🌈 Rainbow Mind Map Relative density

🌈 RAINBOW MIND MAP • PHYSICS

Relative Density

Learn the meaning, formula, density-bottle method, and the reason objects float or sink in water.

Relative density mind map for quick revision.

💡 Introduction

Relative density compares the density of a substance with the density of water at 4°C. For example, the relative density of mercury is approximately 13.6, which means mercury is about 13.6 times as dense as water.

📘 Definition and Main Formula

Relative density, also called specific gravity, is the ratio of the density of a substance to the density of water.

Relative Density = Density of Substance ÷ Density of Water Both densities must be measured in the same unit.

Relative Density = Mass of a Certain Volume of Substance ÷ Mass of the Same Volume of Water Because the two samples have equal volume, the density ratio becomes a mass ratio.

Important: Relative density has no unit because it is a ratio of two quantities having the same unit.

🧪 Density Bottle Method

R.D. of Liquid = (M₃ − M₁) ÷ (M₂ − M₁) M₁ = mass of empty bottle, M₂ = mass of bottle with water, M₃ = mass of bottle with the liquid.

1

Empty Bottle

Measure the mass of the clean, dry density bottle. This is M₁.

2

Bottle + Water

Fill the bottle with water and measure the total mass. This is M₂.

3

Bottle + Liquid

Fill the same bottle with the test liquid and measure the total mass. This is M₃.

⚖️ Relation Between Relative Density and Density

• Density of water at 4°C is approximately 1 g/cm³ or 1000 kg/m³.
• Therefore, density in g/cm³ is numerically equal to relative density.
• Density = R.D. × 1 g/cm³
• Density = R.D. × 1000 kg/m³
• Relative density of water is 1.

🚢 Will an Object Float or Sink?

R.D. < 1 The object usually floats in water.

R.D. = 1 The object is neutrally buoyant under ideal conditions.

R.D. > 1 The object usually sinks in water.

🌍 Quick Examples

Substance	Approx. Relative Density	Behaviour in Water
Cork	Less than 1	Floats
Water	1	Reference substance
Aluminium	About 2.7	Sinks as a solid piece
Mercury	About 13.6	Much denser than water

🖼️ Visual Summary

▶️ Video Representation

🧠 Quick Self-Test

1. Does relative density have a unit?

Answer: No. It is a ratio of two densities measured in the same unit.

2. What happens when R.D. is greater than 1?

Answer: The object usually sinks in water because it is denser than water.

3. A liquid has R.D. 0.8. What is its density?

Answer: Its density is 0.8 g/cm³, or 800 kg/m³.

❓ Frequently Asked Questions

What is relative density?

Relative density is the ratio of the density of a substance to the density of water at 4°C.

Why does relative density have no unit?

It has no unit because it is calculated by dividing two densities expressed in the same unit.

What is the relative density of water?

The relative density of water is 1 because water is compared with itself.

How is relative density measured with a density bottle?

Use the formula (M3 − M1) ÷ (M2 − M1), where M1 is the mass of the empty bottle, M2 is the mass of the bottle with water, and M3 is the mass of the bottle with the test liquid.

When will an object float in water?

An object usually floats when its average relative density is less than 1.

When will an object sink in water?

An object usually sinks when its average relative density is greater than 1.

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