Topic 3.8 Representations of Solutions: use particulate-level diagrams to represent the species present in a solution, distinguishing strong electrolytes, weak electrolytes and nonelectrolytes.

What this topic is asking

The College Board (Topic 3.8) wants you to represent solutions at the particle level, drawing and interpreting particulate diagrams that show the species actually present once a substance dissolves. The central skill is knowing whether a dissolved substance exists as separated ions, as a mixture of ions and molecules, or as intact molecules, which depends on whether it is a strong electrolyte, a weak electrolyte, or a nonelectrolyte.

Electrolytes and the species present

The reason this matters for a diagram is that the dissolved particles look completely different in the three cases. A solution conducts electricity in proportion to how many mobile ions it contains, so strong electrolytes conduct well, weak electrolytes conduct weakly, and nonelectrolytes do not conduct at all.

Drawing a particulate diagram

For example, a diagram of dissolved $\text{MgCl}_2$ should show, for every magnesium ion, two chloride ions, because the formula releases one $\text{Mg}^{2+}$ and two $\text{Cl}^-$ on dissolving. Diagrams should also keep charge balanced overall and show the solvent (water) molecules around the solute. The College Board often asks you to choose, draw or critique such a diagram, so the count and the type of particle both have to be right.

Why dissociation differs

A soluble ionic solid dissociates fully because the ion-dipole attractions between its ions and water molecules are strong enough to pull the lattice apart completely. A weak acid such as acetic acid is a molecular substance whose O-H bond ionizes only slightly in water, so most of it stays as whole molecules. A nonelectrolyte such as glucose has no ionisable bonds; it dissolves through hydrogen bonding with water but stays neutral. This is why a particulate picture of acetic acid looks mostly molecular while one of hydrochloric acid (a strong acid) looks fully ionized, even at the same concentration.

Try this

Q1. Classify each as a strong electrolyte, weak electrolyte or nonelectrolyte: (a) $\text{KNO}_3$, (b) ethanol, (c) ammonia ($\text{NH}_3$). [3 points]

• Cue. (a) strong electrolyte; (b) nonelectrolyte; (c) weak electrolyte (weak base).

Q2. Describe the species present when potassium sulfate ($\text{K}_2\text{SO}_4$) dissolves in water. [1 point]

• Cue. Two $\text{K}^+$ ions and one $\text{SO}_4^{2-}$ ion per formula unit, all fully separated.