Question

1. what is the only sure sign of a chemical change?

Explanation:

A chemical change involves the formation of new substances. While signs like color change, gas production, or temperature change can indicate a chemical change, they can sometimes occur in physical changes too. The formation of a new substance (detected by a chemical reaction producing a substance with different chemical properties) is the only definitive sign, as it implies a change in the chemical composition.

Answer:

The only sure sign of a chemical change is the formation of a new substance (through a chemical reaction that creates a substance with distinct chemical properties from the reactants).