1. Formation of a new substance: This is the most fundamental indicator. If the chemical composition of the starting materials has changed, a new substance has been formed. You can identify this by:
* Change in color: Rust forming on iron (reddish-brown) is a classic example.
* Appearance of a precipitate: When solutions are mixed and a solid forms, it's a sign of a new substance.
* Evolution of gas: Bubbles forming indicate a new gas is being produced.
* Change in odor: The smell of burning wood (a chemical change) is distinct from the smell of wood itself.
2. Release or absorption of energy: Chemical changes often involve the release or absorption of energy, which you can detect as:
* Heat change: A reaction that releases heat is called exothermic (burning wood). A reaction that absorbs heat is called endothermic (dissolving some salts in water).
* Light emission: A chemical reaction that produces light, like a firefly or a glow stick.
Important Note: While these are strong indicators, it's essential to remember that sometimes physical changes can also exhibit some of these characteristics. For example, melting ice (physical change) absorbs heat, but it doesn't form a new substance.
