Here's why:

* Chemical Change: A chemical change involves the formation of new substances with different chemical properties. These changes are usually irreversible. Examples:

* Burning wood: Wood changes into ash, smoke, and gases.

* Rusting iron: Iron reacts with oxygen to form iron oxide (rust).

* Physical Change: A physical change alters the appearance or form of a substance but doesn't change its chemical composition. These changes are often reversible. Examples:

* Melting ice: Water changes state from solid to liquid but remains H₂O.

* Dissolving sugar in water: Sugar disappears but is still present as sugar molecules in the water.

To determine if a change is chemical or physical, consider these factors:

* Formation of new substances: If a new substance is formed with different properties, it's a chemical change.

* Irreversibility: If the change is difficult or impossible to reverse, it's likely a chemical change.

* Energy change: Chemical changes often involve energy release (like heat or light) or absorption.

Example:

* Burning a candle: The candle wax melts (physical change), then burns (chemical change), producing carbon dioxide, water vapor, and soot (new substances).

In conclusion: A change in appearance can be a clue to a chemical change, but it's not a definitive indicator. You need to consider other factors to determine the true nature of the change.