.

Factors that affect the rate of evaporation

  • Surface area: A larger exposed surface means more particles can escape, so evaporation is faster. We spread out wet clothes on a line, and tea cools faster in a wide saucer than in a narrow cup.
  • Temperature: Higher temperature gives more particles enough energy to escape, so evaporation increases. Clothes dry faster on a hot day.
  • Humidity: Humidity is the amount of water vapour already in the air. When humidity is high, the air is nearly saturated and can accept little more vapour, so evaporation slows down. This is why clothes dry slowly on a damp, rainy day.
  • Wind speed: Moving air carries away the escaped vapour particles, keeping the air above the liquid less saturated, so evaporation speeds up. Clothes dry faster on a windy day.

Evaporation causes cooling

Here is the most important consequence. To escape, the fastest (most energetic) particles leave the liquid. The particles left behind have lower average kinetic energy, so the temperature of the liquid falls. To keep evaporating, the liquid then absorbs heat (latent heat) from its surroundings — including from your skin or the container. This is why evaporation produces a cooling effect.

Everyday applications

  • We feel cool when we sweat, because the sweat evaporates and takes heat from our body. This is the body's natural cooling system.
  • Water stays cool in an earthen pot (matka) because water seeps through the tiny pores and evaporates, drawing heat from the water inside.
  • People sprinkle water on a rooftop or courtyard on a hot evening — its evaporation cools the surroundings.
  • We feel a cool sensation when we put a few drops of acetone, petrol or perfume on our palm, because these volatile liquids evaporate quickly and absorb heat from the skin.
  • Wearing cotton clothes in summer helps because cotton absorbs sweat and exposes it for faster evaporation, keeping us cool.
Evaporation: only fast surface particles escape, so the remaining liquid cools and draws heat from its surroundingsliquid surfacefast particles escape as vapourliquid absorbs heat from surroundings → cooling
Solved Examples
1
Worked Example
Why is evaporation called a surface phenomenon, while boiling is not?
Solution
  1. In evaporation, only the fast-moving particles at the surface of the liquid have enough energy to escape into the air.
  2. The particles below the surface do not leave, so the change happens only at the surface.
  3. Boiling, in contrast, occurs throughout the entire bulk of the liquid at a fixed temperature.

Answer: Evaporation involves only surface particles escaping, so it is a surface phenomenon, unlike boiling which happens throughout the liquid.

2
Worked Example
Explain why we feel cool when we apply acetone (nail-polish remover) on the back of our hand.
Solution
  1. Acetone is a volatile liquid that evaporates very quickly.
  2. To evaporate, its particles absorb heat (latent heat) from the surroundings.
  3. The nearest source of heat is our skin, so heat is drawn from the hand.

Answer: The fast-evaporating acetone absorbs latent heat from the skin, lowering its temperature, so we feel cool.

3
Worked Example
List the four main factors that affect the rate of evaporation and state how increasing each one changes the rate.
Solution
  1. Surface area: increasing it increases the rate (more particles can escape).
  2. Temperature: increasing it increases the rate (more particles gain escape energy).
  3. Humidity: increasing it decreases the rate (the air is already near saturation).
  4. Wind speed: increasing it increases the rate (vapour is carried away).

Answer: Higher surface area, higher temperature and higher wind speed all increase evaporation; higher humidity decreases it.

4
Worked Example
Why does water kept in an earthen pot (matka) become cool in summer?
Solution
  1. An earthen pot has many tiny pores in its walls.
  2. Water seeps through these pores to the outer surface and evaporates.
  3. This evaporation absorbs latent heat from the water inside the pot, lowering its temperature.

Answer: Water evaporates from the pores of the matka and takes heat from the water inside, so the water inside stays cool.

5
Worked Example
Why do clothes dry faster on a hot, dry and windy day than on a cold, humid and still day?
Solution
  1. A higher temperature gives more water particles enough energy to escape.
  2. Low humidity means the surrounding air can still absorb a lot of vapour.
  3. Wind continuously carries away the escaped vapour, keeping the air near the cloth unsaturated.
  4. All three factors increase the rate of evaporation, so the clothes dry faster.

Answer: High temperature, low humidity and wind all raise the rate of evaporation, so the clothes dry much faster.

6
Worked Example
Tea or milk poured into a saucer cools faster than the same amount in a cup. Give the reason.
Solution
  1. A saucer has a much larger exposed surface area than a cup.
  2. A larger surface area allows more particles to evaporate per second.
  3. Faster evaporation removes more heat from the liquid, so it cools more quickly.

Answer: The larger surface area of the saucer speeds up evaporation, which removes heat faster and cools the tea more quickly.

Key Points

  • Evaporation is the change of a liquid into vapour at a temperature below its boiling point; only fast surface particles escape, so it is a surface phenomenon.
  • Rate of evaporation increases with larger surface area, higher temperature and higher wind speed.
  • Rate of evaporation decreases when humidity is high, because the air is already nearly saturated with water vapour.
  • Evaporation causes cooling: the escaping high-energy particles leave the liquid cooler, and the liquid absorbs latent heat from its surroundings.
  • Everyday uses: sweating cools the body, water stays cool in a matka, acetone/perfume feels cold on skin, and cotton clothes keep us cool in summer.
Quick Check — 5 MCQs
Tap an option to check your answer0 / 5
Q1.Evaporation takes place:
Explanation: Only the fast-moving surface particles escape, making evaporation a surface phenomenon.
Q2.Which change will INCREASE the rate of evaporation?
Explanation: Wind carries vapour away, keeping the air unsaturated, so evaporation increases.
Q3.Clothes dry slowly on a rainy day mainly because of:
Explanation: High humidity means the air is already nearly saturated, so it accepts little extra vapour.
Q4.Evaporation causes cooling because the liquid:
Explanation: The fastest particles escape and the rest absorb heat from the surroundings, lowering the temperature.
Q5.We feel comfortable in cotton clothes in summer because cotton:
Explanation: Cotton soaks up sweat and exposes it for evaporation, which cools the body.
Practice more MCQs → 📝 Assignment · 30 marks