Solutions: Molarity, Dilution, and Colligative Properties
When salt disappears into water, what really happens? Solubility, the math of molarity and dilution, the like-dissolves-like rule, and how dissolved solute changes the boiling and freezing points.
What is a solution?
A solution is a homogeneous mixture — uniform at the molecular level. Two parts:
- Solute — the substance being dissolved (usually present in smaller amount).
- Solvent — the substance doing the dissolving (usually larger amount).
In saltwater: salt is the solute; water is the solvent.
Solutions can be solid, liquid, or gas. Air is a gas-gas solution (mostly N₂ + O₂). Brass is a solid-solid solution (alloy of Cu + Zn). But "solution" in chemistry usually means a substance dissolved in a liquid — most often water (aqueous).
"Like dissolves like" — the polarity rule
Polar solvents dissolve polar/ionic solutes; nonpolar solvents dissolve nonpolar solutes. The reason: solvent molecules must surround and stabilize solute particles, and they do this best when the polarities match.
- Water (polar) dissolves salt (ionic), sugar (polar), ethanol (polar).
- Water does NOT dissolve oil (nonpolar) — they form separate layers.
- Hexane (nonpolar) dissolves oils and waxes but not salt.
This is why soap works: soap molecules have a polar head AND a nonpolar tail, letting them bridge grease (nonpolar) and water (polar).
The dissolution process
For ionic compounds in water, polar water molecules orient themselves around each ion: positive H ends near anions, negative O ends near cations. This hydration stabilizes the dissolved ions. NaCl(s) → Na⁺(aq) + Cl⁻(aq).
Three factors that SPEED UP dissolving (without changing the maximum amount dissolvable):
- Higher temperature (more kinetic energy)
- Smaller particles (more surface area)
- Stirring (replenishes fresh solvent near the surface)
Saturation levels
- Unsaturated — below the solubility limit; more solute could dissolve.
- Saturated — at the solubility limit; any additional solute won't dissolve.
- Supersaturated — temporarily holds MORE than saturated, achieved by cooling carefully. Unstable; any disturbance triggers crystallization.
Solubility generally INCREASES with temperature for solids in water, but DECREASES for gases (warm soda goes flat faster).
Molarity — the most common concentration unit
Molarity (M) = moles of solute per LITER of solution:
M = mol / L
Example: dissolve 5.85 g of NaCl in enough water to make 0.250 L of solution.
Moles NaCl = 5.85 / 58.5 = 0.100 mol. Molarity = 0.100 / 0.250 = 0.400 M.
Other concentration units:
- Molality (m) = moles solute per KG of SOLVENT. Used for colligative properties because mass doesn't change with temperature.
- Mass percent = (mass solute ÷ mass solution) × 100
- ppm = parts per million; in dilute aqueous solutions, mg solute per L of water
- Mole fraction = mol of one component / total mol of all components
Dilution — adding solvent to lower concentration
When you dilute a solution, the AMOUNT of solute stays the same; only the volume changes. The dilution equation:
M₁V₁ = M₂V₂
Example: How much water to add to 100 mL of 6.0 M HCl to make 0.50 M?
V₂ = (M₁V₁)/M₂ = (6.0 × 100)/0.50 = 1,200 mL (final volume).
Water to add = 1,200 − 100 = 1,100 mL.
SAFETY: when diluting concentrated acids, ALWAYS add acid TO water — never water to acid. Heat is released and water can splash.
Electrolytes — conducting electricity
A solution conducts electricity only if it contains MOBILE IONS. Ionic compounds (NaCl, KOH) dissociate completely into ions when dissolved → strong electrolytes. Molecular compounds (sugar, ethanol) dissolve as intact molecules → don't conduct (non-electrolytes). Weak acids (acetic acid) partially dissociate → weak electrolytes.
Colligative properties — they depend on how MANY particles
Colligative properties depend ONLY on the number of dissolved particles, not their identity:
- Freezing point depression — solute lowers the freezing point of solvent. Salt on icy roads.
- Boiling point elevation — solute raises the boiling point. Salt in pasta water.
- Vapor pressure lowering — dissolved nonvolatile solute reduces the solvent's tendency to evaporate.
- Osmotic pressure — pressure needed to prevent osmotic flow through a semi-permeable membrane.
Ionic solutes that dissociate produce MORE particles per formula unit. 1 mol CaCl₂ → 3 mol particles (1 Ca²⁺ + 2 Cl⁻) → triple the colligative effect of an equivalent non-ionizing solute.