Butene are a unsaturated hydrocarbons. Why?

We all know that, butene contains two carbon atoms connected each other with double covalent bond. It is chemically more reactive also. So, it is called unsaturated hydrocarbons.

What are alcohols? Write down their types and examples of each.

Alcohols are organic compound which contains hydroxyl group (OH). Its types are:
        > Monohydric alcohol ---------> Methanol
        > Dihydric alcohol ---------> Ethylene glycol
        > Trihydric alcohol ----------> Glycerol

What is meant by homologous series? Write down structural formula of 1st five members of homologous series.

The group organic compounds having similar structure and chemical properties in which the successive compounds differ by - CH₂ group is called homologous series.
The first five members are:
    Methane ( CH₄ )        
    Ethane ( C₂H₆ )        
    Propane ( C₃H₈ )        
    Butane ( C₄H₁₀...

Why should all carbon atoms be sp2 hybridized in an aromatic hydrocarbon?

Does a compound necessarily be a derivative of benzene in order to be an aromatic hydrocarbon?

No ! all the benzene derivatives are aromatic , but to be aromatic it's not necessary for a compound to be derivative of benzene. A compound needs to follow Huckel's rule to be aromatic

You ca take examples like Furan , Thiophene, Pyrrole etc.