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Checking purity: melting temperature and chromatography

Edexcel IAL Chemistry · WCH16 Practical Skills in Chemistry II

After a preparation you must show the product is what you think it is, and that it is pure. Melting temperature questions appeared in both January 2022 and October 2024.

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Measuring a melting temperature

1. Make sure the solid is dry and finely powdered.

2. Seal one end of a capillary tube and push in solid to a depth of about 2 to 3 mm.

3. Attach the tube to a thermometer so the solid is level with the bulb, and place it in a Thiele tube of oil (or an oil bath, or an electric melting point apparatus).

4. Heat gently, and slowly (about 1 to 2 °C per minute) near the melting point.

5. Record the temperature when the solid starts to melt and when it is completely liquid.

A water bath cannot be used for a solid melting above 100 °C (October 2024 report).

Interpreting it

A pure solid melts sharply (over about 1 °C) at the data-book value.
An impure solid melts lower and over a wider range.
A mixed melting point (sample mixed with an authentic sample) that is unchanged confirms identity.
For liquids, a boiling temperature close to the data value, over a narrow range, indicates purity; impurities raise and broaden it.

Thin-layer chromatography

Draw the baseline in pencil (ink would dissolve and run). Spot the sample and a pure reference.
Stand the plate in solvent whose level is below the baseline; cover with a lid so the air is saturated with vapour.
Remove when the solvent is near the top; mark the solvent front. Locate colourless spots with UV light or iodine.
Rf = distance moved by spot / distance moved by solvent.
One spot that matches the reference suggests purity; extra spots show impurities.

Worked example

A spot moves 3.6 cm and the solvent front 8.0 cm. Calculate Rf.

Rf = 3.6 / 8.0

Answer: Rf = 0.45

Key idea: Dry, powdered sample in a capillary, oil not water above 100 °C, heat slowly near the melting point, record the range. Pure: sharp, at the data value; impure: lower and broader. Rf = spot distance / solvent distance.

Check yourself

On a TLC plate a spot travels 4.2 cm while the solvent front travels 7.0 cm. Calculate the Rf value.
Show answer
0.60. 4.2 / 7.0 = 0.60.
A compound has Rf = 0.62 in a solvent. The solvent front moves 7.5 cm. How far does the spot move?
Show answer
4.7 cm. 0.62 x 7.5 = 4.65 cm.
Why is the TLC baseline drawn in pencil?
- Pencil marks glow under UV light
- Pencil is easier to see
- Ink would react with the sample
- Pencil does not dissolve in the solvent and move up the plate
Show answer
Pencil does not dissolve in the solvent and move up the plate. Ink is made of dyes that would separate.
A solid has a melting temperature of 131 °C. Why is an oil bath, not a water bath, used?
- Water boils at 100 °C, so it cannot reach the melting temperature
- Oil is less flammable than water
- Water would dissolve the sample
- Oil gives a sharper melting point
Show answer
Water boils at 100 °C, so it cannot reach the melting temperature. The bath must reach a temperature above the melting point.
Why is the sample heated slowly close to its melting temperature?
- To stop the sample burning
- To save energy
- So the thermometer and sample stay at the same temperature and the reading is accurate
- To make the solid melt over a wider range
Show answer
So the thermometer and sample stay at the same temperature and the reading is accurate. Fast heating makes the thermometer lag behind the sample.
How does the melting temperature of an impure sample compare with that of the pure compound?
- Lower and over a wider range
- Higher and sharper
- The same but over a wider range
- Higher and over a wider range
Show answer
Lower and over a wider range. Impurities disrupt the lattice.
Why must the solvent level be below the baseline on a TLC plate?
- To make the plate dry faster
- So the solvent front can be marked
- Otherwise the spots dissolve into the solvent instead of moving up the plate
- To keep the plate upright
Show answer
Otherwise the spots dissolve into the solvent instead of moving up the plate. The samples would wash off into the solvent.

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Last updated 8 November 2026. Spotted a mistake? Tell us.