Use melting and boiling points to judge purity, interpret a chromatogram, and carry out simple tests to identify oxygen, hydrogen, carbon dioxide, water and ammonia

What this dot point is asking

The syllabus wants you to judge whether a substance is pure using its melting and boiling points, to read a chromatogram, and to carry out and describe the simple tests for common gases, oxygen, hydrogen, carbon dioxide, water, and ammonia, giving both the test and the result. These are recall-and-describe skills that appear in Paper 4, where you must write the result clearly, for example the exact colour change.

The answer

Using melting and boiling points to judge purity

A pure substance has a sharp, fixed melting point and a sharp, fixed boiling point. An impurity changes both:

• An impurity lowers the melting point and makes the solid melt over a range of temperatures instead of at one value.
• An impurity raises the boiling point and makes the liquid boil over a range.

So if a solid melts sharply at the expected temperature, it is pure; if it melts low and over a spread of temperatures, it is impure. This is a simple, reliable purity check used for medicines and food chemicals.

Reading a chromatogram

A chromatogram is the paper after chromatography has finished. You read it like this:

• A pure substance gives one spot; a mixture gives two or more spots.
• Two samples that contain the same substance give spots that travel the same distance up the paper.
• Comparing an unknown against known samples lets you identify what the unknown contains.

Tests for gases

You must know the test and the result for each common gas. The result is what you would see or hear.

• Oxygen: a glowing splint relights (bursts back into flame).
• Hydrogen: a lighted splint gives a squeaky pop.
• Carbon dioxide: bubbled through limewater, it turns the limewater milky (a white precipitate).
• Ammonia: it has a sharp smell and turns damp red litmus paper blue, because ammonia is alkaline. It is the only common gas that does this.

Test for water

To test whether a liquid is water, add it to anhydrous copper(II) sulfate, which turns from white to blue. To show that water is pure, check it boils at exactly $100\ ^\circ\text{C}$ at normal pressure.

Examples in context

Example 1. Checking a medicine is pure. A drug company measures the melting point of each batch of a tablet ingredient. A pure batch melts sharply at the known temperature; a batch that melts low and over a range contains an impurity and is rejected. This shows how a melting point is used as a real quality check.

Example 2. Matching a suspect ink. In forensic work, ink from a document is compared with inks from several pens using chromatography. If the document ink gives spots that travel the same distances as one pen's ink, that pen is the likely source. The chromatogram is read by comparing spot positions against known samples.

Try this

Q1. State the result you would see if you tested a sample of pure water with anhydrous copper(II) sulfate. [1 mark]

• Cue. The anhydrous copper(II) sulfate turns from white to blue, showing water is present.

Q2. A solid melts over the range $60\ ^\circ\text{C}$ to $66\ ^\circ\text{C}$. The pure solid should melt at $68\ ^\circ\text{C}$. What does this tell you? [2 marks]

• Cue. The solid is impure: the impurity has lowered the melting point and spread it over a range instead of one sharp value.

Q3. Describe how you would test an unknown gas to show that it is ammonia. [2 marks]

• Cue. Hold damp red litmus paper in the gas; if it is ammonia the litmus turns blue, because ammonia is alkaline (and it has a sharp smell).