Solutions and their properties
Most chemistry happens in solution. How substances dissolve, and how dissolved particles change a solvent's properties, matters from antifreeze to medicine.
Solutions
A solution is a homogeneous mixture of a solute dissolved in a solvent. Water is the most common solvent, sometimes called the universal solvent because its polarity dissolves many ionic and polar substances. Solutions can also be gases (air) or solids (alloys like brass).
The dissolving process
Dissolving involves separating solute particles, separating solvent particles, and forming new solute-solvent attractions. For salt in water, polar water molecules surround each ion (hydration). Whether dissolving releases or absorbs heat depends on the balance: cold packs use endothermic dissolving of ammonium nitrate.
Solubility and factors affecting it
Solubility is the maximum amount that dissolves at a given temperature. Most solids become more soluble in hotter water. Gases become less soluble as temperature rises, which is why warm soda goes flat. Higher gas pressure increases gas solubility (Henry's law), which keeps carbon dioxide dissolved in sealed bottles.
Concentration units
Molarity (mol solute per litre of solution) is common in labs. Molality (mol solute per kg of solvent) does not change with temperature. Mass percent and parts per million (ppm) describe dilute contaminants: 1 ppm lead means 1 mg per kg of water.
Colligative properties
Some properties depend only on the number of dissolved particles, not their identity: vapour pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Salt melts ice on roads by lowering water's freezing point; antifreeze does the same in car radiators.
Freezing point depression
\(\Delta T_f = iK_fm\), where \(m\) is molality, \(K_f\) is a solvent constant (1.86 \(^\circ\)C kg/mol for water), and \(i\) is the number of particles each formula unit produces (2 for NaCl, since it splits into \(\text{Na}^+\) and \(\text{Cl}^-\)). A 1.0 m NaCl solution freezes about \(3.7^\circ\)C below pure water.
\Delta T_f = iK_f mOsmosis
Osmosis is the movement of water through a semipermeable membrane toward higher solute concentration. Red blood cells swell and burst in pure water and shrivel in very salty water, so intravenous fluids must match blood's concentration (isotonic saline is about 0.9% NaCl). Reverse osmosis forces water the other way to desalinate seawater.
Colloids and suspensions
Colloids contain particles larger than in a solution but small enough not to settle, like milk, fog, and gelatin. They scatter light (the Tyndall effect), which is why a beam is visible in fog. Suspensions have large particles that settle, like muddy water.
Key takeaways
Solutions are homogeneous mixtures; 'like dissolves like'. Solid solubility usually rises with temperature, gas solubility falls, and pressure raises gas solubility. Colligative properties depend on particle count: freezing point depression \(\Delta T_f = iK_fm\) and osmosis are key examples.
Flashcard
Homogeneous mixture
Check your understanding
Why is salt spread on icy roads?