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 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 |
| Substances | |
|---|---|
| Physical change | No new substances are formed |
| Chemical change | New substances (known as products) are formed when the original chemicals (known as reactants) react |
| Easy to reverse? | |
|---|---|
| Physical change | It is usually easy to reverse the change (reversible) |
| Chemical change | It is usually difficult to reverse the change (irreversible) |
| Energy change? | |
|---|---|
| Physical change | Usually, there is no significant energy change |
| Chemical change | Energy is always given out (exothermic) or taken in (endothermic) resulting in a temperature change |
| Colour change? | |
|---|---|
| Physical change | Usually there is no colour change |
| Chemical change | This can result in a colour change due to the formation of new products |
| Change of state? | |
|---|---|
| Physical change | The substance usually changes state |
| Chemical change | New products can have a different state to the reactants but not always |
Changes in properties
Image caption, Change in colour
When copper reacts with certain gases in the air, bright copper changes to a green colour
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Changes in energy
Image caption, Warming or cooling
During a chemical change, thermal energy is either absorbed or given off
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The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). is never lost or gained in chemical reactions. It is said that mass is always conserved. In other words, the total mass of A substance formed in a chemical reaction. at the end of the reaction is equal to the total mass of the A substance that reacts together with another substance to form products during a chemical reaction. at the beginning. This is because no All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. are created or destroyed during chemical reactions.