Olive oil is water-insoluble because it is hydrophobic. Why is olive oil hydrophobic ?

Answer 1

The key to hydrophilic/hydrophobic is in the prefix to each--'hydro' or water. It's telling you that the focus is how these substances interact with water. So let's consider water first. Water is a simple molecule, H2O--two hydrogens, one oxygen. Molecule means the atoms are covalently bonded together (sharing electrons). HOW those electrons are shared is everything.

Oxygen 'pulls' harder on electrons than hydrogen does (technically: 'more electronegative'). So the electrons are 'with' the oxygen more often, and the oxygen atom is therefore somewhat negative because these electrons are negative. The hydrogen, 'robbed' of electrons, ends up a little positive. So this is water--every atom is either a little bit positive (hydrogens) or negative (oxygen). So two water molecules are likely to attract--positives on one can 'find' negatives on another.

Different story with molecules made of carbon and hydrogen, though! Carbon and hydrogen 'pull' equally on electrons, so nobody ends up with 'too little' or 'too much' electron, and all atoms remain uncharged or neutral. When a water molecule comes by, there is nothing 'of interest' on a neutral molecule; the two don't attract or connect. So another term for neutral molecules is 'hydrophobic' because water wants nothing to do with them.

So the answer to your question: olive oil is 'hydrophobic' (uncharged) because it is built from carbons and hydrogens and contains few or no oxygen or nitrogen atoms, which would create regions of charge.

Answer 2

Olive oil like every other hydrophobic substances is insoluble in water because the cohesive force between its molecules are greater than it adhesive force, making it molecules stick together even when mixed with water. Though they dissolve in organic solvents.