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๐ฌ Think about it
Picture a crowded bus packed with people versus one with just a few. The packed bus is "dense". A forest with trees close together is dense; far apart, it is less dense. Density is just how much stuff is packed into a space. Let's define it properly.
What is density?
- Density is the mass present in a unit volume of a substance.
- It does not depend on the shape or size of the object.
- It does depend on temperature and pressure.
- Pressure mainly affects gases, hardly solids and liquids.
Example:
A crowded bus has high density of people; an empty bus has low density.
Density = Mass ÷ Volume → \( \text{Density} = \dfrac{\text{Mass}}{\text{Volume}} \)
What is relative density?
- Relative density compares a substance's density to water's density.
- An aluminium block of mass 27 g and volume 10 cm³ has density 2.7 g/cm³.
- So aluminium is 2.7 times denser than water.
- Relative density is a number with no units.
Relative density
=
\( \dfrac{\text{Density of that substance}}{\text{Density of water at that temperature}} \)
๐ฌ Think like a scientist
- Some oil or ghee packs say 1 litre but weigh only about 910 g.
- 1 litre of water weighs about 1000 g.
- So the same volume of oil weighs less than water.
- This means oil is less dense than water.
What are the units of density?
- The SI unit of density is kilogram per cubic metre (kg/m³).
- For liquids, g/mL and g/cm³ are also used.
- The mass of 1 mL of water is close to 1 g.
- So 100 mL of water is about 100 g.
Example:
10 mL of water has a mass of about 10 g at room temperature.
| Quantity | SI unit | Other common units |
|---|---|---|
| Mass | kilogram (kg) | gram (g) |
| Volume | cubic metre (m³) | cm³, mL, litre (L) |
| Density | kg/m³ | g/mL, g/cm³ |
Important Points
- Density = mass ÷ volume; it is independent of shape and size.
- Relative density compares a substance to water and has no units.
- The SI unit of density is kg/m³; 1 mL of water is about 1 g.
Important Definitions
- Density — the mass present in a unit volume of a substance (mass ÷ volume).
- Relative density — the density of a substance divided by the density of water; a number with no units.
๐ NCERT Question 4 — A stone sculpture weighs
A stone sculpture weighs 225 g with a volume of 90 cm³. Find its density and whether it floats or sinks.
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๐ NCERT Question 7 — An object has a
An object has a mass of 400 g and a volume of 40 cm³. What is its density?
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๐ NCERT Question 9 — Object A has a
Object A is 200 g and 40 cm³; Object B is 240 g and 60 cm³. Which object is denser?
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๐ NCERT Question 11 — A block of iron
A block of iron is 600 g with a density of 7.9 g/cm³. What is its volume?
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