Reaction & Changes

Physical changes

When a substance undergoes physical changes, its physical composition changes but its chemical composition remain the same.

Eg: Ice, water and water vapour may exist in different physical states - solid, liquid and gas respectively - but they are still made up of the same chemical composition, H₂O.

Also, physical changes are reversible.

Eg: Water turns into ice when cooled. To get back the water, heat the ice.

Forces like temperature and pressure can effect physical changes.


Chemical changes

The chemical composition of a substance is changed when it undergoes chemical changes. As a result, a new substance is formed. The process is irreversible.

So, how can a chemical change in a substance be identified?

• Change in colour.
• Release or absorption of energy or heat.
• Release of odour.
• Production of gases or solids.

Note: In a chemical reaction, matter is neither destroyed nor created. The particles of an original substance are rearranged to form a new substance. The number of particles that exists before and after the reaction is the same.

Exothermic vs Endothemic

In chemical reactions, heat is either released (exothermic reaction) or absorbed (endothermic reaction). Any change of physical state from solid to liquid is an endothermic reaction as heat is absorbed to push the particles further apart to change its physical state.

However, any change of physical state from liquid to solid is an exothermic reaction as heat is released to push the particles closer together.

To determine whether a reaction is endothermic or exothermic, use a thermometer.