Separating Mixtures: Quick Revision

A brief note on Separating Mixtures. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Every method uses a difference in a property between the parts, and none of them makes a new substance.
• Chromatography separates dyes that travel at different speeds on paper. $R_f = \dfrac{d}{D}$, with no unit, between $0$ and $1$.
• Crystallisation purifies a solid: impurities stay dissolved as the hot solution cools.
• Distillation separates liquids that mix by their boiling points: simple if they are more than about $25\ \text{K}$ apart, fractional if closer.

Which method for which mixture

A worked example

The solvent rises $10.5\ \text{cm}$, and two spots rise $7.35\ \text{cm}$ and $2.52\ \text{cm}$. Find their $R_f$ values, and where each would be with the solvent at $14\ \text{cm}$.

1. The upper spot. $R_f = \dfrac{7.35\ \text{cm}}{10.5\ \text{cm}} = 0.70$.
2. The lower spot. $R_f = \dfrac{2.52\ \text{cm}}{10.5\ \text{cm}} = 0.24$.
3. A longer run. Each dye keeps its $R_f$: $0.70 \times 14\ \text{cm} = 9.8\ \text{cm}$ and $0.24 \times 14\ \text{cm} = 3.36\ \text{cm}$.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Measuring from the top of the paper. Both distances start at the starting line.
• An $R_f$ above $1$. No spot passes the solvent front: you have divided the wrong way round.
• Simple distillation for close boiling points. Below about $25\ \text{K}$ apart, use a fractionating column.
• Filtering a solution. A dissolved solid passes straight through the paper.

Check yourself

1. A spot rises $6.5\ \text{cm}$ while the solvent rises $13\ \text{cm}$. What is its $R_f$?

2. How would you get pure water from muddy, salty water?

3. Which kind of distillation separates two liquids that boil at $340\ \text{K}$ and $352\ \text{K}$?

4. Why can a centrifuge separate cream from milk faster than leaving it to stand?