AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/3/3-8)
Unit 3 · Topic 3.8
3.8 Representations of Solutions
A particle diagram of a solution should show the right ratio of particles and how they interact. Water molecules surround dissolved ions with their partly negative oxygen toward cations and their partly positive hydrogens toward anions.
Key terms
- particle diagram
- solute
- solvent
- dissociation
- ion-dipole interaction
- concentration
What a good solution diagram shows
- Correct ratio of solute particles. CaCl₂ dissolved in water should show twice as many Cl⁻ as Ca²⁺.
- Ions separated from each other, not paired up, for a soluble ionic compound.
- Molecular solutes like glucose kept intact as whole molecules, not split into atoms or ions.
- Solvent molecules oriented sensibly around solute particles.
- Relative concentrations: a more concentrated solution has more solute particles in the same volume.
How water surrounds ions
Water is polar: oxygen is partially negative (δ−) and the hydrogens are partially positive (δ+). When an ionic compound dissolves, water molecules cluster around each ion. This is an ion-dipole interaction.
Around a cation, like Na⁺ or Ca²⁺, water molecules point their oxygen atoms toward the ion. Around an anion, like Cl⁻, water molecules point one hydrogen atom toward the ion. Drawing water the wrong way around (hydrogens toward a cation) is one of the most common diagram errors.
Smaller ions and ions with larger charges attract water molecules more strongly, by Coulomb's law.
Dissolving, step by step
Picture salt dissolving. Water molecules at the surface of the crystal surround an ion at an edge or corner, with the correct ends pointing toward it. When enough water molecules are attracted to it, the ion pulls free of the lattice and moves into the solution, still surrounded by a shell of water molecules. That shell of solvent is called a hydration shell.
Your diagram can show this as each ion with a small ring of 3 to 6 water molecules around it, all oriented correctly. Ions with a larger charge, like Ca²⁺, attract water more strongly than ions like Na⁺.
Showing concentration
In a particle diagram, concentration means the number of solute particles per unit volume. If one beaker shows 4 solute particles and another the same size shows 8, the second is twice as concentrated. When comparing ionic solutions, count ions: 0.10 M CaCl₂ has more particles (0.30 mol of ions per liter) than 0.10 M NaCl (0.20 mol of ions per liter).
Diagrams don't show every water molecule. It's acceptable to show a few water molecules around some ions to illustrate interactions, as long as the solute ratio and orientations are right.
Molecular solutes
Polar molecular solutes like ethanol or glucose dissolve by forming hydrogen bonds or dipole-dipole attractions with water. A diagram should show whole molecules surrounded by water, with hydrogen bonds drawn between an O–H hydrogen and an oxygen lone pair.
You won't be tested on
Colligative properties, such as boiling-point elevation and freezing-point depression, are excluded from the exam, along with molality and percent-by-mass calculations.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Describing a correct diagram
Describe a correct particle-level drawing of a small volume of aqueous K₂SO₄.
Show the solutionHide the solution
- Step 1: K₂SO₄ is ionic and soluble (potassium salts are soluble), so it separates into K⁺ and SO₄²⁻ ions.
- Step 2: The ratio must be 2 K⁺ : 1 SO₄²⁻. Sulfate stays together as one polyatomic ion; its S and O atoms don't separate.
- Step 3: Water molecules near K⁺ point their O atoms toward it; water molecules near SO₄²⁻ point an H atom toward it.
- Step 4: The ions are spread apart among the water molecules, not stuck together.
Answer: Separate K⁺ and intact SO₄²⁻ ions in a 2 : 1 ratio, with water's O facing K⁺ and water's H facing SO₄²⁻.
- Example 2
Spotting the error (classic trap)
A student's diagram of aqueous NaCl shows each Na⁺ surrounded by water molecules with their H atoms pointing at the ion, and each Cl⁻ surrounded by water molecules with their O atoms pointing at it. What's wrong?
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- Step 1: Na⁺ is positive, so it attracts the partially negative end of water, the O atom.
- Step 2: Cl⁻ is negative, so it attracts the partially positive H atoms.
- Step 3: The student has both orientations reversed.
Answer: The orientations are backwards: O should face Na⁺ and H should face Cl⁻.
Common mistakes
- Pointing water's hydrogens toward cations. The oxygen side faces cations.
- Breaking polyatomic ions apart in a drawing. SO₄²⁻, NO₃⁻ and NH₄⁺ stay intact.
- Splitting molecular solutes like sugar into ions.
- Getting the ratio of ions wrong for compounds like CaCl₂ or Na₃PO₄.
On the exam
- Expect to draw or pick a particle diagram of a solution, often as part of a longer free-response question. Points are usually given for the correct ion ratio and correct orientation of water molecules.
- If asked to compare two solutions' concentrations from diagrams, count solute particles in equal volumes.
Connected topics
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Check yourself
4 questions on 3.8 Representations of Solutions. Pick an answer to see if you got it, and why.
A particle diagram of a 0.20 M CaCl₂ solution is drawn showing 6 chloride ions. Which of the following correctly gives the number of calcium ions in the diagram and the actual concentration of Cl⁻ in the solution?
Which of the following best describes how water molecules are arranged around a K⁺ ion in an aqueous solution of KBr?
Which of the following best describes a particle-level picture of a dilute aqueous solution of glucose, C₆H₁₂O₆?
Which of the following solutions has the greatest total concentration of dissolved ions?
0 of 4 answered