Chemistry 20 · Solutions
What is actually in the beaker
Dissolve salt in water and there is no salt in there any more — there are sodium ions and chloride ions, moving independently. Nearly every question in this unit becomes easier once you picture that.
- 1. The words, first
- 2. Dissolving and solubility
- 3. Concentration and dilution
- 4. Ionic equations and precipitates
- 5. What costs marks
The words, first
The idea: Concentration and amount are different quantities, and solubility is about a maximum rather than about speed.
| Word | What it means |
|---|---|
| Solute | The substance dissolved. |
| Solvent | The substance doing the dissolving, usually the one present in greater amount. |
| Aqueous | Dissolved in water — the (aq) label in an equation. |
| Solubility | The maximum mass or amount of solute that dissolves in a given amount of solvent at a stated temperature. |
| Saturated | Holding the maximum it can at that temperature. Adding more leaves it undissolved. |
| Dissociation | An ionic compound separating into free ions as it dissolves. |
| Molar concentration | Amount of solute per litre of solution: c = n/V, in mol/L. |
| Dilution | Adding solvent. The amount of solute does not change, so c₁V₁ = c₂V₂. |
| Precipitate | An insoluble solid formed when two solutions are mixed. |
| Spectator ion | An ion present before and after and unchanged by the reaction. Left out of the net ionic equation. |
Dissolving and solubility
The idea: Water pulls ions out of a lattice because its molecules are polar. Whether that happens depends on whether the attractions water can offer beat the ones already holding the solid together.
Why water works. The oxygen end of a water molecule is partly negative and the hydrogen ends partly positive, so water can attract both positive and negative ions. Enough of those attractions and an ion is pulled free and surrounded.
Like dissolves like. Oil is non-polar, so water molecules have nothing to grip and prefer each other. This one rule explains most solubility questions at this level.
Solubility rules worth knowing. All nitrates, all sodium, potassium and ammonium compounds are soluble. Most chlorides are soluble except silver, lead and mercury(I). Most sulfates are soluble except barium, lead and calcium. Most carbonates, phosphates and hydroxides are insoluble except with the alkali metals and ammonium.
Solubility versus rate. Stirring and heating make a solid dissolve faster; only temperature changes how much can dissolve in total. Confusing the two is common.
Concentration and dilution
The idea: c = n/V, with V the volume of the finished solution in litres. Every other calculation in this unit is a rearrangement of that.
Finding concentration. 0.50 mol in 2.0 L → c = 0.25 mol/L.
Finding an amount. 50.0 mL of 0.200 mol/L → n = (0.200)(0.0500) = 0.0100 mol. Convert millilitres to litres first; skipping it costs a factor of a thousand.
Finding a mass. To make 500.0 mL of 0.200 mol/L NaCl: n = (0.200)(0.5000) = 0.100 mol, so m = (0.100)(58.44) ≈ 5.8 g.
Dilution. The amount of solute does not change, so c₁V₁ = c₂V₂. Diluting 50.0 mL of 2.00 mol/L to 200.0 mL gives (2.00)(50.0)/200.0 = 0.500 mol/L. Quadrupling the volume divides the concentration by four, which is a fast check.
Ion concentrations follow the subscripts. In 0.10 mol/L Na₂SO₄, each formula unit gives two sodium ions, so [Na⁺] = 0.20 mol/L while [SO₄²⁻] = 0.10 mol/L.
Ionic equations and precipitates
The idea: Write what is really there. Soluble ionic compounds are free ions; anything insoluble is a solid; anything unchanged is a spectator.
Worked example: AgNO₃(aq) + NaCl(aq).
- Complete equation: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
- Total ionic: Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻
- Net ionic: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Sodium and nitrate ions are floating free before and after, so they are spectators and drop out. The net ionic equation is a statement about what actually changed, and it is the same whichever soluble silver salt and whichever soluble chloride you started with.
Predicting a precipitate. Swap the partners, then check both products against the solubility rules. If both are soluble, no reaction happens at all — the beaker just contains four kinds of ion.
What costs marks
The idea: Volume units and the difference between concentration and amount.
- Millilitres used as litres. The most expensive habit in this unit.
- Using the volume of solvent added instead of the final volume of solution.
- Forgetting the subscript when asked for an ion concentration.
- Leaving spectator ions in a net ionic equation. If it appears unchanged on both sides, it goes.