Unit 3 · Topic 3.10 Beta

Solubility

A substance dissolves when the attractions it forms with the solvent are comparable to the solute–solute and solvent–solvent attractions it replaces, so like dissolves like.

Practice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Sugar vanishes into tea, but oil and vinegar separate however hard you shake the bottle. Grease will not wash off with water but comes off with soap. Whether something dissolves is decided by the attractions between particles.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which molecule can hydrogen-bond to water?

  1. CH₃OH
  2. CH₄
  3. CCl₄
Show the answer

Methanol has an O–H group and lone pairs on O.

  • Correct: CH₃OH:
  • CH₄:
  • CCl₄:

2. In a dissolved ionic compound, which end of water faces a cation?

  1. The oxygen end
  2. A hydrogen end
  3. Neither end
Show the answer

Water's δ− oxygen is attracted to a positive ion.

  • Correct: The oxygen end:
  • A hydrogen end:
  • Neither end:

Part 4 · See it

See it first

Dissolving overcomes solute-solute and solvent-solvent attractions and forms solute-solvent attractions. A substance dissolves well when the new attractions are comparable to the ones it replaces.
Dissolving overcomes solute-solute and solvent-solvent attractions and forms solute-solvent attractions. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Dissolving overcomes solute–solute and solvent–solvent attractionsit must form solute–solvent attractions of comparable strength to proceed
  2. Polar and ionic solutes form hydrogen bonds or ion-dipole forces with watermany of them dissolve well in water
  3. Nonpolar solutes attract water only weakly, and fitting them in costs water–water hydrogen bondsthey do not dissolve well in water but do in nonpolar solvents
  4. Dissolved gas molecules escape more easily when they move fastergases dissolve less as temperature rises, and more as their partial pressure rises

Part 6 · Key ideas

Key ideas

  • Dissolving trades attractions: overcome solute–solute and solvent–solvent, form solute–solvent.
  • Like dissolves like: polar and ionic in polar, nonpolar in nonpolar.
  • Oil and water separate because water attracts itself far more strongly, not because they repel.
  • Gas solubility in water rises with partial pressure and falls with temperature.

Part 7 · Misconception

A common mistake

The wrong idea: Nonpolar molecules do not dissolve in water because they repel water molecules.

What actually happens: They attract water weakly. Dissolving them would cost water many hydrogen bonds and give back much weaker attractions, so the water molecules stay together.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Graph

How much alcohol dissolves in water

Solubility at 20 °C of straight-chain alcohols with one –OH group, plotted against the number of carbon atoms. Methanol, ethanol and 1-propanol mix with water in all proportions and are not plotted.

01.534.567.594567Carbon atoms per moleculeSolubility (g per 100 g water)
Data table
Carbon atoms per moleculeSolubility
47.7
52.2
60.59
70.17

1. Which statement describes the trend in the graph?

  1. Solubility in water falls as the carbon chain gets longer.
  2. Solubility in water rises as the carbon chain gets longer.
  3. Solubility in water is about the same for each alcohol.
  4. Solubility rises to a maximum at 6 carbons, then falls.
Show the answer

From 4 to 7 carbons the solubility drops from 7.7 to 0.17 g per 100 g water, each bar lower than the one before.

  • Correct: Solubility in water falls as the carbon chain gets longer.: Right: a steady fall.
  • Solubility in water rises as the carbon chain gets longer.: This reverses the trend.
  • Solubility in water is about the same for each alcohol.: The values span a factor of about 45.
  • Solubility rises to a maximum at 6 carbons, then falls.: The 6-carbon bar is lower than the 4- and 5-carbon bars.

2. Which explanation accounts for the trend?

  1. A longer nonpolar chain costs more water–water hydrogen bonds; the one –OH adds the same.
  2. Longer alcohols have more –OH groups, which repel the water molecules around them.
  3. Longer alcohols are heavier, and heavier molecules sink rather than dissolving.
  4. Longer chains are more polar, so they are attracted to water too strongly to dissolve.
Show the answer

Each alcohol has one –OH that hydrogen-bonds to water. The growing carbon–hydrogen part attracts water only weakly, and fitting it in costs water–water hydrogen bonds, so solubility falls.

  • Correct: A longer nonpolar chain costs more water–water hydrogen bonds; the one –OH adds the same.: Right: the nonpolar part grows; the polar part does not.
  • Longer alcohols have more –OH groups, which repel the water molecules around them.: Each of these alcohols has one –OH group, and –OH attracts water.
  • Longer alcohols are heavier, and heavier molecules sink rather than dissolving.: Dissolving depends on attractions, not on whether a molecule is heavy.
  • Longer chains are more polar, so they are attracted to water too strongly to dissolve.: Carbon–hydrogen chains are nonpolar; longer chains make the molecule less polar overall.

3. 1-Hexanol dissolves only slightly in water, but it mixes completely with hexane (C₆H₁₄). Which explanation is best?

  1. Its chain attracts hexane by dispersion forces like those hexane loses; in water it costs hydrogen bonds.
  2. Hexane hydrogen-bonds to the –OH group of 1-hexanol more strongly than water can.
  3. 1-Hexanol becomes nonpolar when mixed with hexane, because its –OH group breaks off.
  4. Hexane molecules repel water, so they push 1-hexanol out of the water and into the hexane.
Show the answer

Like dissolves like: hexane is nonpolar, and the long chain of 1-hexanol interacts with it by dispersion forces of similar strength to those it replaces. In water the same chain disrupts strong hydrogen bonds.

  • Correct: Its chain attracts hexane by dispersion forces like those hexane loses; in water it costs hydrogen bonds.: Right: comparable attractions in hexane; a poor trade in water.
  • Hexane hydrogen-bonds to the –OH group of 1-hexanol more strongly than water can.: Hexane has no H on N, O or F and cannot hydrogen-bond.
  • 1-Hexanol becomes nonpolar when mixed with hexane, because its –OH group breaks off.: Mixing does not break covalent bonds; the –OH stays on the molecule.
  • Hexane molecules repel water, so they push 1-hexanol out of the water and into the hexane.: Water and hexane do not repel; water molecules just attract each other much more.

Data table

Oxygen dissolved in water

The concentration of O₂ dissolved in water that is in contact with air (O₂ partial pressure 0.21 atm) at several temperatures.

Dissolved oxygen in water under air
Water temperature (°C)Dissolved O₂ (mg/L)
014.6
1011.3
209.1
307.6
406.4

4. How does the solubility of O₂ in water change as the temperature rises?

  1. It increases.
  2. It decreases.
  3. It stays the same.
  4. It increases, then decreases.
Show the answer

The dissolved O₂ falls from 14.6 mg/L at 0 °C to 6.4 mg/L at 40 °C.

  • It increases.: This reverses the trend in the table.
  • Correct: It decreases.: Right: lower at each higher temperature.
  • It stays the same.: The values change by more than half.
  • It increases, then decreases.: Every value is lower than the one before.

5. Which particle-level explanation accounts for the trend?

  1. Faster dissolved O₂ molecules escape their weak attractions to water more easily.
  2. At higher temperature, water molecules hydrogen-bond more strongly to O₂ and pull it apart into atoms.
  3. At higher temperature, O₂ molecules become polar and are repelled by the polar water molecules.
  4. At higher temperature, the air contains less oxygen, so less of it can dissolve in the water.
Show the answer

O₂ is held in water only by weak dipole-induced dipole and dispersion forces. Faster-moving molecules break away and escape into the air more readily.

  • Correct: Faster dissolved O₂ molecules escape their weak attractions to water more easily.: Right: more kinetic energy, more escape.
  • At higher temperature, water molecules hydrogen-bond more strongly to O₂ and pull it apart into atoms.: O₂ cannot hydrogen-bond, and dissolving does not break the O=O bond.
  • At higher temperature, O₂ molecules become polar and are repelled by the polar water molecules.: O₂ is nonpolar at any temperature.
  • At higher temperature, the air contains less oxygen, so less of it can dissolve in the water.: The O₂ partial pressure in air is 0.21 atm throughout.

6. Which pairs of liquids are likely to mix completely? Select all that apply.

  1. Water and methanol, CH₃OH
  2. Hexane and octane, C₈H₁₈
  3. Water and hexane
  4. Water and carbon tetrachloride, CCl₄
Show the answer

Like dissolves like. Water and methanol both hydrogen-bond. Hexane and octane are both nonpolar. Water with nonpolar hexane or CCl₄ separates into layers.

  • Correct: Water and methanol, CH₃OH: Right: both hydrogen-bond.
  • Correct: Hexane and octane, C₈H₁₈: Right: both nonpolar, attracting by dispersion.
  • Water and hexane: Hexane would cost water its hydrogen bonds.
  • Water and carbon tetrachloride, CCl₄: CCl₄ is symmetric and nonpolar.

Part 9 · Summary

Summary

A substance dissolves when the attractions it forms with the solvent are comparable to the solute–solute and solvent–solvent attractions it replaces, so like dissolves like. Long nonpolar chains lower solubility in water. Gases dissolve more at higher partial pressure and less at higher temperature.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections