Which of these is an indication that a chemical reaction has occurred?

a. freezing of a substance
b. melting of a substance
c. boiling of a substance
d. formation of a gas

The correct option is D.

A chemical change is a type of change in which a new substance is formed and which is always irreversible. Chemical changes involves the breaking and forming of bonds and the original composition of the substances involved are usually altered. The indicators that show that a chemical change has occur include: change in temperature, change in colour, formation of precipitate and formation of gas bubbles.

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.