♨️ Boiling Point

 Certainly, Mayank! Here's a student-friendly, complete explanation of “Boiling Point” for Class 9 CBSE Science, including scientific concept, real-life examples, latent heat of vaporisation, and Kelvin–Celsius temperature conversion — structured perfectly for classroom teaching.

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♨️ Boiling Point (Class 9 – CBSE Science)

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📘 Definition:

The boiling point of a liquid is the temperature at which it changes into vapour (gas) throughout its volume, not just from the surface.

✅ At this temperature, vapour pressure of the liquid equals atmospheric pressure.

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🌡️ What Happens During Boiling?

• As a liquid is heated, its particles gain kinetic energy.

• They start moving rapidly, pushing against each other.

• At a certain temperature, they gain enough energy to overcome the intermolecular forces and escape into the air as vapour.

• Boiling occurs throughout the liquid — not just on the surface (unlike evaporation).

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📏 Boiling Point of Water:

Temperature in °C	Temperature in K
100°C	373 K

✅ At sea level (1 atm pressure), water boils at 100°C or 373 K.

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🔥 Latent Heat of Vaporisation

📘 Definition:

Latent heat of vaporisation is the amount of heat required to convert 1 kg of a liquid into vapour at its boiling point, without changing the temperature.

💡 During boiling, temperature stays constant, because all the heat energy is used to break bonds between the particles, not to increase temperature.

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💧 Difference Between Boiling and Evaporation

Boiling	Evaporation
Happens at fixed temperature	Can happen at any temperature
Occurs throughout the liquid	Occurs only from the surface
Requires external heat	Can occur naturally
Fast process	Slow process

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🧪 Real-Life Examples of Boiling Point:

• Water boils at 100°C (373 K)

• Alcohol boils at around 78°C

• Cooking oil has a higher boiling point (~200°C+)

• Pressure cookers raise the boiling point of water → food cooks faster

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📎 Note on Temperature Conversion:

🧠 Kelvin is the SI unit of temperature.

To Convert	Formula	Example
°C → K	K = °C + 273	100°C = 373 K
K → °C	°C = K – 273	373 K = 100°C

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🧠 Key Takeaway:

• Boiling point is a physical property of liquids.

• It depends on atmospheric pressure — water boils at a lower temperature at higher altitudes due to lower pressure.

• The heat used during boiling (without temperature change) is the latent heat of vaporisation.

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💬 Ask Your Students:

1. What is the boiling point of water in °C and K?

2. Why does water not boil at 100°C in the mountains?

3. What is latent heat of vaporisation?

4. Why does boiling water stay at the same temperature?

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🎤 Closing Line for Class:

“Boiling is not just bubbling water — it’s energy in action, breaking bonds and transforming liquid into invisible vapour.”

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Would you like a worksheet, diagram, or boiling vs evaporation slide for this topic?