Most alcohols undergo dehydration to form alkenes, when heated with a strong acid. → Elimination reaction.

(Difficulty in dehydration) 1 $°$ > 2 $°$ > 3 $°$ . → This is due to stabilization of carbocation. (Rearrangement is possible.)

Dehydration of Secondary and Tertiary alcohols.

The mechanism is an $E 1$ reaction wherein the substrate is a protonated alcohol. → (Step 1)

The hydroxyl group is protonated (generally happens quickly) by a hydronium ion (or any other acid catalyst.)
This causes the formation of a very strong electron withdrawing group, $- O^{+} H_{2}$ . → (Step 2)
The C-O bond, weakened due to the electron withdrawing nature, breaks heterolytically. This forms a carbocation and is the rate determining step.
(Formation of carbocation generally always is the rate determining step.) Hence it is this step that determines the overall reactivity of alcohols toward dehydration. Since a 1 $°$ or a 0 $°$ carbocation is not stable (hyperconjugation), their corresponding alcohols are extremely difficult to dehydrate in this manner. (Free energies of activation for the formation of carbocations from protonated alcohols: tertiary < secondary << primary.) → (Step 3)
A water molecule removes a proton from the $β$ -carbon of the carbocation. (When removing this $β$ -Hydrogen keep in mind the number of $α$ -Hydrogens of the resultant double bond.)

Rearrangements

For the transition state, more stable carbocation is to be formed.
During the removal of $β$ -Hydrogen, the more favored product is dictated by the stability of the alkene being formed. (Zaitsev’s rule.)
Rearrangements where the carbocations have approximately equal energy are also possible.

Since $E 1$ mechanism for primary alcohols is relatively unstable, dehydration proceeds through $E 2$ mechanism.

The first step proceeds like that of $E 1$ and the hydroxyl group gets protonated.
Then with $- O^{+} H_{2}$ being a good leaving group, a Lewis base in the reaction mixture removes a $β$ -Hydrogen simultaneously with the formation of double bond and the departure of the protonated hydroxyl group.