The Chemical Detective

Pure means something narrower here

A chemist handed an unknown white powder cannot simply look at it. Identifying a substance means gathering evidence and reasoning from it, which is why this topic is really about detective work. Three kinds of evidence come up, and they are the three parts of this module. Whether the substance is pure. Whether it is a formulation, meaning a mixture built on purpose. And what a chromatogram shows about what it contains. Before any of that, one word has to be fixed, because it causes more lost marks on this topic than anything else. In everyday English, pure means nothing has been added: pure orange juice, pure wool. In chemistry it means something much narrower. A pure substance is a single element or a single compound, not mixed with anything else at all. By that definition orange juice is not pure, however natural it is, because it is a mixture of many compounds. Nor is air, nor sea water, nor most things you would casually call pure. Hold that distinction, because the whole first half of this topic depends on it, and because examiners test it directly almost every year.

Words this topic needs

Six terms to use precisely. The two phases are the pair most often mixed up, so read them together and notice that one moves and one does not.

The everyday meaning is wrong

A carton of milk is labelled "pure". Is milk a pure substance in the chemical sense?

• No, because milk is a mixture of many different compounds, and a pure substance must be a single element or compound
• Yes, because nothing has been added to it
• Yes, as long as it has been filtered
• No, because milk is not an element

One temperature, or a range

How melting data lets you tell the two apart. This is the practical test for purity, and it is also why formulations behave the way they do.

Match each formulation to its purpose

Which two are true of a formulation

Select the TWO statements that are true of a formulation.

• Each component is present in a measured quantity, chosen so the product does its job
• It melts over a range of temperatures rather than at one fixed point
• It is a pure substance, because it is made to a precise recipe
• Its components are always elements rather than compounds

Purity and formulations in words

In chemistry a _____ substance is a single element or compound, which is narrower than the everyday meaning. A pure substance melts at one _____ temperature, whereas a mixture melts over a _____. A _____ is a mixture designed as a useful product, with each component present in a _____ quantity. Fuels, paints, medicines and alloys are all examples.

Two phases, one separation

Chromatography separates the substances in a mixture, and the whole method rests on two phases with different jobs. The stationary phase does not move. In paper chromatography it is the paper. The mobile phase does move: it is the solvent, which travels up the paper and carries dissolved substances along with it. Separation happens because the substances in the mixture are not equally attracted to the two phases. A substance that is more attracted to the moving solvent, and dissolves in it readily, is carried a long way up the paper. A substance more attracted to the paper is held back and travels only a short way. Since different substances have different balances of attraction, they end up in different places, and a mixture that looked like a single spot at the start becomes several spots. That is why the technique identifies as well as separates. A pure substance gives ONE spot on a chromatogram, whichever solvent is used. A mixture gives more than one. And because the same substance always travels the same fraction of the way in the same conditions, you can compare an unknown against a known reference and see whether they match.

Set up the experiment

Required practical 6. Put the steps of a paper chromatography experiment into the correct order.

• Draw a baseline near the bottom of the paper, using pencil
• Place a small spot of each mixture or reference substance on the baseline
• Pour solvent into the container so its level is below the baseline
• Stand the paper in the solvent and leave it so the solvent travels up the paper
• Remove the paper before the solvent reaches the top, and mark the solvent front
• Let the paper dry, then measure the distances needed to calculate Rf values

Why pencil, why below

The practical decisions that carry marks. Work out the reason behind each one.

• Why is the baseline drawn in pencil rather than pen?
• Why must the solvent level start below the baseline?
• A student's chromatogram shows one spot for their sample. What can they conclude?
• They want to identify the substance rather than just test its purity. What do they need?

Find the Rf value

On a chromatogram, a spot has travelled 3.6 cm from the baseline while the solvent has travelled 8.0 cm. Calculate the Rf value. Give your answer as a decimal, remembering that an Rf value has no units.

An Rf value, interpreted

Here is the calculation done properly, and then the part students often leave out: what the number is for. A chromatogram is run and the solvent front is measured at 9.0 cm from the baseline. One spot has travelled 5.4 cm. The Rf value is the distance moved by the spot divided by the distance moved by the solvent: 5.4 divided by 9.0, which is 0.6. Three things about that answer are worth checking every time. It has no units, because it is one distance divided by another and the centimetres cancel. It is less than 1, and it always must be, because a spot cannot travel further than the solvent that carries it. And it is quoted to a sensible number of figures rather than a long decimal. Now the interpretation. On its own, 0.6 identifies nothing. Its value is that the same substance gives the same Rf in the same conditions, so if a known reference substance run on the same paper in the same solvent also gives 0.6, the unknown may well be that substance. If the reference gives 0.3, it is not. Notice the careful wording: may well be. Two different substances can happen to share an Rf value, which is why a match is good evidence rather than proof, and why saying so earns credit in an evaluation question.

The claim about purity that fails

Four sentences from answers on this topic. Select the ONE that is incorrect.

• A pure substance melts at a sharp, fixed temperature, whereas a mixture melts over a range.
• An Rf value has no units and is always less than 1.
• A formulation is a pure substance, because the quantity of each component is measured precisely.
• A pure substance produces a single spot on a chromatogram, whichever solvent is used.

Explain purity, formulations and chromatography

A chemist is given an unknown white solid. Explain how they could find out whether it is pure, what a formulation is and how it differs, and how chromatography would help identify what the solid contains.

• Define a pure substance in the chemical sense, and say how that differs from everyday use
• Explain how melting point data shows whether a substance is pure
• Define a formulation, including that each component is present in a measured quantity
• Give two examples of formulations and say what their proportions are chosen for
• Name the mobile and stationary phases in paper chromatography and say which moves
• Explain why different substances travel different distances up the paper
• Give the equation for Rf and state that it has no units and is less than 1
• Explain why the baseline is drawn in pencil and why the solvent starts below it