Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- A particle diagram of a solution shows a small volume of it. It must get two things right: the relative numbers of particles (concentration) and the interactions between them.
- Ionic solutes are drawn as separate ions in the formula ratio. Polyatomic ions (SO₄²⁻, NO₃⁻, CO₃²⁻, NH₄⁺) stay whole.
- Molecular solutes (glucose, ethanol, methanol) stay as whole molecules.
- Water is polar: O is δ−, H is δ+. Ion–dipole attractions make water point O towards cations and H towards anions.
- Solutes with O–H or N–H groups form hydrogen bonds with water; nonpolar solutes in nonpolar solvents interact by London dispersion forces only.
- Use labels, a key or distinct shapes, never colour alone.
Key relationships
| Idea | How it shows in a diagram |
|---|---|
| Ion ratio | Counts follow the formula, e.g. Ca(NO₃)₂ → 1 Ca²⁺ : 2 NO₃⁻ |
| Charge balance | Total + charge = total − charge in the box |
| Concentration | Particles ÷ volume represented |
| Dilution | Same number of solute particles, larger volume |
| Moles from a diagram | n = (number drawn) × (moles each particle stands for); then M = n ÷ V |
Assumptions behind the pictures
- Only a few water molecules are drawn; the real solution has far more.
- The solution is homogeneous, so the box is a fair sample of the whole.
- Ionic solutes are assumed to dissolve completely into separate ions.
Mistakes to avoid
- Ion pairs or lattice pieces in solution.
- Broken polyatomic ions, such as separate S and O atoms for sulfate.
- Molecular solutes drawn as ions.
- Water reversed around an ion.
- Wrong ratio or charges that do not balance.
- Comparing particle counts in boxes of different volumes without dividing by volume.
- Spending time on molality or percent by mass. They are not assessed in calculations for this course.
Quick self-check
- In a diagram of Na₃PO₄(aq), what is the ratio of Na⁺ to PO₄³⁻? (3 : 1; the PO₄³⁻ ion stays whole.)
- Which atom of water faces a Mg²⁺ ion? (Oxygen.)
- Box X (0.10 L) shows 5 solute particles; box Y (0.20 L) shows 8 particles of the same solute. Which is more concentrated? (X: 50 per L against 40 per L.)
- A box of ethanol solution is drawn with C₂H₅O⁻ and H⁺ ions. What is wrong? (Ethanol is molecular; it dissolves as whole C₂H₅OH molecules.)
Next: practice questions.