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.

geosphere and hydrosphere.

this is because the geosphere has to do with geological events, while the hydrosphere controls all water on earth.