Questions: Solutions and Solubility: Factors Affecting Dissolution

Question 1

You want to dissolve a nonpolar organic compound in a solvent. Which solvent choice best applies the principle of 'like dissolves like'?

• A. Water, because it is a universal solvent that dissolves most substances
• B. Ethanol, because it is a liquid at room temperature and mixes with everything
• C. Hexane, because its London dispersion forces match the nonpolar solute's intermolecular forces
• D. Acetone, because it has a high boiling point that keeps the solute dissolved

Answer: (C)

'Like dissolves like' means the solute and solvent must have the same *type* of intermolecular forces. A nonpolar solute interacts via London dispersion forces, so a nonpolar solvent like hexane — which also relies on dispersion forces — provides the energetically favorable solute-solvent interactions needed for dissolution. Water is polar and forms strong hydrogen bonds; it would not accommodate nonpolar molecules because water molecules prefer each other. Ethanol has a polar hydroxyl group and does not universally dissolve nonpolar compounds. Boiling point is irrelevant to solubility.

Question 2

A sealed bottle of soda is cold and fizzy. You warm it to room temperature and shake it. What happens to the dissolved CO₂, and why?

• A. More CO₂ dissolves because warmer water has more kinetic energy to absorb gas molecules
• B. The same amount of CO₂ stays dissolved because Henry's law only applies to very high pressures
• C. CO₂ escapes from solution because gas solubility in liquids decreases as temperature rises
• D. CO₂ stays dissolved until you open the bottle because temperature does not affect gas solubility

Answer: (C)

Gas solubility in liquids decreases with increasing temperature — the reverse of the trend for most solid solutes. As temperature rises, gas molecules gain enough kinetic energy to escape the solution more readily, shifting the dissolution equilibrium toward the gas phase. This is why a warm soda goes flat quickly and why boiling water degasses it. Opening the bottle releases the pressure, but warming already reduces solubility even while sealed.

Question 3

A saturated solution can seldom dissolve any additional solute under any circumstances.

• T. True
• F. False

Answer: False

A saturated solution holds the maximum amount of solute *at that temperature and pressure*. Changing conditions — especially raising the temperature for most solid solutes — increases solubility, allowing more solute to dissolve. A supersaturated solution is even a case where more than the equilibrium amount is temporarily dissolved. 'Saturated' describes an equilibrium at specific conditions, not a permanent ceiling.

Question 4

In a saturated solution, dissolution has stopped and the system is static — no solute is entering or leaving the solid.

• T. True
• F. False

Answer: False

Saturation is a *dynamic* equilibrium, not a static one. Solute molecules continue to leave the solid surface and enter solution (dissolution), while dissolved molecules continue to return to the solid (recrystallization) at equal rates. The net concentration remains constant, but both processes proceed continuously. This is the same type of dynamic equilibrium seen in other reversible processes like vapor pressure equilibrium.

Question 5

Why does oil not dissolve in water, even though both are liquids?

Model answer: Oil molecules interact through weak London dispersion forces, while water molecules form strong hydrogen bonds with each other. For dissolution to occur, solute-solvent interactions must be comparable in strength to the solute-solute and solvent-solvent interactions they replace. Inserting nonpolar oil molecules into water would disrupt water's hydrogen-bond network without providing comparable compensating interactions, making dissolution energetically unfavorable. Water molecules effectively exclude the oil, which clusters together instead.

The 'like dissolves like' principle reflects an energy and intermolecular force argument, not just a polarity label. The key is whether the energy required to break existing solute-solute and solvent-solvent interactions is offset by the energy gained from new solute-solvent interactions. When the force types are mismatched (London dispersion vs. hydrogen bonding), they cannot compensate each other, and dissolution does not occur.