Density and Relative Density

Density and Relative Density

Middle School

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Learning Objectives

Key Vocabulary

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Density

A measure of how much mass is packed into a certain volume of a given substance.

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Mass

The total amount of matter that an object contains, which is constant regardless of its location.

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Volume

The total amount of three-dimensional space that is occupied by a substance or an object.

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Relative Density

The comparison of a substance's density to the density of a reference fluid, like water.

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What are Mass, Volume, and Density?

Mass and Volume

• Mass is the amount of matter an object has, measured in grams (g) or kilograms (kg).

• An object's mass is not affected by gravity and does not change based on its location.

• Volume is the amount of space an object occupies, measured in cubic centimeters (cm³) or milliliters (mL).

Density

• Density is the measurement of how tightly packed the matter is within an object.

• It shows the relationship between an object’s mass and its volume.

• The density of a substance is a specific physical property that does not change.

Multiple Choice

Which of the following statements best describes the key difference between mass and volume?

• Mass is measured in milliliters, while volume is measured in grams.

• Mass changes depending on gravity, while volume is always constant.

• Mass is the space an object occupies, while volume is the amount of matter it has.

• Mass is the amount of matter in an object, while volume is the amount of space it takes up.

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How to Calculate Density

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Multiple Choice

An object has a mass of 100 grams. When placed in a graduated cylinder, it causes the water level to rise from 50 mL to 75 mL. What is the density of the object?

• 2 g/mL

• 4 g/mL

• 1.33 g/mL

• 0.25 g/mL

Relative Density: Sink or Float?

• Relative density determines if an object sinks or floats.

• An object that is denser than the fluid will sink.

• An object that is less dense than the fluid will float.

Multiple Choice

The density of water is 1.0 g/mL. An object with a density of 0.85 g/cm³ is placed in a beaker of water. What will happen?

• It will sink to the bottom.

• It will float on the surface.

• It will split into two pieces.

• It will dissolve in the water.

How Density is Affected

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States of Matter

• ​Solids are generally the densest state of matter.

• ​​Liquids are typically less dense than their solid form.

• ​Gases are the least dense state of matter by far.

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Particle Arrangement

• ​In solids, particles are packed tightly together in a fixed position.

• ​​In liquids, particles are close but can slide past one another.

• ​In gases, particles are spread far apart and move around freely.

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Effect of Temperature

• ​Increasing the temperature of a substance usually decreases its density.

• ​​The particles move faster and spread out, increasing the substance's volume.

• ​Water is an exception; solid ice is less dense than liquid water.

Multiple Choice

What is the general relationship between temperature and density for most substances?

• As temperature increases, density increases.

• Density is highest at medium temperatures.

• As temperature increases, density decreases.

• Temperature has no effect on density.

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Common Misconceptions

Misconception	Correction
Big or heavy objects always sink.	Floating depends on density, not size or weight.
Mass and weight are the same thing.	Mass is the amount of matter. Weight is the force of gravity.
Ice is denser than liquid water.	Ice is less dense than liquid water, which is why it floats.

Multiple Choice

The density of seawater is 1.03 g/mL, and the density of pure water is 1.00 g/mL. Which of the following substances will float in seawater but sink in pure water?

• A substance with a density of 1.05 g/mL

• A substance with a density of 1.00 g/mL

• A substance with a density of 0.98 g/mL

• A substance with a density of 1.02 g/mL

Multiple Choice

An object has a mass of 90 grams. You use the water displacement method to find its volume. The initial water level in a graduated cylinder is 60 mL, and it rises to 120 mL after the object is submerged. What is the density of the object?

• 0.67 g/mL

• 1.5 g/mL

• 2.0 g/mL

• 0.75 g/mL

Multiple Choice

You have a cube of aluminum (density 2.70 g/cm³) and a cube of lead (density 11.34 g/cm³). If both cubes have the exact same volume, which statement is correct and why?

• The lead cube is lighter because it has less volume.

• The lead cube has more mass because it is more dense.

• Both cubes have the same mass because they have the same volume.

• The aluminum cube has more mass because it is less dense.

Multiple Choice

A liquid is cooled down significantly, causing its particles to move closer together. How does this affect its density, and what would likely happen if it were layered with a liquid of slightly higher density?

• Its density decreases, and it would likely rise above the other liquid.

• Its density increases, and it would likely sink below the other liquid.

• Its density does not change, so its position would remain the same.

• Its density increases, but it would still float on top of the other liquid.

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Summary

Poll

On a scale of 1-4, how confident are you with calculating density and predicting if an object will sink or float?

• 1 - Not confident at all

• 2 - A little confident

• 3 - Mostly confident

• 4 - Very confident