Solutions & Concentration
Where does salt go when it dissolves — and how much can water hold? Enter saturation, solubility curves and molarity.
Builds on: 1.3 Metallic Bonds & the Bond Spectrum2.2 The Mole: Chemistry's Dozen
Dissolving is disassembly
Stir salt into water and it “vanishes” — but you know better by now. Water’s polar molecules (Unit 1) pry Na⁺ and Cl⁻ ions off the lattice, surround each one in a jacket of oriented dipoles, and carry them away. The salt is still there, disassembled into invisible, hydrated ions. The water is the solvent, the salt the solute, and the mixture a solution — transparent because the dissolved pieces are smaller than wavelengths of light.
Saturation: the parking garage fills up
Water’s capacity is finite. Keep adding salt and at some point the solution is saturated — every new crystal just sinks undissolved. The limit is the solubility, usually quoted in g per 100 mL, and for most solids it rises with temperature (hotter water = more energetic prying). Different substances have wildly different curves: NaCl’s is almost flat (36 g cold, 39 g hot), while saltpetre’s rockets from 13 g to over 240 g.
Steep curves enable a beautiful trick: dissolve much solute hot, cool the solution, and the excess must exit — as crystals. That is how you’ll grow salt crystals in the Unit 4 capstone, and how sugar becomes rock candy.
Molarity: the chemist’s concentration
Grams-per-litre depends on the substance; moles-per-litre speaks particle language. If a bottle says 0.1 M HCl, you know exactly how many reactive particles each millilitre delivers — which is what makes the titration in Unit 4 an act of counting, not guessing.
Solids dissolve better hot; gases dissolve better cold (heat helps them escape). Warm cola goes flat fast, and warming oceans hold less CO₂ and less O₂ — one solubility curve with planetary consequences.
⚗️ Lab — The Saturation Point
Add solute, heat the water, and watch the solubility curve decide what dissolves.
- Add 50 g of NaCl at 20 °C — a pile stays on the bottom. Now heat: does it help much?
- Switch to KNO₃: 50 g won’t dissolve cold but vanishes completely at 45 °C.
- Dissolve 150 g of KNO₃ hot, then slide the temperature down — crystal rain.
Work it by hand — 0 / 3
No multiple choice here: compute the value and type it. Suffixes like 2.5k, 20m or 100µ are understood; answers within ±2% count.