Hydrocarbons are compounds created when carbon and hydrogen atoms combine together. Carbon has 4 free electrons in the outer most shell and hydrogen has one free electron. Hydrogen forms single bonds with carbon. There are three different types of bonds that can form between two carbon atoms, single bond, double bond and triple bond. Hydrocarbons with only single bonds are known as alkanes, those with at least one double bond are known as alkenes and if there is a triple bond the compound is known as an alkyne.
Octane is a alkane with 8 carbon atoms and the chemical formula C_8H_18. Bromine water is brown in color. When an alkene is added to bromine water, bromine is added to the compound with double bonds breaking open. This causes the bromine water to lose its brown color. Alkanes on the other hand do not react with bromine water as there is no double bond present.
When bromine water is added to octane no color change is expected to take place.
Alkanes e.g. octane do not react with bromine. So the red color of bromine will remain unchanged. However alkenes do undergo addition reaction with bromine.
Bromine is used to check whether a hydrocarbon(an organic compound made of only hydrogen and carbon) is an alkane(single bond) or an alkene(carbon- carbon double bond). Bromine reacts differently with both. A bromine solution is originally brown in color. Adding bromine water to an alkane would not cause any color change, so the color of the solution remains brown. Whereas if you add an alkene to bromine water, the solution becomes colorless. Therefore, when bromine water is added to an octane, the solutions color stays the same (brown).
When alkenes combine with a colored substance like bromine water (brown) they will create a colorless solution. When bromine is added to an alkene double bond it is known as an addition reaction. Usually an addition reaction takes place using bromine and causes the reddish brown color of the 'bromine solution' to disappear.
Alkenes do not react with bromine, so octane will not cause any colour change. However, the addition reaction causes the bromine solution to turn colorless.