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๐ฌ Think about it
Why does a hot air balloon rise? Heat the air inside and it spreads out, becoming lighter than the cool air around it. So warming a substance lowers its density. Let's see why.
How does temperature change density?
- Density usually decreases on heating and increases on cooling.
- On heating, particles move apart and spread out.
- This increases the volume but the mass stays the same.
- Since density = mass ÷ volume, density decreases.
Example:
Hot air rises because it is less dense than the cool air around it — how a hot air balloon works.
Heating Lowers Density
Heat the substance
Particles move apart and spread.
↓
Volume increases
Mass stays the same.
↓
Density decreases (so hot air rises)
Important Points
- Heating spreads particles, raising volume but not mass.
- So density decreases on heating and increases on cooling.
- Hot air rises because it is less dense than cool air.
Important Definitions
- Effect of temperature on density — density generally decreases on heating (volume rises) and increases on cooling.
๐ NCERT Question 12 — You are provided with
A test tube of water is heated to ~70°C and the level in the glass tube rises. How does it affect density?
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