Effect of Change of Pressure

🧪 1.4.2 – Effect of Change of Pressure

(Can pressure change the state of matter?)

🎤 Start with a Thought-Provoking Question

“What do you think will happen if we start pushing a gas into a smaller and smaller space? Will the particles come closer together?”

Let students respond.

Now say:

✅ Yes — by increasing pressure, we can force the particles of a gas closer together.
And if we lower the temperature at the same time, the gas can even turn into a liquid!

🔍 Key Concept:

📘 Gases can be liquefied by:

  • Increasing pressure, and

  • Lowering temperature

💡 Real-Life Examples:

  • LPG (Liquefied Petroleum Gas) used at home is a gas stored as a liquid in cylinders.

  • CNG (Compressed Natural Gas) in vehicles is gas compressed under very high pressure.

  • Oxygen tanks in hospitals also store gas in liquefied form.

❄️ Dry Ice – A Special Case

Ask students:

“Have you heard of dry ice? It looks like ice, but it’s not made of water!”

📘 Dry Ice is solid carbon dioxide (CO₂).

  • It is stored under high pressure.

  • When pressure is reduced to 1 atmosphere, it doesn’t melt — it sublimes (solid → gas directly)!

🎯 That’s why it's called "dry ice" — it doesn't leave any liquid behind.

📘 Definition:

"Sublimation is the direct change of state from solid to gas without becoming a liquid."
Dry ice undergoes sublimation when pressure is reduced.

📏 Understanding Pressure Units

  • Atmosphere (atm) is a unit to measure gas pressure.

  • 1 atm = 1.01 × 10⁵ Pascal (Pa)

  • At sea level, the air exerts 1 atm of pressure — this is called normal atmospheric pressure.

🧠 Key Takeaway:

The state of matter (solid, liquid, or gas) depends not just on temperature —
but also on pressure.

  • High pressure + Low temperature = Gas → Liquid

  • Low pressure = Solid CO₂ (dry ice) → Gas

💬 Ask Your Students:

  1. How is dry ice different from regular ice?

  2. Why can gas be turned into a liquid under pressure?

  3. What does 1 atmosphere mean?

🎤 Closing Line for Class:

“With the right pressure and temperature, we can squeeze a gas into a liquid, or watch a solid like dry ice vanish into thin air — literally! Science lets us control the states of matter.”

Would you like a diagram, pressure vs. state change worksheet, or animated explanation to visually support this topic?

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