AP® Chemistry · Unit 3 · Topic 3.8
Representations of Solutions
A particulate drawing of a solution shows what's actually floating around at the particle level: separate ions for strong electrolytes, whole molecules for nonelectrolytes, and a mix of both for weak electrolytes.
When NaCl dissolves, water pulls the Na+ and Cl- ions apart because the solid was already an ionic lattice of charged particles held together by attraction, not a collection of neutral NaCl units. Water molecules surround each ion and stabilize the separated charges, so the correct drawing shows individual Na+ and Cl- dots scattered through the solvent, never paired up as NaCl. Glucose is different: it's a molecular compound, built from covalent bonds with no ions to begin with. It dissolves whole, so the drawing shows intact glucose molecules, just spread out and surrounded by water. Acetic acid sits in between — it's a weak acid, so only a small fraction of its molecules ionize into H+ and acetate. Its drawing shows mostly intact CH3COOH molecules with just a few separated ions mixed in.
Concentration shows up as density of dots, not as a number on the page. A more concentrated solution gets more particles drawn in the same size box; a drawing of 1.0 M NaCl should show twice as many ion dots as 0.5 M NaCl, counting Na+ and Cl- separately since each formula unit releases two ions. Scale matters: if a key says one dot equals 0.5 mol/L of ion, a 2 M solution that releases 2 ions per unit needs (2 x 2)/0.5 = 8 dots.
This connects directly to double-replacement reactions: mixing AgNO3(aq) with NaCl(aq) means drawing Ag+, NO3-, Na+, and Cl- all loose at first, then showing solid AgCl particles settling out as a precipitate while Na+ and NO3- remain dissolved as spectator ions.
Key terms
- Dissociation
- The separation of an ionic compound into its individual ions when it dissolves in water.
- Strong electrolyte
- A substance that dissociates completely into ions in solution, like NaCl or a strong acid.
- Weak electrolyte
- A substance that only partially ionizes in solution, leaving mostly intact molecules with a few separated ions.
- Nonelectrolyte
- A molecular compound like glucose that dissolves as whole, intact molecules without forming ions.
- Spectator ion
- An ion present in solution that does not take part in a precipitation reaction and stays dissolved.
Watch it explained
- How it worksDraw the particles present in solutions of NaCl, of glucose, and of acetic acid at the same molarity.
- How it worksHow does a particulate drawing communicate concentration without writing a number?
- Common mistakeWhy isn't every dissolved compound broken into ions when it goes into water?
- Going furtherWhat changes in the drawing when you mix a solution of AgNO3 with one of NaCl?
Check yourself
End of topic.