How do you calculate and use Rf values?

Rf compares how far a substance moves with how far the solvent moves, and identifies unknowns. Rf = \frac{distance moved by substance}{distance moved by solvent}

Purity, Formulations and Chromatography Notes

AQA GCSE Chemistry: Revision notes

Key facts

A pure substance has a fixed melting point and boiling point; a mixture melts and boils over a range.
A formulation is a mixture designed, in set proportions, for a useful purpose, such as a medicine, alloy, fuel or fertiliser.
Chromatography separates a mixture; one spot means pure, more than one means a mixture.
$R_f$ = distance moved by the substance ÷ distance moved by the solvent. It is always between 0 and 1.
Use a pencil baseline, and compare $R_f$ values with standards in the same solvent.

Pure or mixture?

A pure substance has a fixed melting and boiling point; a mixture melts and boils over a range.

A pure substance is a single element or compound with nothing else mixed in. Its melting point and boiling point are fixed values.

A mixture has two or more substances that are not chemically bonded. Each component has its own melting and boiling point, so the mixture changes state over a range of temperatures.

Pure substance Mixture Composition Single element or compound Two or more substances Melting point Fixed, specific value Range of temperatures Boiling point Fixed, specific value Range of temperatures

	Pure substance	Mixture
Composition	Single element or compound	Two or more substances
Melting point	Fixed, specific value	Range of temperatures
Boiling point	Fixed, specific value	Range of temperatures

Pure substance

Composition:: Single element or compound
Melting point:: Fixed, specific value
Boiling point:: Fixed, specific value

Mixture

Composition:: Two or more substances
Melting point:: Range of temperatures
Boiling point:: Range of temperatures

Pure substance

Mixture

A pure substance has one kind of particle; a mixture has more than one

A solid melts over the range 112 °C to 118 °C. What does this show?

Formulations

A formulation is a mixture designed, in precise proportions, to have useful properties for a purpose.

A formulation is a designed mixture. It can combine the best properties of several substances, which a single pure substance cannot.

It is a physical mixture, not a compound, so it can be separated.

Made of Purpose Medicines Active ingredient plus fillers and binders Right dose reaches the body Fuels Blended hydrocarbons Specific energy and burning Alloys Metals (or a metal and carbon) Stronger and harder than pure metals Fertilisers Nitrogen, phosphorus and potassium compounds Plant growth

	Made of	Purpose
Medicines	Active ingredient plus fillers and binders	Right dose reaches the body
Fuels	Blended hydrocarbons	Specific energy and burning
Alloys	Metals (or a metal and carbon)	Stronger and harder than pure metals
Fertilisers	Nitrogen, phosphorus and potassium compounds	Plant growth

Made of

Medicines:: Active ingredient plus fillers and binders
Fuels:: Blended hydrocarbons
Alloys:: Metals (or a metal and carbon)
Fertilisers:: Nitrogen, phosphorus and potassium compounds

Purpose

Medicines:: Right dose reaches the body
Fuels:: Specific energy and burning
Alloys:: Stronger and harder than pure metals
Fertilisers:: Plant growth

Why is steel, an alloy of iron and carbon, described as a formulation?

Paper chromatography

Different substances dissolve in the solvent to different extents, so they travel up the paper at different rates.

Substances differ in how soluble they are in the solvent and how strongly they stick to the paper. The solvent rises by capillary action, carrying components at different rates.

Draw the baseline in pencil, never pen, because ink would separate. Stand the paper in a little solvent below the baseline. Mark the solvent front in pencil before the paper dries.

1
Draw a baseline
in pencil, near the bottom
2
Add the spot
a small spot of the mixture
3
Stand in solvent
level below the baseline
4
Solvent rises
carrying components at different rates
5
Mark the solvent front
in pencil, before it dries
6
Read the chromatogram
each spot is a different substance

Paper chromatography method

Why must the baseline be drawn in pencil rather than pen?

Rf values

Rf compares how far a substance moves with how far the solvent moves, and identifies unknowns.

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

Measure both from the baseline, to the centre of the spot and to the solvent front. $R_f$ has no units and is always between 0 and 1.

The same substance has the same $R_f$ in the same solvent, so run standards alongside the unknown.

$R_f$ $\dfrac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

Worked example

A spot travels 6 cm and the solvent front 10 cm from the baseline. Calculate the $R_f$ value.

1
Baseline to centre of the spot: 6 cm.
2
Baseline to solvent front: 10 cm.
3
$R_f = 6 \div 10$ .

Which $R_f$ value is impossible?

Reading a chromatogram

Count the spots for purity, and match Rf values to standards for identity.

One spot means the substance is pure. Two or more spots means a mixture.

Match each unknown spot to a standard with the same $R_f$ in the same solvent. Only position identifies a substance; the size or darkness of a spot does not.

Pure substance Mixture Spots One Two or more Conclusion Pure Mixture of that many components

	Pure substance	Mixture
Spots	One	Two or more
Conclusion	Pure	Mixture of that many components

Pure substance

Spots:: One
Conclusion:: Pure

Mixture

Spots:: Two or more
Conclusion:: Mixture of that many components

Worked example

Food colouring shows two spots, at $R_f$ 0.3 and 0.6. Red dye has $R_f$ 0.3 and yellow dye has 0.6 in the same solvent. What is the sample?

1
Two spots, so it is a mixture.
2
0.3 matches red dye; 0.6 matches yellow dye.

A chromatogram of a drink shows a single spot. What can you conclude?

Try an exam question

A student runs a chromatogram. A spot moves 4.8 cm and the solvent front moves 8.0 cm from the baseline. Calculate the $R_f$ value and explain why the baseline is drawn in pencil.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.