Density Definition in Science

By definition, density is mass per unit volume. In other words, it is the amount of matter packed into a space.

The symbol most commonly used for density is ρ (the lower case Greek letter rho). The capital letter D is also common. Density is an intensive property of matter, which means the size of a sample does not affect its value. It is a physical property of matter, meaning you measure density without requiring any chemical reaction. Density units include grams per milliliter and kilograms per liter.

Key Takeaways: Density

• Density is mass per unit volume, meaning how much matter packs into a given space.
• The formula for density is ρ = m/V.
• Density is an intensive property, so it does not depend on the size of the sample.
• Common units include g/mL, g/cm³, and kg/m³.
• Solids are usually more dense than liquids, and liquids are usually more dense than gases.
• Temperature and pressure affect density, especially for gases.
• Density determines whether an object floats or sinks in a fluid.

Free Lab Density Worksheet

This free density lab worksheet helps students measure mass and volume, calculate density, and predict whether objects float or sink. It works well for middle school and high school science classes.

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What Is Density?

Density describes how much matter is packed into a given amount of space. A substance with high density contains a large amount of mass in a small volume, while a substance with low density contains less mass in the same volume.

At the particle level, density reflects how closely atoms or molecules are packed together. In most solids, particles are tightly packed, so density is relatively high. In liquids, particles are close together but can move past one another, so density is lower than in solids. For gases, particles are far apart, which gives gases very low densities.

It is important to distinguish between mass and weight. Mass measures the amount of matter in an object, while weight is the force of gravity acting on that mass. Density depends on mass, not weight.

Units of Density and Conversions

Density units depend on the system of measurement and the type of substance.

Common units of density:

The units g/mL and g/cm³ are equivalent because 1 milliliter equals 1 cubic centimeter.

Useful conversions:

• 1 g/cm³ = 1000 kg/m³
• 1 g/mL = 1 g/cm³

Example conversion:

Convert 2.7 g/cm³ to kg/m³:

2.7 g/cm³ × 1000 = 2700 kg/m³

Density Formula

The equation for density is:

density = mass / volume

ρ = m/V

The reciprocal of density (V/m) is its specific volume.

How to Calculate Density

You can rearrange the density formula to solve for mass, volume, or density:

• Density: ρ = m/V
• Mass: m = ρV
• Volume: V = m/ρ

Example 1: Solve for Density

A sample has a mass of 50 g and a volume of 10 mL. What is its density?

Known:
m = 50 g
V = 10 mL

ρ = m/V = 50 g / 10 mL = 5 g/mL

Example 2: Solve for Volume

A metal has a density of 8 g/mL and a mass of 40 g. What is its volume?

Known:
ρ = 8 g/mL
m = 40 g

V = m/ρ = 40 g / 8 g/mL = 5 mL

Example 3: Solve for Mass

A liquid has a density of 1.2 g/mL and a volume of 15 mL. What is its mass?

Known:
ρ = 1.2 g/mL
V = 15 mL

m = ρV = 1.2 g/mL × 15 mL = 18 g

Density Examples

Density tends to be higher for solids than for liquids, which are in turn more dense than gases. For example, the density of iron is 7.87 g/ml. The density of pure water (a defined value) is 1 gram per milliliter. The density of air is only about 0.012 g/ml.

Density of Common Substances

The density of a substance is characteristic and can help identify it.

Water has a density of 1 g/mL at 4°C, which makes it a useful reference for comparing other substances.

Density and Temperature and Pressure

Density often changes with temperature and pressure.

For most substances, increasing temperature causes particles to move farther apart. This increases volume and decreases density. Cooling has the opposite effect.

Water is an important exception. It reaches its maximum density at 4°C. When water freezes into ice, it expands and becomes less dense, which is why ice floats.

Pressure has a small effect on solids and liquids but a large effect on gases. Compressing a gas reduces its volume and increases its density.

Why Density Determines Floating and Sinking

Density determines whether an object floats or sinks in a fluid.

• If an object is less dense than the fluid, it floats.
• If an object is more dense than the fluid, it sinks.

This behavior is explained by Archimedes’ principle, which states that a fluid exerts an upward force on an object equal to the weight of the fluid displaced.

For example, wood floats in water because its density is less than 1 g/mL, while a rock sinks because its density is greater than that of water.

How Density Is Measured

Scientists measure density by determining mass and volume.

Regular-shaped objects:

• Measure mass using a balance
• Measure dimensions and calculate volume
• Use the density formula

Irregular-shaped objects:

• Measure mass with a balance
• Submerge the object in water
• Measure volume by water displacement

Liquids:

• Measure mass and volume using a graduated cylinder

Gases:

• Calculate density using known relationships or measure under controlled conditions

Density vs Related Terms

Students often confuse density with other related quantities.

Specific gravity has no units because it is a ratio.

Real-World Applications of Density

Density plays an important role in many everyday and scientific applications.

• Ship design relies on average density to ensure floating.
• Hot air balloons rise because heated air is less dense than cooler air.
• Oil floats on water because it has a lower density.
• Earth’s layers are arranged by density, with denser materials at the core.
• Scientists use density to identify unknown materials.

Density Column Explained

A density column forms when liquids of different densities are layered.

The most dense liquid settles at the bottom, while less dense liquids float above it. If the liquids do not mix, they form distinct layers.

For example, honey, water, oil, and alcohol form a layered column because each has a different density. Objects placed in the column will sink or float until they reach a layer with a similar density.

Common Student Mistakes

• Confusing mass with weight
• Forgetting to include or convert units
• Using inconsistent units in calculations
• Assuming larger objects are always more dense
• Forgetting that temperature affects density
• Assuming all solids are more dense than all liquids

FAQs

Why is the density of water 1 g/mL?
Pure water has a density of 1 g/mL at 4°C by definition. This value is used as a reference for other substances.

Why does ice float?
Ice floats because it is less dense than liquid water. When water freezes, it expands and forms an open crystal structure.

Does density depend on shape?
No. Density is an intensive property and does not depend on the shape or size of the sample.

Can density be negative?
No. Density cannot be negative because mass and volume are both positive quantities.

What is the densest element?
Osmium and iridium are the densest elements, with densities of about 22.5 g/cm³.

Common Misspellings

The two most common misspelling of density are dencity and densety.

References

• Serway, Raymond; Jewett, John (2005). Principles of Physics: A Calculus-Based Text. Cengage Learning. ISBN 0-534-49143-X.
• Sharma, P. V. (1997). Environmental and Engineering Geophysics. Cambridge University Press. ISBN 9781139171168 doi:10.1017/CBO9781139171168
• Young, Hugh D.; Freedman, Roger A. (2012). University Physics with Modern Physics. ISBN 978-0-321-69686-1.