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.

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !