To dehydrate an alcohol (make it lose a water molecule) it needs to be heated in the presence of a suitable catalyst. Usually acids like H2SO4 or H3PO4 are used. Secondary and tertiary alcohols are dehydrated by an E1 mechanism since they can form stable carbocations, but watch out for rearrangements. Primary alcohols undergo E2 reactions since primary carbocations are unstable. During the dehydration of a primary alcohol a competing SN2 reaction will occur. If multiple alkenes can be formed, the most stable alkene will be the main product.
Δ, H₃PO₄
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Practice