electrolytes

Meaning: An electrolyte produces mobile ions in water, so the solution can carry current. Strong electrolytes (soluble ionic salts like NaCl, strong acids, strong bases) dissociate essentially completely. Weak electrolytes (weak acids and bases such as CH₃COOH and NH₃) ionize only slightly, setting up an equilibrium. Nonelectrolytes (glucose, sucrose, ethanol) dissolve as intact molecules.

Equation: ΔTf = i Kf m, where ΔTf is the freezing point depression (K), i is the van't Hoff factor (unitless), Kf is the solvent's molal freezing point constant (K·kg/mol), and m is molality (mol/kg). Ideally i equals the number of ions per formula unit: 1 for glucose, 2 for NaCl, 3 for CaCl₂.

On the MCAT: Passages ask you to rank equal-concentration solutions by conductivity, boiling point, freezing point, vapor pressure or osmotic pressure. Count particles: 0.1 M CaCl₂ gives the most, then NaCl, then acetic acid (i only slightly above 1), then glucose. In electrochemistry, ions in solution and the salt bridge complete the circuit.

Common confusion: Strong describes extent of dissociation, not solubility. AgCl is a strong electrolyte in the small amount that dissolves, yet its saturated solution conducts poorly because few ions are present. Real i values fall a bit below the ideal whole number because of ion pairing, especially at high concentration.