Here's why:

* Compounds are formed when two or more different elements chemically combine in a fixed ratio. The chemical bonds holding these elements together are strong and require chemical reactions to break them.

* Physical changes only alter the appearance or state of matter, not the chemical composition. Examples include melting ice (solid to liquid), crushing a rock, or dissolving sugar in water.

* Chemical changes involve the formation of new substances with different properties.

Examples of compounds:

* Water (H₂O) is made of hydrogen and oxygen.

* Salt (NaCl) is made of sodium and chlorine.

* Sugar (C₁₂H₂₂O₁₁) is made of carbon, hydrogen, and oxygen.

You can't break down a compound like water into hydrogen and oxygen simply by freezing it, boiling it, or crushing it. You need a chemical reaction, like electrolysis, to separate the elements.