Explain how amino acids can be separated and identified by paper/TLC chromatography, including intermolecular forces/solubility in mobile and stationary phase and retention (RF) values.

A student performs thin-layer chromatography (TLC) on two amino acids, leucine and serine, using a non-polar solvent as the mobile phase and a silica gel plate as the stationary phase. Leucine has a non-polar side chain, while serine has a polar hydroxyl group in its side chain. Explain why leucine travels further up the TLC plate than serine.