Oil and water are famously immiscible. Which explanation best captures why, in terms of intermolecular forces?
AOil molecules are too large to fit between water molecules
BWater molecules form strong hydrogen bonds with each other; accommodating nonpolar oil would cost more energy than nonpolar oil-water interactions can provide
COil and water have different densities, so they naturally separate by gravity
DOil molecules carry a net negative charge that repels water's partial charges
Dissolution is an energy competition. Water's hydrogen-bonding network is highly stabilized. To dissolve oil, you'd have to disrupt that network and get back enough energy from oil-water interactions to compensate — but nonpolar oil can only offer weak London dispersion forces to water, nowhere near enough. The density/gravity explanation (C) describes the result, not the cause: even if you force them to mix, they separate because the thermodynamics are unfavorable.
Question 2 Multiple Choice
When NaCl dissolves in water, each Na⁺ ion ends up surrounded by a hydration shell. How are water molecules oriented in that shell?
ARandomly — the orientation depends on local turbulence
BWith hydrogen atoms pointing toward Na⁺, forming hydrogen bonds with it
DWith oxygen atoms pointing away from Na⁺ to minimize repulsion
Na⁺ is a positive ion. Water's oxygen atom carries a partial negative charge (δ−), so it orients toward the cation to form an attractive ion-dipole interaction. The hydrogen atoms (δ+) orient toward anions like Cl⁻. This directional arrangement — not random orientation — is precisely what solvation means: a structured stabilizing shell formed by favorable electrostatic alignment.
Question 3 True / False
Dissolution is generally exothermic — it releases heat as solute-solvent interactions form.
TTrue
FFalse
Answer: False
Many substances dissolve endothermically (absorbing heat). Ammonium nitrate dissolving in water is a classic example — the pack gets cold. Dissolution is thermodynamically favorable when the overall free energy decreases, which requires considering both enthalpy (energy of breaking and forming interactions) and entropy (increase in disorder). Endothermic dissolution can proceed spontaneously when the entropy gain is large enough. The energy gained from solute-solvent interactions need not exceed the energy required to separate solute and solvent particles.
Question 4 True / False
Grease dissolves in mineral spirits (a nonpolar solvent) but not in water because grease molecules can only form London dispersion interactions.
TTrue
FFalse
Answer: True
Grease is a nonpolar substance and can only participate in London dispersion forces — it has no permanent dipoles, no -OH groups, no charged regions. Mineral spirits is also nonpolar, so grease-mineral spirits dispersion interactions are comparable in energy to the grease-grease and mineral spirits-mineral spirits interactions being broken, making dissolution energetically favorable. Water, by contrast, is held together by strong hydrogen bonds; disrupting that network to accommodate nonpolar grease would cost far more energy than weak grease-water dispersion forces can recover.
Question 5 Short Answer
Explain why ethanol (which has an -OH group) is miscible with water in all proportions, while hexane (a nonpolar hydrocarbon) separates from water immediately.
Think about your answer, then reveal below.
Model answer: Ethanol's -OH group can form hydrogen bonds with water molecules, providing strong solute-solvent interactions. The energy gained from these ethanol-water hydrogen bonds is sufficient to compensate for disrupting the water-water and ethanol-ethanol hydrogen bonds being broken. Hexane is nonpolar and can only offer London dispersion forces to water. These are far too weak to compensate for disrupting water's strong hydrogen-bonding network, so the energy balance strongly disfavors dissolution. This is the 'like dissolves like' principle in operation: matching interaction types makes dissolution thermodynamically favorable.