* New substances are formed: When sodium or potassium reacts with water, it produces sodium hydroxide (NaOH) or potassium hydroxide (KOH), hydrogen gas (H2), and heat. These are entirely different substances from the original sodium or potassium and water.
* Irreversible: The reaction is irreversible. You can't simply separate the products back into sodium/potassium and water.
* Energy change: The reaction releases significant amounts of heat, often causing the hydrogen gas to ignite and explode. This energy change is a strong indicator of a chemical reaction.
In contrast, a physical change would involve a change in the state or appearance of the substance, but not its chemical composition. For example, melting ice is a physical change because it only changes the state of water from solid to liquid. The chemical formula (H2O) remains the same.
