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 carbocation intermediate. If HCl is used , the chloride `(Cl^-)` ion being a good nucleophile brings about substitution reaction to give an alkyl halide.
`{:(underset"Alcohol"(R-CH_2-CH_2-OH)underset(-H_2O)overset(H^+)to,underset"Carbocation"(R -CH_2-overset+CH_2),underset(-H^+)to underset"(Elimination product)"underset"Alkene"(R-CH=CH_2)),(,underset"(Substitution product)"underset"Alkyl chloride"overset(darr +Cl^-)(R-CH_2-CH_2-Cl),):}`
If `H_2SO_4` is used, then the bisulphate `(HSO_4^-)` ion being non-necleophilic cannot bring out the substitution reaction. Instead , the carbocation loses a proton to form an alkene. Conc. `HNO_3` is not used because it being a strong oxidising agent, preferentially brings about the oxidation of alcohols first to aldehydes or ketones and then to carboxylic acids. Thus, dehydration of alcohols to form alkenes is always carried out with such strong acids (e.g., `H_2SO_4 , H_3PO_4,H_3PO_4, KHSO_4`) which have non-nucleophilic anion)