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Unit 3: Forces and energy

Cooling By Evaporation — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 15 of 16 1 quiz

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Cooling by evaporation — HIA INTERNATIONAL notes: Explain how evaporation cools a liquid and its surroundings using particle ideas.Evaporation = the change of state from liquid to gas.Temperature — it happens at temperatures well below the boiling point, e.g.

  • Random motion
  • Different energies
  • Pulled back
  • Average energy
  • Energy source
  • Clay water cooler

Section 1 of 8Evaporation

Evaporation
Evaporation the change of state from liquid to gas.
Temperature
  • Temperature it happens at temperatures well below the boiling point, e.g. water at 15 °C.
Surface
  • Surface it happens only at the surface of the liquid, unlike boiling which happens throughout the liquid.
Rate
  • Rate it is faster at higher temperature, with larger surface area, with air movement and in low humidity.

Section 2 of 8Particle explanation

Particle explanation
Random motion
  • Random motion the particles in a liquid move in random (unpredictable) directions and at random speeds.
Different energies
  • Different energies some particles move faster and have more energy than others.
Escape
  • Escape the highest-energy particles at the surface can escape and become gas.
Pulled back
  • Pulled back medium-energy particles that escape may be pulled back into the liquid.
Remain
  • Remain the lowest-energy particles stay in the liquid.

Section 3 of 8Why evaporation causes cooling

Average energy
  • Average energy when the highest-energy particles leave, the average energy of the remaining particles falls.
Temperature
  • Temperature temperature measures the average energy of the particles, so the liquid cools.
Energy source
  • Energy source the energy to change state comes from the liquid and its surroundings, e.g. your skin.
Example
  • Example after leaving a swimming pool, water evaporating from your skin makes you feel cold.

Section 4 of 8Examples of cooling by evaporation

Sweat
  • Sweat sweat evaporates, so its temperature drops and it takes thermal energy from the skin, lowering skin temperature.
Humid air
  • Humid air in very humid conditions sweat may not evaporate, so the body can overheat.
Clay water cooler
  • Clay water cooler water seeps through porous clay and evaporates outside, cooling the water inside.
Air cooler
  • Air cooler a fan blows warm air through a wet sponge;
    • the air supplies the thermal energy to evaporate the water, so the air cools.

Section 5 of 8Different liquids

LiquidForces between particlesEvaporationFeel on skin
Perfume or isopropanolWeaker than waterFasterFeels colder than water
WaterMediumMediumCool
Liquid soapStronger than waterSlowerFeels less cold than water
  • Safety
    • apply only water, soap or perfume to skin and wash soap off with plenty of water.

Section 6 of 8Making an air cooler

1
Measure the room air temperature with a thermometer in a stand, not held in your hand.
2
Switch on a fan and measure the temperature of the air it blows.
3
Soak a towel in water and hang it on a frame with its bottom edge in a tray of water.
4
Place the fan about 10 cm from the wet towel and measure the air temperature between them.
5
Compare readings; the air through the wet towel is cooler.
  • Safety
    • keep water away from electric cables, sockets and the fan.

Section 7 of 8Key terms

TermMeaning
EvaporationThe change of state from liquid to gas at the surface of a liquid.
RandomUnpredictable.
PorousAllowing water to slowly soak through.
Average energyThe mean energy of all the particles in a substance.
HumidityThe amount of water vapour in the air.
Thermal energyHeat energy that moves from hot to cold.

Section 8 of 8Exam technique

1
Evaporation only happens at 100 °C — it happens at any temperature, at the surface.
2
The liquid on the skin alone cools it — cooling happens when the liquid evaporates and takes thermal energy.
3
All the particles leave equally — only the highest-energy particles escape.
4
Evaporation warms the liquid — it lowers the average energy, so it cools.

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