Dehydration of alcohol to form an alkene is always carried out with concentrated `H_(2)SO_(4)` and not with concentrated `HCl` or `HNO_(3)`. Explain.

Explanation

Dehydration of alcohols to alkenes occurs through the formation of carbocation intermediate. If HCl is used then chloride (Cl) on being a good nucleophile will result into substitution reaction forming alkyl chloride.
`underset(("Elimination product"))underset("Alkene")(R-CH=CH_(2)) overset(HSI_(4)^(-)) underset(-H_(2)SO_(4))larr underset("Carbocation")(R-CH_(2)-CH_(2)^(+)) overset(Cl^(-))to underset(("Substitution proudct"))underset("Alkyl halide")(R-CH_(2)-CH_(2)Cl)`
However, if `H_(2)SO_(4)` isused, then the bisulphate `(HSO_(4)^(-))` ion being very weak nucleophile cannot result into substitution reaction. Rather the carbocation loses a proton to form elimination product alkene. On the other hand, conc. `HNO(3)` also cannot be used because it is a strong oxidising Agent and will preferentially result into the oxidation of alcohols to aldehydes, ketones or carboxylic acids. Thus, the dehydration of alcohols to form allenes is always carried out with such strong acids `(H_(2)SO_(4), H_(3)PO_(4)` etc.) which have non-nucleophilic anions.