Properties are the characteristics that give a substance its identity and enable us to distinguish one kind of substance from another. No two substances have exactly the same set of properties. To identify a substance, chemists observe two distinct types of properties: physical and chemical properties.

Chemical properties are characteristics that describe the ability of a substance to form new substance. Examples of chemical properties include burning of a substance and rusting of iron.

Physical Properties Detected by Sense Organs

The properties of the substance you observed in the above activity are properties that can be perceived by the sense organs. Some examples are color, odor and taste.

Color: The color of a substance results from its interaction with light. Substances can be identified by their colors. For example, chalk is white, water is colorless, gold is yellow and so on.

Odor: Refers to the property of a substance perceived by the sense of smell. Terms commonly used to describe the odor of a substance are pungent, fragrant, spicy, fruity and odorless. For example, water is odourless, flowers are fragrant, orange smells fruity.

Measurable Physical Properties

Measurable physical properties are the properties of a substance that can be measured using an appropriate apparatus. Examples are melting point, boiling point, density and electrical conductivity.

Melting Point: The temperature at which a solid substance changes to its liquid state. For example, ice is the solid form of water.

Boiling Point: The temperature at which the vapor pressure of the liquid equals the surrounding atmospheric pressure.

Electrical Conductivity: Electrical conductivity is the ability of a substance to conduct electricity.

Changes around Us

The substances in our environment constantly undergo changes. The decay of plants and animals, the burning of gasoline, the evaporation of lakes and seas, and the melting of snow are some of the changes that occur in our environment.

Physical change: Changes that do not alter the composition of the substances are called physical changes.

Characteristics of a Physical Change

• It is a change in the physical property of the substance.
• No new substance is formed (the identity of the substance does not change).
• The change is easy to reverse (It is a reversible process).
• No change in the mass of the substance is involved.
• No change in composition occurs.
• Energy changes (heat changes) are not necessarily involved.

Chemical Changes: After burning, the composition of the sample is changed because it no longer exists as a paper. Such type of change that results in the formation of new substances with new properties and composition is known as a chemical change

Characteristics of a Chemical Change

• The change in which the molecular composition is completely altered and a new product is formed is called a chemical change.
• Chemical changes create a new product.
• Chemical changes are irreversible and permanent.
• The mass of the substance is altered during a chemical change. Either the mass is added or removed.