Is Density a Physical or Chemical Property? A Clear Explanation

Density is a physical property. It describes how much mass a substance has in a given volume, and it can be measured without changing the substance’s chemical identity. Measuring a copper sample, for instance, does not turn the copper into a different substance.

A chemical property describes how a substance can change into something new. Flammability and reactivity with acid are examples. Observing these properties may involve a chemical reaction that forms different substances. Measuring density does not require that kind of change.

Why density is physical

Physical properties can be observed or measured without changing a substance’s composition. Common examples include color, melting point, boiling point, mass, volume, and density LibreTexts Chemistry.

Density is calculated by dividing mass by volume:

density = mass ÷ volume

For example, a sample with a mass of 54 grams and a volume of 20 cubic centimeters has a density of 2.7 grams per cubic centimeter. The calculation uses measurements of the sample and does not change what the sample is.

The distinction depends on chemical identity, not on how easy a property is to measure or whether it requires a calculation. Measuring density involves weighing the sample and measuring its volume, but neither step ordinarily turns the substance into something new. The method must suit the sample. For example, a solid that dissolves in water cannot be measured accurately by submerging it and recording the water displacement.

Chemical changes produce substances with different chemical identities. Burning wood produces materials such as gases and ash, while rusting changes iron into iron oxides Khan Academy. To tell whether a change is chemical, ask whether the substance’s composition changes. Measuring density does not require a change in composition.

Density can change while remaining a physical property

A substance’s density is not always a single fixed number. It can vary with conditions, especially temperature and pressure, and it can differ between physical states. These changes do not automatically make density a chemical property.

For example, heating a gas usually makes it expand. If its mass stays the same while its volume increases, its density decreases. The gas changes physically, but its chemical identity can stay the same. Water and ice also have different densities, although both are made of H₂O. Freezing changes water’s physical state, not its chemical formula.

A physical property’s value can change when conditions change, while the property remains physical. A change in density alone does not show that a chemical reaction has occurred. To determine whether a change is chemical, look for a change in composition, not just a change in appearance, state, temperature, or measured density.

How density is measured

The basic method is to measure a sample’s mass and volume, then divide the mass by the volume. The units depend on the measurements. Grams per cubic centimeter (g/cm³) and grams per milliliter (g/mL) are common for solids and liquids. Since a milliliter and a cubic centimeter represent the same volume, those units can be used interchangeably in this context.

For a regular solid, calculate its volume from its dimensions. For an irregular solid that does not react with or dissolve in water, use water displacement: measure the water level, submerge the object, and measure the change in level. That difference is the object’s volume. Materials that react with the liquid, dissolve in it, or trap air need a different method.

Density is often called an intensive property. This means it does not depend on the amount of a uniform sample. If the sample is divided, its mass and volume both decrease, while its density stays the same under the same conditions. This classification does not make density chemical. “Intensive” describes how a property relates to sample size; “physical” describes whether measuring or observing it changes chemical identity. The terms answer different questions.

Key Takeaways

• Density is a physical property because it can be measured without changing a substance’s chemical identity.
• Density equals mass divided by volume.
• Density can vary with temperature, pressure, or physical state without becoming a chemical property.
• Chemical properties describe how substances can form new substances, as in burning or rusting.
• Use a measurement method that does not react with or dissolve the sample.