Mixtures and Separation of Mixtures (Advanced)

Separating by differences: column chromatography, simple and fractional distillation chosen by boiling points, crude oil fractions, and percentage composition.

Key ideas

Chromatography (Tswett, 1906) separates components that move at different rates over a stationary phase.
In a column, silica gel or alumina is the stationary phase and the eluent is the mobile phase.
The least strongly adsorbed component comes out first; the most strongly held comes out last.
Simple distillation separates miscible liquids whose boiling points differ by at least 25 °C; closer ones need fractional distillation.
A fractionating column gives surfaces for repeated condensation and vaporisation, so the vapour gets richer in the lower-boiling liquid.
The column is hottest at the bottom: liquids come over in order of rising boiling point, and petroleum gas is collected at the top.

Remember

Choosing the method	difference < 25 °C → fractional; ≥ 25 °C → simple distillation
Order of collection	lowest boiling point first
Crude oil, top to bottom	petroleum gas, petrol, kerosene, diesel, fuel oil, lubricating oil
Percentage composition	part ÷ whole × 100

Watch out

Saying chromatography separates by boiling point.
Thinking the most strongly adsorbed dye comes out first.
Using simple distillation for liquids that boil only a few degrees apart.

Worked example

Fractional distillation of 130 litres of crude oil gives 39 L of diesel, 21 L of fuel oil, 40 L of lubricating oil, and the rest is residue. Find (a) the percentage of diesel and (b) the percentage of lubricating oil. (c) Which of these three fractions is collected highest in the column? (Give the answer to 2 decimal places where needed.)

Working: (a) 39 ÷ 130 × 100; (b) 40 ÷ 130 × 100; (c) the lowest boiling point rises highest

Answer: (a) 30%; (b) 30.77%; (c) diesel