❄️ Evaporation Causes Cooling
Excellent, Mayank! Here’s a complete and concept-rich explanation of the CBSE Class 9 Science topic “Evaporation Causes Cooling”, integrating all the points you've provided — including daily life examples, cotton clothes, water droplets on cold glass, and latent heat of vaporisation — in a student-friendly format.
❄️ Evaporation Causes Cooling
(Section 1.5.2 – Matter in Our Surroundings)
🔍 What is Evaporation? (Quick Recap)
Evaporation is the process where particles at the surface of a liquid escape into vapour at temperatures below the boiling point.
🌬️ How Does It Cause Cooling?
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When liquid particles evaporate, they take away energy (heat) from their surroundings.
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The fastest-moving (most energetic) particles escape first.
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The average energy of remaining particles drops, lowering the temperature of the surface.
🧠 This heat taken from surroundings is equal to the latent heat of vaporisation.
🧪 Everyday Examples That Show Evaporation Causes Cooling
✅ 1. Acetone or Nail Polish Remover on Skin
When you pour some acetone on your palm:
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It evaporates quickly
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Absorbs heat from your skin
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Makes your palm feel cold
✅ 2. Sprinkling Water on Roofs or Ground in Summer
After a hot day, people sprinkle water on the ground or terrace:
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Water evaporates using the surface’s heat
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This cools down the hot cement or concrete
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Used in villages and urban areas alike
✅ 3. Wearing Cotton Clothes in Summer
Cotton is ideal for summer because:
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It absorbs sweat
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Exposes sweat to air for easy evaporation
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During evaporation, sweat takes heat from the body
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This cools the body and prevents overheating
✅ 4. Sweat Keeps the Body Cool
Sweating is our body’s natural cooling system:
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Sweat evaporates
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Heat is drawn from skin
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Body temperature is regulated
💧 Why Do We See Water Droplets on a Glass of Cold Water?
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The air around us contains water vapour.
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When this vapour comes in contact with a cold surface (like a glass of ice-cold water), it loses energy.
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The vapour condenses into tiny liquid water droplets on the outer surface of the glass.
✅ This is an example of condensation, the opposite of evaporation.
📘 Important Concept: Latent Heat of Vaporisation
The amount of heat required to change 1 kg of liquid to vapour at constant temperature is called latent heat of vaporisation.
✅ Water requires a large amount of heat to evaporate, making it very effective for cooling.
📊 Summary Chart
| Example | What Happens? |
|---|---|
| Acetone on skin | Evaporates fast → Absorbs heat from palm → Palm feels cold |
| Sprinkling water on hot ground | Water evaporates → Uses heat from surface → Ground cools down |
| Cotton clothes in summer | Absorb sweat → Help it evaporate → Keep body cool |
| Sweat | Evaporates using body heat → Lowers body temperature |
| Cold glass forms water droplets | Water vapour condenses on cold surface → Forms liquid drops |
💬 Ask Your Students:
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Why do we feel cold after applying perfume or sanitiser?
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How does sweating help regulate body temperature?
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Why do droplets form on a cold bottle even when it's dry?
🎤 Closing Line for Class:
“Evaporation isn’t just about drying — it’s nature’s smart way of cooling. From sweating to sprinkling water, it's the science of comfort, working silently all around us.”
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