Colligative Properties

Forty Q&A flashcards summarising key facts, equations and everyday applications of colligative properties, suitable for exam revision.

What is the definition of a colligative property?

A property that depends only on the relative number of solute and solvent particles present, not on the chemical identity of the solute.

List the four classical colligative properties.

(1) Vapour-pressure lowering, (2) Boiling-point elevation, (3) Freezing-point depression, and (4) Osmosis.

How does adding a non-volatile solute affect the vapour pressure of a solvent?

It lowers the vapour pressure because fewer solvent molecules are able to escape into the vapour phase.

State Raoult’s Law in its simplest algebraic form.

Psolution = Psolvent × X_solvent

In Raoult’s Law, what does X_solvent represent?

The mole fraction of the solvent in the liquid phase of the solution.

Under what condition is Raoult’s Law obeyed exactly?

When the solution is ideal—that is, solute–solvent interactions equal solvent–solvent and solute–solute interactions.

What causes a positive deviation from Raoult’s Law?

Weaker solute–solvent interactions than solvent–solvent interactions, leading to a higher vapour pressure than predicted.

Which thermodynamic quantity decides the most stable phase at a given temperature?

Gibbs Free Energy (G).

Write the expression for the Gibbs free energy change of vaporisation.

ΔGvap = ΔHvap – TΔS_vap

Why is ΔS_vap smaller for a solution than for the pure solvent?

Because the liquid phase of a solution is already more disordered (higher entropy) due to mixing, so the entropy gain when molecules enter the gas phase is smaller.

What is the consequence of a smaller ΔS_vap on vapour pressure?

It makes ΔG_vap less negative, so fewer molecules evaporate and the vapour pressure is lower.

Why does a solution have a higher boiling point than the pure solvent?

A higher temperature is required for ΔGvap to reach zero because ΔSvap is reduced; this shifts the boiling point upward.

During boiling-point elevation, which thermodynamic term remains essentially unchanged by the solute?

ΔH_vap (the enthalpy of vaporisation).

Complete the equality that holds exactly at the boiling point: ΔHvap = .

T × ΔS_vap

Why does spreading salt on icy roads melt ice?

Salt lowers the freezing point of water (freezing-point depression), so ice can melt even when ambient temperatures are below 0 °C.

How does antifreeze protect a car’s cooling system in winter?

Ethylene or propylene glycol depresses the freezing point and raises the boiling point of the coolant, preventing freezing and boil-over.

Write the general equation used to calculate freezing-point depression.

ΔTf = kf × m × i

What is the purpose of the van ’t Hoff factor, i, in colligative equations?

It adjusts for the actual number of dissolved particles produced by electrolytes (ions vs. ion pairs).

Why can drinking Lucozade® cause darker-coloured urine?

Its high glucose concentration draws water osmotically, concentrating urinary pigments and making the urine appear darker yellow.

Give a concise definition of osmosis.

The net flow of solvent through a semipermeable membrane from a region of lower solute concentration to one of higher solute concentration.

Provide the equation for osmotic pressure.

Π = M R T

What does the symbol Π represent in colligative chemistry?

Osmotic pressure, the external pressure required to stop net solvent flow across a semipermeable membrane.

How does renal dialysis exploit colligative principles?

It uses a membrane permeable to both solvent and small solutes so waste diffuses out while osmotic balance keeps essential ions and water in the blood.

Why is water homeostasis in the body considered an application of osmosis?

Kidney tubules create osmotic gradients that regulate how much water is reabsorbed or excreted, controlling blood volume and concentration.

Why can two HFA metered-dose inhalers (e.g., QVAR and Ventolin) feel different in the mouth?

Differences in propellant composition alter vapour pressure and cooling (enthalpy of vaporisation), both colligative effects perceived as mouth-feel.

Name an everyday example of boiling-point elevation.

Adding salt to cooking water slightly raises its boiling point, though the effect is small.

Name an everyday example of vapour-pressure lowering.

Adding a non-volatile solute such as sugar to water lowers its vapour pressure, slowing evaporation (e.g., syrup staying moist).

Define molality.

The number of moles of solute per kilogram of solvent (mol kg⁻¹).

List two salts commonly used for winter road grit.

Sodium chloride (NaCl) and calcium chloride (CaCl₂).

Which two glycols are commonly used as antifreeze agents?

Ethylene glycol and propylene glycol.

What property of liposome drug formulations must be controlled via colligative considerations?

Osmotic balance/isotonicity to prevent liposome swelling, shrinkage, or cell damage.

In an ideal solution, how do solute–solvent interactions compare with solute–solute interactions?

They are equal in strength, leading to perfect adherence to Raoult’s Law.

How does ΔG_vap of a solution compare with that of the pure solvent below the boiling point?

It is more positive, making vaporisation even less spontaneous for the solution.

Is the melting/freezing point of a solution higher or lower than that of the pure solvent?

Lower; this is the freezing-point depression effect.

Explain thermodynamically why freezing-point depression occurs when a solute is added.

The solute raises liquid-phase entropy more than solid-phase entropy, so a lower temperature is required for ΔG_melt to reach zero.

What does it mean for a solute to be ‘non-volatile’?

It has negligible vapour pressure and therefore does not enter the gas phase under the conditions considered.

From which Greek root does the word “osmosis” derive, and what does it mean?

From Greek “osmos,” meaning “push.”

Give one industrial or clinical use of vapour-pressure lowering.

Predicting and controlling propellant behaviour in aerosol inhalers (e.g., adjusting ethanol content in HFA formulations).

How does the presence of ethanol influence the vapour pressure of an aqueous solution?

By lowering the mole fraction of water (and altering intermolecular forces), ethanol reduces water’s partial vapour pressure according to Raoult’s Law.