Physical and Chemical Properties

Scientists classify properties of matter as physical or chemical. Physical properties are those we can observe without altering the identity of the substance. For example, the melting or boiling point of a substance is a physical property because they do not alter the identity of the substance. 

We can only observe chemical properties when we alter the identity of the substance. Rusting is an example of a chemical property because it is a chemical reaction that changes the composition of the substance.

Read this text. Pay attention to the chart at the end of the section, which gives examples of some physical and chemical properties of the element sodium (Na).

Since chemistry is partly the study of the transformations that matter can undergo, we can also assign to any substance a set of chemical properties that express the various changes of composition the substance is known to undergo. Chemical properties also include the conditions of temperature, etc., required to bring about the change, and the amount of energy released or absorbed as the change takes place.

The properties that we described above are traditionally known as physical properties, and are to be distinguished from chemical properties that usually refer to changes in composition that a substance can undergo.

For example, we can state some of the more distinctive physical and chemical properties of the element sodium:

Physical Properties (25°C)
Chemical Properties

  • Appearance: a soft, shiny metal
  • Density: 0.97g cm3
  • Melting point: 97.5°C
  • Boiling point: 960°C

  • Forms an oxide Na2O and a hydride NaH
  • Burns in air to form sodium peroxide Na2O2
  • Reacts violently with water to release hydrogen gas
  • Dissolves in liquid ammonia to form a deep blue solution

Another Dubious Dichotomy?

The more closely you look at the distinction between physical and chemical properties, the more blurred this distinction becomes. For example, the high boiling point of water compared to that of methane, CH4, is a consequence of the electrostatic attractions between O-H bonds in adjacent molecules, in contrast to those between C-H bonds; at this level, we are really getting into chemistry!

So although you will likely be expected to distinguish between physical and chemical properties on an exam, do not take it too seriously – this turns out to be a rather dubious dichotomy, beloved by teachers, but of limited usefulness!

Source: Stephen Lower,
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Last modified: Monday, November 29, 2021, 5:50 PM