Physical Properties of Matter

Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.

The physical properties of matter are any properties that can be perceived or observed without changing the chemical identity of the sample. In contrast, chemical properties are those that can only be observed and measured by performing a chemical reaction, thus changing the molecular structure of the sample.

Because physical properties include such a wide array of characteristics, they are further classified as either intensive or extensive and either isotropic or anisotropic.

Intensive and Extensive Physical Properties

Intensive physical properties do not depend on the sample's size or mass. Examples of intensive properties include boiling point, state of matter, and density. Extensive physical properties depend on the amount of matter in the sample. Examples of extensive properties include size, mass, and volume.

Isotropic and Anisotropic Physical Properties

Isotropic physical properties do not depend on the orientation of the specimen or direction from which it is observed. Anisotropic properties depend on the orientation. While any physical property could be assigned as isotropic or anisotropic, the terms are usually applied to help identify or distinguish materials based on their optical and mechanical properties.

For example, one crystal might be isotropic with respect to color and opacity, while another might appear a different color depending on the viewing axis. In a metal, grains might be distorted or elongated along one axis compared with another.

Examples of Physical Properties

Any property you can see, smell, touch, hear, or otherwise detect and measure without performing a chemical reaction is a physical property. Examples of physical properties include:

• Color
• Shape
• Volume
• Density
• Temperature
• Boiling point
• Viscosity
• Pressure
• Solubility
• Electric charge

Physical Properties of Ionic vs. Covalent Compounds

The nature of chemical bonds plays a role in some physical properties displayed by a material. The ions in ionic compounds are strongly attracted to other ions with opposite charge and repelled by like charges. Atoms in covalent molecules are stable and not strongly attracted or repelled by other parts of the material. As a consequence, ionic solids tend to have higher melting and boiling points compared with the low melting and boiling points of covalent solids.

Ionic compounds tend to be electrical conductors when they are melted or dissolved, while covalent compounds tend to be poor conductors in any form. Ionic compounds are usually crystalline solids, while covalent molecules exist as liquids, gases, or solids. Ionic compounds often dissolve in water and other polar solvents, while covalent compounds are more likely to dissolve in nonpolar solvents.

Chemical Properties

Chemical properties encompass characteristics of matter that can be observed only by changing the chemical identity of a sample—examining its behavior in a chemical reaction. Examples of chemical properties include flammability (observed from combustion), reactivity (measured by readiness to participate in a reaction), and toxicity (demonstrated by exposing an organism to a chemical).

Chemical and Physical Changes

Chemical and physical properties are related to chemical and physical changes. A physical change alters only the shape or appearance of a sample and not its chemical identity. A chemical change is a chemical reaction, which rearranges a sample on a molecular level.