Physical and chemical changes

Physical change

A physical change is when a substance changes its state (solid, liquid, gas) or appearance, but does not form a new substance. The substance stays the same, only its form changes.

Example: water

1. Ice is water in a solid state. When you heat ice, it melts and turns into liquid water.
2. If you heat liquid water, it boils and becomes water vapor, which is a gas.
3. If you cool water vapor, it turns back into liquid water, and if you cool liquid water, it freezes to form ice again.

All these changes are physical because water (whether ice, liquid, or vapor) is still the same substance. These changes just change the state of matter (solid, liquid, or gas). You can change water back and forth from one state to another by heating or cooling it.

Chemical change

A chemical change is when a substance changes into a completely new substance that cannot be changed back into the original substance. During this change, the chemical structure of the substance is altered.

Examples of chemical changes

1. Rusting of iron: When iron is exposed to water and air, it reacts and forms rust. Rust is a new substance, and you can't turn it back into the original iron.
2. Souring of milk: When milk goes bad, it forms a sour smell and taste, and this is a new substance.
3. Burning paper: When you burn a piece of paper, it turns into ash and smoke, which are new substances. You cannot turn ash back into the original paper.

Differences between physical and chemical changes

Physical change	Chemical change
No new chemical substance is formed.	A new chemical substance is formed.
The original substance can be easily regenerated after the change because the change is temporary (like freezing and melting).	A new substance cannot be changed back to the original substance because the change is permanent (like burning paper).
The mass of the substance before and after the change is the same (like when ice melts into water).	The mass of the substance after the change is not the same (like when a piece of wood burns and turns into ash).
A little energy is used or produced (like when you heat water to make it boil).	A lot of energy is used or produced (like when paper burns and releases heat and light).
The substance before and after the change has the same properties (like when you tear a piece of paper, it's still paper).	The new substance has different properties from the original substance (like when milk turns sour, it tastes and smells different).