What is the first step in the reaction of dehydration of alcohols to form alkenes by heating with concentrated $H_2SO_4$?

Which of the following compounds on oxidation gives ethyl methyl ketone?

Ethanol cannot be dried by anhydrous $CaCl_2$ due to the formation of the following solvated product:

Which among the following compounds has the lowest boiling point?

$A$ substance $C_4H_{10}O$ yields on oxidation a compound $C_4H_8O$ which gives an oxime and a positive iodoform test. The original substance on treatment with conc. $H_2SO_4$ gives $C_4H_8$. The structure of the compound is:

Consider the following alcohols:
$(I)$ $C_6H_5CH_2OH$
$(II)$ $O_2N-C_6H_4-CH_2OH$ (para-nitro)
$(III)$ $CH_3O-C_6H_4-CH_2OH$ (para-methoxy)
$(IV)$ $Br-C_6H_4-CH_2OH$ (para-bromo)
The order of decreasing reactivities of these alcohols towards nucleophilic substitution with $HBr$ is: