Physical and Chemical Change

≈ 25 min

A physical change changes only the appearance or state — no new substance forms and it is usually reversible. Melting ice, dissolving sugar, tearing paper, boiling water.
A chemical change (a reaction) forms one or more new substances and is usually hard to reverse. Signs: a colour change, a gas produced, a precipitate, heat/light given off. Burning wood, rusting iron, baking bread.

A precipitation reaction happens when two solutions mix and an insoluble solid (a precipitate) forms. Example (reactions in aqueous solution):

\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)

The white $\text{AgCl}$ solid is the new substance — proof of a chemical change.

Which of these is a chemical change?

Melting candle wax

Iron rusting

Dissolving salt in water

Crushing a can

When $\text{AgNO}_3(aq)$ is added to $\text{NaCl}(aq)$ , a white solid appears. This white solid is called a…

filtrate

precipitate

solvent

mixture

Boiling water to make steam — is this a physical or a chemical change? (one word)