C10 RP Required Practical: Chromatography

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: required practical 12: using paper chromatography to separate and tell apart coloured substances, and calculating Rf values. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how to carry out paper chromatography to separate coloured substances
• Identify the variables, hazards and sources of error in the practical
• Calculate Rf values and use them to identify the substances in a mixture
• Analyse results, calculate means and uncertainties, and suggest improvements

Key words

• Independent variable: The variable you change: here, the coloured substance being tested.
• Dependent variable: The variable you measure: the distance each spot moves (used to find Rf).
• Control variable: A variable kept the same so the test is fair, e.g. solvent, paper, temperature.
• Uncertainty: The range in which the true value is likely to lie: ± half the range of repeat results.
• Risk assessment: Identifying hazards and how to reduce the risk of harm.

Aim, apparatus and method

Aim: to use paper chromatography to separate and tell apart coloured substances, such as inks or food colourings, and to calculate Rf values.

Method 1. Draw a pencil line about 1.5 cm from the bottom of a strip of chromatography paper. 2. Use a capillary tube to put a small, concentrated spot of each sample and each reference dye on the line, spaced apart. Let the spots dry. 3. Pour solvent (e.g. water) into a beaker to a depth of about 1 cm, so it is below the baseline. 4. Hang the paper so its bottom dips into the solvent without touching the sides. Put on a lid. 5. When the solvent is near the top, remove the paper and mark the solvent front in pencil. Let it dry. 6. Measure from the baseline to the centre of each spot and to the solvent front. Calculate the Rf values.

• independent; In this practical the coloured substance (sample) being tested
• dependent; In this practical distance moved by each spot, used to calculate Rf
• control; In this practical type of paper; solvent; temperature; height of baseline; size of spots; lid on

Results and analysis

Worked example: analysing the chromatogram

Spots of the purple ink moved 2.4 cm and 5.6 cm. The solvent moved 8.0 cm.
Rf = 2.4 ÷ 8.0 = 0.30 and 5.6 ÷ 8.0 = 0.70. These match dye 1 (0.30) and dye 3 (0.70).
Repeat Rf values for the upper spot: 0.69, 0.72, 0.69. Mean = 2.10 ÷ 3 = 0.70; uncertainty = ± (0.72 − 0.69) ÷ 2 = ± 0.015

Answer: the purple ink contains dyes 1 and 3 (red and blue)

Key fact: A sample that gives one spot may be pure. A substance that stays on the baseline is insoluble in that solvent – try a different solvent.

Errors, improvements and safety

• spots too big or too dilute; Effect spots spread out and overlap; hard to measure; Improvement small, concentrated spots; dry between applications
• solvent above the baseline; Effect samples dissolve into the solvent in the beaker; Improvement shallower solvent; baseline higher
• no lid; Effect solvent evaporates; front moves unevenly; Improvement cover the beaker
• measuring to the edge of a spot; Effect Rf values inaccurate; Improvement measure to the centre of each spot
• two substances with the same Rf; Effect cannot tell them apart; Improvement repeat with a different solvent

Resolution: a ruler marked in millimetres measures distances to the nearest 0.1 cm, so Rf values should be given to 2 decimal places, not more.

Safety: Some solvents (e.g. ethanol) are flammable – keep them away from flames. Wear eye protection. Some dyes stain skin.

Common mistakes

• Drawing the baseline in ink, or putting the solvent above the baseline.
• Measuring Rf distances from the bottom of the paper instead of the baseline.
• Calling the solvent the “independent variable”. In this practical the solvent is a control variable.
• Forgetting to mark the solvent front before it evaporates.

Revision questions

Try these, then check your answers below each one.

Question 1. Why must the solvent level be below the baseline? (Choose from: So the spots do not dissolve into the solvent in the beaker, So the solvent evaporates more slowly, So the paper does not tear, So the Rf values are bigger.)

Answer: So the spots do not dissolve into the solvent in the beaker

Working: If the spots are under the solvent, they wash into the beaker instead of moving up the paper.

Question 2. In this practical, which is a control variable? (Choose from: The solvent used, The ink being tested, The distance moved by each spot, The Rf value.)

Answer: The solvent used

Working: The solvent (and paper, temperature, baseline height) is kept the same. The ink is the independent variable; the distance moved is the dependent variable.

Question 3. Three repeats gave Rf values of 0.36, 0.40 and 0.38 for one dye. Calculate the mean Rf value.

Answer: 0.38

Working: (0.36 + 0.40 + 0.38) ÷ 3 = 1.14 ÷ 3 = 0.38.

Related lessons

• Previous lesson: C10.3 Testing for Gases
• Next lesson: C11.1 History of Our Atmosphere
• In this chapter: C10.1 Pure Substances and Mixtures
• In this chapter: C10.2 Analysing Chromatograms

Continue in the full course

This lesson is section C10 RP (AGCSC2704) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.