Liquids, Solids, Solutions, and Colloids

This is Unit Eight of Chemistry, Part Three: solutions and colligative properties, and the last part of the unit. Parts One and Two dealt with pure substances — what holds a liquid together, what holds a solid together. This part is what happens when one substance dissolves in another. The through-line is that once something is dissolved, several of the solvent's properties shift in ways that depend only on how many solute particles are present and not at all on what they are — which is a genuinely strange result, and the reason the unit ends by counting particles rather than identifying them. By the end you will be able to say what "like dissolves like" is and is not; say what makes a property colligative; tell molality from molarity; say which direction each colligative effect acts; and say what the van't Hoff factor corrects for.

Two entries. What actually has to happen for one substance to dissolve in another, and the familiar three-word rule that summarises it — which the book is careful to label a heuristic rather than a law.

Old attractions, new ones.

Chem 1520 The Dissolution Process

Dissolving a solute means replacing solute-solute and solvent-solvent attractions with new solute-solvent attractions, and it is favourable only when those new attractions can compete with the ones being given up. Dissolution is a competition between old and new attractions rather than passive mixing — which is why some pairs never form a stable solution at all.

A rule of thumb.

Chem 1523 "Like Dissolves Like" as a Structural Heuristic

"Like dissolves like" is a heuristic rather than a strict law: polar solvents favour polar solutes, and nonpolar solvents favour nonpolar ones, because similar substances form comparable attractions. It predicts trends rather than certainties — some polar and nonpolar pairs dissolve partially instead of refusing outright.

Three entries. Whether a dissolved substance produces ions, how completely it does so, and the three states a solution can be in relative to its solubility limit — one of which is not stable at all.

Does it conduct.

Chem 1526 Electrolytes vs. Nonelectrolytes

An electrolyte produces ions when dissolved, which lets its solution conduct electricity; a nonelectrolyte dissolves as intact, nonconducting molecules. Conductivity depends on whether ions are present rather than on how much solute dissolved — a dilute electrolyte still conducts, only more weakly.

How completely.

Chem 1529 Strong vs. Weak Electrolytes, Extended to the Van't Hoff Factor

Strong electrolytes dissociate completely and weak ones only partially, and that distinction now sets how many particles each dissolved formula unit actually contributes — which is what the colligative material later depends on. Strong and weak describe how completely a substance dissociates, never how much is dissolved: a weak electrolyte can still be concentrated.

Three states.

Chem 1533 Saturated, Unsaturated, and Supersaturated Solutions

A solution is saturated at its solubility limit, unsaturated below it, and supersaturated above it — that last one in an unstable, non-equilibrium state holding more solute than it should. A supersaturated solution can be disturbed into sudden precipitation of its excess solute.

Two entries. Temperature, which pushes solids and gases in opposite directions, and pressure, which matters for gases only.

Opposite directions.

Chem 1536 Temperature's Effect on Solid and Gas Solubility

Most solids grow more soluble as temperature rises, while gases grow less soluble as temperature rises — two opposite structural trends, which is why a warm carbonated drink holds less gas and a warm cup of tea dissolves more sugar. The solid-versus-gas contrast is a general trend rather than a universal rule; some solids buck it entirely.

Pressure above the liquid.

Chem 1539 Henry's Law for Gas Solubility

Henry's law states that a gas's solubility in a liquid is directly proportional to that gas's partial pressure above the liquid, at fixed temperature. It covers solubility as pressure changes at a fixed temperature — not a simultaneous temperature change, which the previous entry's trend governs instead.

Three entries. What the word colligative means and why it is surprising, the concentration measure these properties require, and how to get there from the one you already know.

Number, not identity.

Chem 1544 Colligative Properties Defined

A colligative property of a solution depends on the number of dissolved solute particles present, not on their chemical identity. Colligative means the particle count is what matters — swapping one nonelectrolyte solute for another at equal molality changes nothing about the effect, which is an unusual kind of chemical result.

Two concentration measures.

Chem 1547 Mole Fraction and Molality

Mole fraction is a component's moles divided by the total moles present; molality is solute moles divided by kilograms of solvent, and unlike molarity it does not shift with temperature, since it uses mass rather than volume. Molality divides by the mass of solvent alone, not the whole solution — confusing the two gives a close but wrong value.

From one to the other.

Chem 1550 Converting Between Molarity and Molality

Converting molarity to molality means using the solution's density to find the mass of one litre of solution, subtracting the dissolved solute's mass to isolate the solvent's mass, then dividing the known moles of solute by that solvent mass in kilograms. Subtracting the solvent's mass from the total instead of the solute's inverts the calculation and produces a molality with no correct meaning.

Eight entries. Four things that change when a solute is added — vapour pressure falls, boiling point rises, freezing point falls, and osmotic pressure appears — each with its calculation, and then a use for them: measuring an unknown solute's molar mass.

Vapour pressure falls.

Chem 1555 Raoult's Law and Vapor Pressure Lowering

Raoult's law states that a solution's vapour pressure equals the pure solvent's vapour pressure times the solvent's mole fraction, so adding a nonvolatile solute lowers it. This form assumes that solute is nonvolatile — a volatile second component would contribute its own partial pressure instead of only diluting.

Which fraction.

Chem 1558 Calculating a Solution's Vapor Pressure Under Raoult's Law

Calculating a solution's vapour pressure means finding the solvent's mole fraction from the moles of solvent and solute present, then multiplying that fraction by the pure solvent's known vapour pressure. Using the solute's mole fraction instead of the solvent's inverts which fraction does the multiplying and produces a vapour pressure far too low.

Boiling point rises.

Chem 1561 Boiling Point Elevation

Boiling point elevation is the rise in a solution's boiling point above its pure solvent's, proportional to solute molality through a solvent-specific constant. That constant depends on the solvent's identity, not the solute's — different solvents give different elevations for the same solute at the same molality.

Freezing point falls.

Chem 1564 Freezing Point Depression

Freezing point depression is the drop in a solution's freezing point below its pure solvent's, proportional to solute molality through a separate constant. The freezing and boiling constants differ for one and the same solvent — using one in place of the other swaps in the wrong number entirely.

Which way each goes.

Chem 1568 Calculating Boiling Point Elevation or Freezing Point Depression

Calculating either effect means multiplying the solute's molal concentration by the appropriate solvent-specific constant, then adding that change to the boiling point or subtracting it from the freezing point of the pure solvent. Adding to find a new freezing point, or subtracting to find a new boiling point, reverses the direction each effect acts in — easy to do when both constants appear in one problem.

Across a membrane.

Chem 1573 Osmosis and Osmotic Pressure

Osmosis is the net movement of solvent across a semipermeable membrane from lower to higher solute concentration, and osmotic pressure is the pressure that would stop that movement. The membrane lets solvent cross both ways — osmosis is a net flow toward the more concentrated side, not a one-way gate.

A familiar equation.

Chem 1576 Calculating Osmotic Pressure

Calculating osmotic pressure means substituting a solution's molarity and kelvin temperature into the same gas-constant relationship the ideal gas law uses — osmotic pressure equals molarity times the gas constant times temperature. Using Celsius directly produces a pressure with no physical meaning, the same unit error this course flagged for gases.

Identify an unknown.

Chem 1580 Determining Molar Mass from a Colligative Property

Determining an unknown solute's molar mass means calculating its concentration from an observed freezing point change, boiling point change or osmotic pressure, then dividing the solute's known mass by the moles that concentration implies. The procedure assumes the solute does not ionise — applying it to an electrolyte without correcting for dissociation understates the true molar mass.

Five entries closing the unit. The factor that repairs the assumption the last entry had to make, a third class of mixture sitting between solutions and suspensions, and then the capstone running a single salt from dissolution to a corrected freezing point.

Counting the real particles.

Chem 1584 The van't Hoff Factor

The van't Hoff factor is the measured ratio of dissolved particles to formula units dissolved, correcting a colligative calculation for ionisation — sodium chloride predicts two, magnesium chloride three. A measured factor visibly below its predicted whole number is not measurement error: it reflects real residual attraction between ions still pairing up in solution.

Apply the factor.

Chem 1587 Freezing Point Depression for an Electrolyte Solution

Calculating an electrolyte solution's freezing point depression means multiplying the ordinary formula by the solute's van't Hoff factor, to account for the extra particles ionisation produces. Omitting it, and treating an electrolyte as though it were a nonelectrolyte, understates the depression by roughly the same factor as the ionisation would have added particles.

Between the two.

Chem 1591 Colloids Distinguished from Solutions and Suspensions

A colloid has particles larger than a true solution's but small enough that, unlike a suspension, they never settle out on standing. That places it between the two: a solution's particles are too small to settle and a suspension's are large enough to, while a colloid's stay dispersed indefinitely despite being visibly larger than dissolved molecules.

A visible beam.

Chem 1594 The Tyndall Effect

The Tyndall effect is light scattering by colloidal particles, which are large enough to interact with light where a solution's particles are not. A flashlight beam is visible crossing a foggy room as water droplets scatter it, while the same beam stays invisible through clear salt water. A coloured solution's beam also stays invisible — its particles are too small to scatter, however strongly they absorb.

Magnesium chloride, end to end.

Chem 1598 Capstone: One Aqueous Solution from IMF to Corrected Freezing Point

Magnesium chloride dissolves in water by ion-dipole attraction, water's polar molecules surrounding and stabilising the ions as the lattice breaks apart. Because it separates into ions it is a strong electrolyte, predicted to give three particles per formula unit — one magnesium ion and two chloride. A 0.200 molal solution, with that factor of three and water's freezing constant, gives a depression of about 1.12 degrees, so a freezing point near minus 1.12 — substantially colder than the 0.37 degrees the same solution would show if the correction were skipped. Treating dissolution, classification and calculation as three unrelated facts is the fragmented reading the capstone exists to correct.

Five things to carry out of this part, and out of the unit. "Like dissolves like" is a heuristic summarising a real mechanism — dissolution succeeds when new solute-solvent attractions can compete with the solute-solute and solvent-solvent attractions being given up — but it is not a strict law, and some polar and nonpolar pairs dissolve partially rather than not at all. A property is colligative when it depends on the number of dissolved solute particles and not on their chemical identity, which means swapping one nonelectrolyte for another at equal molality changes nothing. Molality is solute moles per kilogram of solvent, where molarity is moles per litre of solution — two differences that matter: molality uses the solvent alone rather than the whole solution, and because it uses mass rather than volume it does not shift with temperature, which is why the colligative formulas use it. Each colligative effect acts in a fixed direction: vapour pressure falls, boiling point rises, freezing point falls — so the calculated change is added to a boiling point and subtracted from a freezing point, and reversing that is easy when both constants sit in one problem. And the van't Hoff factor corrects for ionisation, counting the particles a formula unit actually produces rather than assuming one: without it an electrolyte's freezing point depression is understated by roughly the ionisation factor. A measured value slightly below the predicted whole number is not error but real ion pairing in solution. Unit Nine takes up kinetics.