Understanding Gases – Compressibility, Diffusion, and Pressure
Here's a complete, engaging explanation and activity walkthrough for Activity 1.11 and the associated CBSE Class 9 content on the gaseous state, compressibility, diffusion, and gas pressure, designed to help your students understand through hands-on learning and real-life connections.
💨 Understanding Gases – Compressibility, Diffusion, and Pressure
🧑🔧 Real-Life Observation: Balloon Seller
🎤 Ask your students:
“Have you ever seen a balloon seller filling dozens of balloons from one small cylinder? Isn’t that amazing?”
✅ Encourage responses, then explain:
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The gas is often helium or hydrogen (for floating balloons).
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These gases are compressed at high pressure into a metal cylinder.
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One cylinder can fill over 100–200 balloons because gases can be compressed into a small volume.
🧪 Activity 1.11: Testing Compressibility of Solids, Liquids, and Gases
🧰 Materials Required:
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Three 100 mL syringes
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Rubber corks to seal the tips
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Water, chalk pieces, air (leave one syringe empty)
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A little vaseline for smoother piston movement
🧪 Steps:
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Label the syringes:
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Syringe A: Air (gas) — leave it empty
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Syringe B: Water (liquid) — fill with 100 mL of water
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Syringe C: Chalk pieces (solid) — fill loosely with small chalk bits
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Seal all nozzles tightly using rubber corks.
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Slowly insert the pistons back into each syringe (apply vaseline for ease).
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Try pushing the piston in each one and observe.
👁️ Observations:
| Syringe | Contents | Can you compress it? | Why? |
|---|---|---|---|
| A | Air (Gas) | ✅ Easily compressed | Large space between gas particles |
| B | Water (Liquid) | ❌ Barely compressed | Particles are close together |
| C | Chalk (Solid) | ❌ Not compressed | Particles tightly packed |
🧠 Conclusion:
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Gases are highly compressible – that’s why we can store so much gas in a small cylinder.
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Liquids and solids are nearly incompressible due to tightly packed particles.
🌫️ Why Does the Smell of Food Reach Us Quickly?
“Ever walked into your home and smelled food cooking before entering the kitchen?”
✅ That’s because of a process called diffusion.
📘 Diffusion: The spreading of particles from an area of higher concentration to lower concentration.
In gases, diffusion is fastest because:
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Particles have maximum kinetic energy
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They are widely spaced
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They move randomly at high speed
💡 That’s why gases mix quickly, and we can smell aromas, gas leaks, or perfumes in seconds.
💨 Pressure in Gases – How is it Created?
🧠 In a container:
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Gas particles are constantly moving.
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They collide with the container walls.
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These collisions exert force per unit area — this is gas pressure.
✅ That’s why:
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Tyres feel hard (air pressure)
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Gas cylinders are heavy and sealed tightly
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Balloons expand and stretch when inflated
🧠 Key Takeaways for Students
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Gases are highly compressible; solids and liquids are not.
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Gases diffuse fastest due to their high speed and low attraction.
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Gases exert pressure because their particles constantly hit container walls.
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These properties make gases easy to store, transport, and use in daily life (LPG, CNG, oxygen, helium).
🎤 Closing Line for Class:
“From the smell of food to flying balloons, gases may be invisible — but they’re always in motion, always creating pressure, and always surprising us with science!”
Would you like a lab record sheet, worksheet, or animated classroom slide to go with this activity?
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