Gas

Gaseous State of Matter

What you'll learn

Gases have neither fixed shape nor fixed volume — they fill any closed container.
Gases are compressible — syringe with air, bicycle pump.
Examples: air we breathe, water vapour, LPG at home (used safely by adults).
NCERT Looking Around 5, Chapter 7 — evaporation produces water vapour in air.

Key concepts

Level 1 — Properties of gases

Verbal: Gas particles are far apart and move freely in all directions.

Symbolic: Gas → no fixed shape + no fixed volume (expands to fill space).

Property	Gas behaviour	Example
Shape	Fills container	Balloon shape = rubber, air inside
Volume	Expands to available space	Open perfume spreads in room
Compressibility	Easy to squeeze	Pump inflates tyre
Visibility	Often invisible	Air around us

NCERT link: Ch 7 — wet clothes dry as water evaporates into vapour (gas) in air.

Level 2 — Air and water vapour

Verbal: Air is a mixture of gases (mainly nitrogen and oxygen) essential for breathing and fire.

Real-life: Steam from hot rice is water vapour; fog is tiny water droplets (liquid) in air — related but different.

Gas	Where	Safety note
Oxygen	Air ~21%	Needed for respiration
Water vapour	After boiling/evaporation	Makes air humid
LPG	Kitchen cylinder	Adult handles; smell added for leak detection

Worked example

Why does air in a syringe compress when you push the plunger?

Step 1 — Gas particles have large empty space between them.
Step 2 — Push reduces space → pressure rises → volume shrinks.
Answer: Gases are highly compressible.

Wet floor dries — where does the water go?

Step 1 — Liquid water evaporates.
Step 2 — Becomes invisible water vapour in air (gas).
Answer: Enters air as water vapour (Ch 7).

Common mistakes

Mistake	Why it happens	Fix
"No gas around us"	Air invisible	Air is gas mixture everywhere
Steam is still liquid	Seeing white cloud	White cloud is condensed droplets; vapour is invisible
Gases have no mass	Light feeling of air	Air has mass — balloon comparison
Only harmful things are gases	LPG fear only	Oxygen is essential gas

Quick check

Do gases have fixed shape and volume?
Name the gas we breathe in most (nitrogen ~78%, oxygen used from air).
Why can you squeeze a empty plastic bottle with cap on?
Stretch: Balloon in hot sun gets bigger — why? (Air inside expands.)

Revision tip: Observe steam from hot idli — note it spreads. Link to evaporation from Ch 7 experiments.

Open the Practice tab for graded questions on Gaseous State of Matter.

Gas

Gas