Gases
States of Matter: Gases
Gases
Gases and Their Properties
What you'll learn
- Define a gas and list its main properties.
- Explain why gases spread out to fill their container.
- Identify common gases around us.
- Connect gases to everyday Indian examples like LPG and balloons.
Key concepts
Level 1 - Meaning of a gas
A gas is a material that has no fixed shape and no fixed volume. Air is a gas (actually a mixture of gases) that spreads out to fill any space it is given, such as a room or a balloon.
Level 2 - Why gases spread out
The tiny particles in a gas are far apart and move around freely and quickly. This is why a gas spreads to fill all the space in its container instead of staying in one place like a solid or settling like a liquid.
Level 3 - Gases can be squeezed
Because the particles in a gas are far apart, a gas can be compressed into a smaller space, such as air pumped into a bicycle tyre. When more air is pumped in, more gas is packed into the same tyre.
Level 4 - Indian context
LPG cooking gas used in Indian kitchens is stored inside a metal cylinder under pressure. When the valve is opened, the gas escapes and spreads out, which is why a gas leak can be smelled across a whole room.
NCERT anchor: Looking Around 4, general observation of air and its properties (air as a gas around us)
Worked example
Filling a balloon with air
Step 1 - Take an empty, flat balloon.
Step 2 - Blow air into it steadily.
Step 3 - Watch the balloon expand and become round.
Step 4 - The air (a gas) spreads out to fill all the space inside the balloon.
Answer: A gas has no fixed shape or volume; it expands to fill whatever container it is given.
Smelling perfume across a room
Step 1 - Spray a little perfume in one corner of a room.
Step 2 - Stand at the opposite corner of the room.
Step 3 - Wait for a minute or two.
Step 4 - Notice the smell reaches you even though you did not move.
Answer: The perfume gas particles spread out on their own to fill the whole room.
Common mistakes
| Mistake | Why it happens | Fix |
|---|---|---|
| Gases have no weight at all | Gases feel invisible and weightless | Gases do have some weight, but they are usually much lighter than solids and liquids of the same size |
| A gas always stays where it is put | Confusing gas with solid behaviour | A gas spreads out on its own to fill all the space available to it |
| Only air is a gas | Air is the most familiar gas | Steam, helium, and carbon dioxide are also gases we meet in daily life |
| Gases cannot be squeezed at all | Gases seem to disappear when compressed | Gases can be compressed into a smaller space, as in a filled bicycle tyre or gas cylinder |
Quick check
- What are the two main properties of a gas?
- Name two gases found in the air around us.
- Why does a smell spread across a whole room?
- Give one example of a gas being squeezed into a smaller space.
- Stretch: Explain why a helium balloon floats while an air-filled balloon does not.
Revision tip: A gas has no fixed shape and no fixed volume; it spreads out to fill its container completely.
Open the Practice tab for graded questions on Gases.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Worked example
- Common mistakes
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