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.
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 change||Chemical change|
|Substances||No new substances are formed||New 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 change||Energy is always given out (exothermic) or taken in (endothermic) resulting in a temperature change|
|Colour change?||Usually there is no colour change||This can result in a colour change due to the formation of new products|
|Change of state?||The substance usually changes state||New products can have a different state to the reactants but not always|
Change in colour
Warming or cooling
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.