When two substances are mixed together, a color change is observed. does this mean that a chemical reaction has definitely occurred?

- yes because any change indicates a chemical reaction.
- no because some physical changes produce color changes.
- no because color changes are not indicators of chemical reactions.
- yes because color changes occur only when new compounds are formed.

No because some physical changes produce color changes.  

Explanation:-

A physical reaction is defined as the reaction in which a change in shape, size takes place. No new substance gets formed in these reactions.

Example: Addition of food colors to water is a physical process and lead o color change

A chemical reaction is defined as the reaction in which a change in chemical composition takes place. A new substance is formed in these reactions.

Example: Oxidation of magnesium leads to formation of white magnesium oxide and is a chemical change.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.