Here's why:
* Chemical reactions involve the breaking and forming of chemical bonds. This means the arrangement of atoms within molecules changes.
* The atoms themselves don't disappear or change into other elements. However, they rearrange to create new substances with different properties.
Example:
* Burning wood: Wood (made of cellulose, a complex carbohydrate) reacts with oxygen. The chemical bonds in both substances break, and new bonds form to create carbon dioxide, water, and ash. The composition of the wood and oxygen has fundamentally changed.
Important Note: While the composition of matter *always* changes, the total mass of the reactants and products remains the same (Law of Conservation of Mass).
