Which of the following substances would have the greatest solubility in water?   hexane (CH3CH2CH2CH2CH2CH3)   carbon dioxide (CO2)   butane (CH3CH2CH2CH3)   methanol (CH3OH)

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t-nez eNotes educator| Certified Educator

Of these substances methanol, CH3OH, has the greatest solubility in water because it forms hydrogen bonds with water. Hydrogen bonding is a strong intermolecular attraction that occurs between molecules that have an H bonded to an F, O or N. These bonds are very polar, resulting in a dipole. In both water and methanol the oxygen has a slightly negative charge and the hydrogen bonded to it has a slightly positive charge because the electrons are more attracted to oxygen. The positive ends of methanol molecules are attracted to the negative end of water molecules, and vice versa. Solubility in all proportions occurs because the solvent-solute interactions are similar in strength to the solute-solute and solvent-solvent interactions.

Hexane and butane are both non-polar so neither dissolves well in water.  Both have stronger solute-solute than solvent-solute interactions due to dispersion forces, which are caused by temporary dipoles that arise from random movement of electrons. Carbon dioxide is also a non-polar molecule and is a gas. Its solubility in water is about 0.034 moles/Liter at 25 degrees C and 1 atm pressure. This varies with temperature and pressure but is limited unlike the solubility of methanol in water.