Substance in Chemistry: Definition, Types & Properties

Chemistry can be confusing but it often becomes easier to understand when we can clearly identify and classify the matter around us. Let us begin by understanding what a substance is today.

A substance is a form of matter with a definite composition and characteristic properties. In chemistry, the term generally refers to a pure substance, which has a constant composition throughout a sample. Pure substances are classified as elements or compounds. An element contains only one type of atom and cannot be broken down into simpler substances by ordinary chemical changes. A compound contains two or more different elements chemically combined in a fixed proportion.
For example, iron, oxygen and copper are elements, while water and sodium chloride are compounds. A mixture such as salt water is not a pure substance because its composition can vary.

In basic chemistry, substances are broadly classified into elements and compounds.
ElementsAn element is a pure substance made up of only one type of atom. It cannot be decomposed into simpler substances through a chemical change. Each element has its own chemical symbol and characteristic properties.
Examples include:
• Hydrogen (H)
• Oxygen (O)
• Iron (Fe)
• Copper (Cu)
• Gold (Au)
Elements may exist as individual atoms or as molecules containing atoms of the same element. For example, oxygen gas exists as O₂ molecules but is still classified as an element because each molecule contains only oxygen atoms.
CompoundsA compound is a pure substance formed when atoms of two or more different elements combine chemically in a fixed ratio. A compound can be broken down into simpler substances through chemical changes.
Examples include:
• Water (H₂O): Contains hydrogen and oxygen in a fixed ratio.
• Carbon dioxide (CO₂): Contains carbon and oxygen.
• Sodium chloride (NaCl): Contains sodium and chlorine.
• Ammonia (NH₃): Contains nitrogen and hydrogen.
A compound generally has properties different from those of the elements from which it is formed.

The properties of a substance can be studied through physical and chemical characteristics. These properties help identify a substance and understand how it behaves under different conditions.
Physical PropertiesA physical property can be observed or measured without changing the chemical composition of the substance. Common examples include:
• Colour
• Odour
• Density
• Melting point
• Boiling point
• Electrical conductivity
• Physical state
• Hardness
For a pure substance, characteristic intensive properties such as melting point, boiling point and density do not depend on the amount of the sample.
Physical properties can also be classified as intensive or extensive:
• Intensive properties: Do not depend on the amount of substance. Colour, density and boiling point are examples.
• Extensive properties: Depend on the amount of substance. Mass and volume are examples.
Chemical PropertiesA chemical property describes how a substance behaves when it undergoes a chemical change. Examples include flammability, reactivity with acids, oxidation and the ability to undergo decomposition or other chemical reactions. Observing a chemical property involves the formation of one or more different substances.

Examples of substances can be found in laboratories, natural materials and everyday products. Some common examples are:

Different substances are used according to their physical and chemical properties. Their applications range from biological processes to construction, manufacturing, and technology.
• Oxygen: Used in medical oxygen supply and supports combustion.
• Iron: Used to manufacture steel and in construction, machinery and tools.
• Copper: Its high electrical conductivity makes it useful for electrical wiring and components.
• Water: Used for drinking, sanitation, agriculture, industrial processes and many chemical reactions.
• Sodium chloride: Used as food seasoning and preservation and in several industrial processes.
• Carbon dioxide: Used in carbonated drinks and some fire-extinguishing systems.
• Ammonia: Used extensively in the production of nitrogen fertilisers.
The usefulness of a substance depends on its specific properties. For example, copper's electrical conductivity makes it suitable for electrical applications, while iron's mechanical properties make it useful in structural materials.

Understanding a substance begins with recognising whether it is a pure substance or a mixture. Pure substances have a constant composition and are classified as elements or compounds. Their physical and chemical properties help distinguish one substance from another and determine how each can be used. Knowing these classifications also helps students identify materials correctly and understand how substances behave during physical and chemical changes.