The dehydration of alcohols to form alkenes occurs when heated with protic acids like concentrated $H_2SO_4$ or $H_3PO_4$. The reaction proceeds via the following mechanism:

1. Protonation: The alcohol oxygen accepts a proton to form an alkyloxonium ion.
2. Carbocation Formation: Loss of a water molecule forms a carbocation intermediate. This is the slow, rate-determining step. The stability order of carbocations is $3^{\circ} > 2^{\circ} > 1^{\circ}$.
3. Elimination: A proton is removed from a $\beta$-carbon to form a double bond. According to Saytzeff's Rule (Zaitsev rule), the major product is the most highly substituted alkene (the one with the greater number of alkyl groups attached to the double-bonded carbon atoms).

If the starting material allows for carbocation rearrangement (e.g., hydride or methyl shift) to a more stable carbocation, the major product will be derived from the rearranged intermediate.