Here's why:

* Chemical Reactions: Involve the breaking and forming of chemical bonds between atoms. This results in a change in the arrangement of atoms, leading to the formation of entirely new substances with different properties.

* Physical Changes: Only alter the appearance or state of a substance (e.g., melting ice), but the chemical composition remains the same.

Examples of Chemical Reactions:

* Burning wood (wood + oxygen -> ash + carbon dioxide + water)

* Baking a cake (flour + eggs + sugar + butter -> cake)

* Rusting of iron (iron + oxygen -> iron oxide)

Let me know if you'd like more examples or want to explore a specific chemical reaction!