Final answer:
Alcohols spread on water because they can form hydrogen bonds with water molecules, while hydrocarbons cannot due to their nonpolar nature.
Step-by-step explanation:
The reason alcohols spread on water while hydrocarbons like various decanes do not is due to the presence of an OH group in alcohols which allows for hydrogen bonding with water molecules. This affinity for water decreases as the carbon chain length increases, making longer alcohols less soluble in water. In contrast, hydrocarbons like decane are nonpolar and only associated through weak dispersion forces and thus do not dissolve in water, which is a polar solvent.
Hydrogen bonding is significant because it requires a relatively large amount of energy to break these intermolecular attractions, contributing to alcohols' higher boiling points and solubility in water when compared to hydrocarbons of similar molar mass. Short-chain alcohols, up to four carbon atoms, are highly soluble in water due to their ability to engage in hydrogen bonding, while larger alcohols and hydrocarbons, lacking this ability, become increasingly insoluble.