Physical and Chemical Changes: Understand the differences and concepts

Physical and Chemical Changes: Understand the concepts and differences. Find study notes, questions and answers.

Explanation of physical change:

Physical changes are usually about physical states of matter. It is a type of change in which the form of matter is altered but one substance is not transformed into another. The size or shape of matter may be changed. They are usually reversible. No energy change takes place.

Explanation of Chemical change:

Chemical changes happen on a molecular level when two or molecules interact. It happens when atomic bonds are broken or created during chemical reactions. Most chemical changes are irreversible. Chemical change is generally accompanied with either liberation or absorption of heat.

Common phenomenon observed in nature having physical changes are:
Freezing, melting, vapourisation, condensation, sublimation and dissolution.

Some chemical changes are: Rusting, burning and curdling.

• Exothermic reaction: It is a chemical reaction that releases energy through light or heat.
• Endothermic reaction: It is a chemical reaction that is accompanied by the absorption of heat.

Here are some examples to explain these changes.

Digestion of food is a chemical change because when food is digested in our body, numerous enzymes take part in the process. Complex food is converted into simplest form with different chemical composition and the change is irreversible.

Changing of water to water vapour is considered a physical change because when we cool down water vapour we get back water. This change is reversible and temporary, so it is a physical change.

When we mix cement with water it hardens and we cannot get back cement powder again, as it is a permanent change. Hence, it is irreversible.

Useful Definitions

• Melting- Melting or fusion is liquefaction process in which a solid is converted to liquid after receiving heat.
• Sublimation- Sublimation is the process of conversion of a solid directly into gaseous state.
• Rusting- Rusting is the process of continues disintegration of iron with respect to time.
• Endothermic reaction- A chemical reaction in which heat is absorbed, is called endothermic reaction.
• Undesirable change- Changes that are not beneficial to us.

Useful Links

Physical and Chemical Changes is a topic covered in Class 7 Chemistry subject.