IB CHEM 1 - Lesson 1 Part 2: Separating Mixtures

Q: What determines the choice of separation method?

A: The difference in physical properties between components (e.g., boiling point, solubility, adsorption, magnetism, density).

Q: Define solvation.

A: Process where solvent particles surround & attract solute particles, allowing dissolution; in water it’s called hydration.

Q: Why is solvent choice critical for separations?

A: It must dissolve the target but not the others, enabling selective removal (e.g., salt dissolves in water; sand doesn’t).

Q: Steps: separate salt + sand.

A: Add water → stir → filter (sand = residue, salt solution = filtrate) → evaporate water → collect salt.

Q: Define filtration.

A: Separation of insoluble solid from liquid/solution using filter paper (filtrate passes; residue remains).

Q: When to use vacuum filtration?

A: For fine/compact solids or when gravity filtration is slow—uses reduced pressure to speed collection.

Q: When to use centrifugation?

A: To separate tiny or dense particles from liquid when filtration is ineffective; separation by density under rotation.

Q: Define crystallisation (crystallization).

A: Recovery of a dissolved solid by evaporating solvent and cooling so solubility decreases and crystals form.

Q: Key steps in crystallisation.

A: Gently heat to concentrate → cool to saturation → seed/test on cold rod → cool slowly → filter, wash, dry.

Q: Two critical cautions in crystallisation.

A: Don’t boil to dryness; and cool slowly for larger, purer crystals.

Q: Define recrystallisation.

A: Purifying an impure solid: dissolve in minimum hot solvent, remove insoluble impurities (hot filtration), cool slowly to crystallise, vacuum filter, wash cold solvent, dry.

Q: Why “minimum hot solvent” in recrystallisation?

A: Minimizes product loss; ensures target recrystallises upon cooling.

Q: Why wash crystals with cold solvent?

A: To remove soluble impurities without redissolving the product.

Q: Define simple distillation.

A: Separation of a solvent from solute or liquid with a much lower bp than others; vapor → condenser → distillate.

Q: When is fractional distillation required?

A: For miscible liquids with similar boiling points (e.g., ethanol 78 °C vs water 100 °C).

Q: Purpose of the fractionating column.

A: Provides repeated vaporization–condensation cycles (theoretical plates) for better separation.

Q: Safety: heating with flammable liquids.

A: Use an electric heater (not open flame) to avoid ignition.

Q: Define paper chromatography (paper TLC conceptually).

A: Separates dissolved substances by differential solubility in the mobile phase and adsorption to the stationary phase (paper).

Q: Why mark the baseline with pencil?

A: Graphite does not dissolve or migrate; ink would smear and contaminate the chromatogram.

Q: Why must sample spots be above solvent level initially?

A: Prevents dissolving into the reservoir; forces capillary rise to carry analytes up the paper.

Q: What makes a spot travel further in paper chromatography?

A: Higher solubility in the mobile phase and weaker adsorption to the paper.

Q: What makes a spot move less?

A: Stronger adsorption to paper and lower solubility in the solvent.

Q: (Extension) Define R_f ​ value.

A: R_f ​ = “distance solute travels/distance solvent front travels” (0–1). Characteristic in given conditions.

Q: Example mapping: air separation method + property.

A: Fractional distillation of liquefied air; components have different boiling points.

Q: Example mapping: food dyes method + property.

A: Chromatography; dyes differ in solubility/adsorption.

Q: Example mapping: iron + sulfur method + property.

A: Magnetic separation; iron is ferromagnetic, sulfur is not.

Q: Quick diagnostic: mixture vs compound in a particle diagram.

A: Mixture: varied, unbonded particles intermingled. Compound: identical bonded units in fixed ratio.

Q: After solvation, why filter?

A: To remove undissolved solids (residue) and collect solution (filtrate) for further steps.

Q: Order of operations: dissolve → filter → crystallise vs filter → dissolve → crystallise?

A: Dissolve → filter → crystallise is correct; filtration works after selective dissolution.

Q: Column choice: simple or fractional for 78 °C ethanol + 100 °C water?

A: Fractional (close bps), heat to ~78 °C first to collect ethanol.