Relative Density

Relative Density

Middle School

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Learning Objectives

• Define what relative density is and understand why we use it.

• Explain how relative density helps us know if an object sinks or floats.

• Use the formula to calculate the relative density of different substances.

• Understand that relative density is a ratio and does not have any units.

Key Vocabulary

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Relative Density

A ratio that compares a substance's density to the density of a reference substance, usually water.

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Density

The measure of how much mass is packed into a certain amount of space or volume.

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Sinking

This happens when an object is denser than the liquid it is in, like water.

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Floating

This happens when an object is less dense than the liquid it is in, like water.

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What Is Relative Density?

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Multiple Choice

What is the primary purpose of calculating relative density?

• To compare the density of a substance with the density of water.

• To find the exact mass of an object.

• To determine the temperature of a substance.

• To measure the volume of an irregular solid.

Predicting Sinking and Floating

• An object with a relative density greater than 1 will sink in water.

• An object with a relative density less than 1 will float in water.

• An object with a relative density of exactly 1 will hover in water.

Multiple Choice

An object has a relative density of 0.75. What will happen when it is placed in water?

• Its relative density will increase to 1.0.

• It will hover in the middle of the water.

• It will float on the surface.

• It will sink to the bottom.

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Common Misconceptions About Density

Misconception	Correction
Heavy objects always sink.	Floating depends on density, not just an object's weight.
Relative density has units like g/cm	Relative density is a ratio, so it is a unitless number.
An object that floats in water will float in any liquid.	An object might sink if the liquid is less dense than the object.

Multiple Choice

Why is water commonly used as the reference substance when calculating relative density?

• Because water is the densest substance on Earth.

• Because water's density never changes with temperature.

• Because all substances dissolve in water.

• Because water is abundant and its density (1 g/cm³) provides a simple baseline for comparison.

Multiple Choice

A rock is found to be 3.5 times denser than water. What is its relative density, and will it sink or float?

• RD = 0.35, it will float.

• RD = 3.5 g/cm³, it will sink.

• RD = 3.5, it will sink.

• RD = 1.0, it will hover.

Multiple Choice

You are given two balls of the exact same size. Ball A sinks in water, while Ball B floats. What must be true about their properties?

• Both balls have the same mass but different relative densities.

• Ball A has a relative density greater than 1 and more mass.

• Ball A has a lower relative density and less mass.

• Ball B has a higher relative density and more mass.

Multiple Choice

A block of wood has a density of 0.8 g/cm³ and a block of rubber has a density of 1.2 g/cm³. Predict what will happen if both are placed in a large tank of water.

• The wood will sink and the rubber will float.

• Both will sink because they are solids.

• The wood will float and the rubber will sink.

• Both will float because they are close to the density of water.

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Summary

Poll

On a scale of 1-4, how confident are you in explaining why some objects float and others sink using relative density?

• 1

• 2

• 3

• 4