Which of the following is not an indication of a chemical reaction?
1.) heat is generated when two substances are combined.
2.) a solid quickly forms when two liquids are combined.
3.) a substance melts when sitting in the sun.
4.) a substance sours when sitting in the sun.

Option (3) is the correct answer.

Explanation:

A reaction due to which there occurs change in chemical composition of reactants is known as a chemical reaction.

For example, evolution of heat when two substances combine together shows a chemical reaction has taken place between the substances.

Whereas a reaction which does not bring any changes in chemical composition of the reactants is known as a physical reaction.

For example, boiling point, melting point, mass, volume etc are all physical properties.

Thus, we can conclude that out of the given options a substance melts when sitting in the Sun, is not an indication of a chemical reaction.

c.

explanation: