* New substances are formed: When sodium reacts with water, it produces sodium hydroxide (NaOH) and hydrogen gas (H2). These are entirely different substances from the original sodium and water.

* Energy is released: The reaction is exothermic, meaning it releases heat and often causes the hydrogen gas to ignite.

* The change is irreversible: You can't easily get back the original sodium and water from the resulting sodium hydroxide and hydrogen gas.

In contrast, a physical change involves a change in form or appearance but not in the chemical composition of the substance. For example, melting ice is a physical change because the water molecules remain the same, just in a different state of matter.