Tuesday

Rocket arrives at the Bench with a list in his fist. "Four clues," he announces. "If I see one, it is a chemical change. Done."

Raven reads the list and frowns. "What do you notice about the ice cube from yesterday? It took in energy from the room and melted."

"So energy moved," Rocket says. "That is a clue. So melting is chemical?"

"It is still water," Raven says. "We checked."

Nova dims her light thoughtfully. "A clue is a reason to look closer," she says. "It is not a verdict. What would a verdict need?"

Rocket taps the list. "Proof that the stuff is different afterward." He adds a fifth line at the bottom and underlines it twice.

How can a scientist tell if a chemical reaction is taking place? There are four visual clues that a reaction is likely occurring.

First, a change in color. Second, a gas is produced. Third, a solid product called a precipitate forms and settles out of a liquid.

Fourth, a transfer of energy. When zinc reacts with hydrochloric acid in a laboratory, the test tube becomes very warm. Other reactions absorb energy.

A precipitate is a solid product that forms from a reaction and settles out of a liquid solution.

Clues are not proof. Physical changes also involve a transfer of energy. Melting a solid absorbs energy, and condensing a gas releases energy.

The only way to be certain that a chemical reaction has taken place is to test the substances after the change.

If their properties are different from the starting substances, new substances formed. That is the evidence this week's standard asks for.

So the Bench rule is: see a clue, then check the properties.

Sharp thinking. Tomorrow is Bench Lab: four changes, four verdicts, and your own data table.