Using particulate models for mixtures: (i) represent interactions between components, and (ii) represent concentrations of components.
Particulate representations of solutions communicate two things: the relative concentrations of the components, and the interactions among them.
Drawing concentration. If solution B is twice as concentrated as solution A in the same volume, B's diagram must contain twice as many solute particles. If two beakers hold the same solution at different volumes, the density of particles is what stays the same.
Drawing dissociation. This is where most points are lost. A strong electrolyte must be drawn as separated ions, and in the correct ratio. A drawing of 0.1 M CaCl₂ should show one Ca²⁺ for every two Cl⁻, drawn apart from each other — never as "CaCl₂ units". A molecular (nonelectrolyte) solute like glucose stays intact.
Drawing interactions. When the question asks for interactions, show the solvation shell with correct orientation: water molecules turn their partially negative oxygen ends toward a cation and their partially positive hydrogen ends toward an anion. Those are ion–dipole interactions, and naming them earns the point.
Some AP conventions worth knowing: water molecules are often omitted for clarity (the prompt will say so), and you should still respect relative particle sizes — anions larger than cations, as in Unit 2.
Describe a correct particulate drawing for 200 mL of 0.10 M K₂SO₄, and explain how it would differ from a drawing of 200 mL of 0.10 M KNO₃.
K₂SO₄: potassium sulfate is a soluble strong electrolyte, so it exists in solution entirely as separated ions: K₂SO₄(aq) → 2 K⁺(aq) + SO₄²⁻(aq). The drawing must show twice as many K⁺ ions as SO₄²⁻ ions — for example, 6 K⁺ and 3 SO₄²⁻ — distributed randomly, never touching in pairs. Sulfate should be drawn as a single polyatomic unit (it does not break apart) and larger than K⁺.
KNO₃: also a strong electrolyte, but it dissociates 1 : 1: KNO₃(aq) → K⁺(aq) + NO₃⁻(aq). At the same 0.10 M concentration the drawing shows equal numbers of K⁺ and NO₃⁻ — for example, 6 of each.
Consequence: at the same molarity, K₂SO₄ produces 3 ions per formula unit versus 2 for KNO₃, so the K₂SO₄ solution contains more total dissolved particles and conducts electricity better.