Question

chemical reactions produce new substances with different properties than the substances that reacted. sometimes, the evidence for these new substances is easy to see. other times, it is not quite so obvious.

which of the following can be a clue that a chemical reaction has taken place?

i. bubbles form when two chemicals are mixed.
ii. a solution suddenly changes color.
iii. the chemicals are mixed at room temperature, and the solution heats up.

a. i and iii only
b. ii only
c. i and ii only
d. i, ii, and iii

Explanation:

Analyze statement I

The formation of bubbles indicates the production of a gas, which is a common indicator of a chemical change.

Analyze statement II

A sudden, unexpected color change indicates that a new substance with different optical properties has been formed.

Analyze statement III

An increase in temperature without external heating indicates an exothermic chemical reaction releasing energy.

Answer:

• (A) I and III only
• (B) II only
• (C) I and II only
• (D) I, II, and III (Correct answer)