Chemical and physical change

Chemical change

When a chemical reaction occurs, atoms are neither created nor destroyed. Instead, the atoms rearrange themselves to form new chemicals. This is known as a chemical change.

Physical change

The other kind of change in chemistry is a physical change, which is when a substance changes without becoming a new chemical. These physical changes are often changes in state such as melting, freezing, boiling, condensing and dissolving.

The differences between the two types of change are summarised below.

Physical changeChemical change
SubstancesNo new substances are formedNew substances (known as products) are formed when the original chemicals (known as reactants) react
Easy to reverse?It is usually easy to reverse the change (reversible)It is usually difficult to reverse the change (irreversible)
Energy change?Usually, there is no significant energy changeEnergy is always given out (exothermic) or taken in (endothermic) resulting in a temperature change
Colour change?Usually there is no colour changeThis can result in a colour change due to the formation of new products
Change of state?The substance usually changes stateNew products can have a different state to the reactants but not always

Changes in properties

Close up of the Statue of Liberty's head and shoulders with green discolouration visible on its copper surface

Change in colour

When copper reacts with certain gases in the air, bright copper changes to a green colour

Changes in energy

A close up of a burning match

Warming or cooling

During a chemical change, thermal energy is either absorbed or given off

Mass is never lost or gained in chemical reactions. It is said that mass is always conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning. This is because no atoms are created or destroyed during chemical reactions.

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