12.7 Required Practicals: Chemical Analysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: aQA required practicals: paper chromatography with Rf values, and identifying the ions in unknown compounds. 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

• Plan and carry out paper chromatography to separate coloured substances and calculate Rf values
• Identify the variables, hazards and sources of error in the chromatography practical
• Plan a sequence of chemical tests to identify the positive and negative ions in an unknown single ionic compound
• Analyse results, calculate means and uncertainties, and suggest improvements

Key words

• Independent variable: The variable you change.
• Dependent variable: The variable you measure.
• Control variable: A variable kept the same so that the test is fair.
• 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.

Chromatography practical: 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 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

Chromatography practical: results and evaluation

Worked example: analysing the chromatogram

Spots of the orange colouring moved 1.8 cm and 5.4 cm. The solvent moved 7.2 cm.
Rf = 1.8 ÷ 7.2 = 0.25 and 5.4 ÷ 7.2 = 0.75. These match dye 1 (0.25) and dye 2 (0.75).
Repeat Rf values for the upper spot: 0.74, 0.77, 0.74. Mean = 2.25 ÷ 3 = 0.75; uncertainty = ± (0.77 − 0.74) ÷ 2 = ± 0.015

Answer: the colouring contains dyes 1 and 2 (red and yellow)

• 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

Safety: Hazards: some solvents (e.g. ethanol) are flammable – keep away from flames; wear eye protection; some dyes stain skin.

Identifying ions practical: method

Aim: to use chemical tests to identify the positive ion and the negative ion in an unknown single ionic compound. Dissolve a little of the solid in distilled water, and use a fresh portion of the solution for each test.

• flame test; Method clean nichrome loop (acid, then flame); dip in solid; hold in blue flame; Positive results Li⁺ crimson; Na⁺ yellow; K⁺ lilac; Ca²⁺ orange-red; Cu²⁺ green
• hydroxide; Method add dilute NaOH a few drops at a time, then in excess; Positive results Cu²⁺ blue; Fe²⁺ green; Fe³⁺ orange-brown; Al³⁺, Ca²⁺, Mg²⁺ white (only Al(OH)₃ dissolves in excess)
• carbonate; Method add dilute HCl; pass gas into limewater; Positive results fizzes; limewater turns milky
• halide; Method add dilute HNO₃, then AgNO₃(aq); Positive results Cl− white; Br− cream; I− yellow
• sulfate; Method add dilute HCl, then BaCl₂(aq); Positive results white precipitate

Identifying ions practical: safety and evaluation

• sodium hydroxide solution is an irritant / corrosive to eyes; Precaution wear eye protection; use dilute solution
• silver nitrate is an irritant and stains skin; Precaution wear gloves / avoid contact; wash hands
• barium chloride is toxic; Precaution avoid skin contact; wash hands; dispose of it safely
• hot Bunsen burner and wire; Precaution tie hair back; let the wire cool on a heatproof mat

• Contamination: a dirty wire loop or test tube can give a false result. Clean the loop until it gives no flame colour; use clean tubes.
• Mixtures and faint colours: sodium’s yellow can mask other colours; some colours are hard to judge by eye. Flame emission spectroscopy is more reliable.
• Adding too much NaOH at once can dissolve an aluminium hydroxide precipitate before it is seen.

Exam tip: These tests are qualitative (they tell you which ions, not how much). There is no dependent variable to measure, so marks come from a logical order of tests, correct reagents, correct observations and safe working.

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 usually a control variable.
• Using hydrochloric acid before silver nitrate, or sulfuric acid before barium chloride.
• Testing the same portion of solution for every ion: the reagents added in one test can interfere with the next.

Revision questions

Try these, then check your answers below each one.

Question 1. In the chromatography required practical, what is the dependent variable? (Choose from: The distance moved by each spot, The type of paper, The coloured substance being tested, The temperature.)

Answer: The distance moved by each spot

Working: You measure how far each spot moves (and use it to calculate R_f).

Question 2. True or false? The pencil baseline should be below the level of the solvent.

Answer: False

Working: The baseline must be above the solvent, or the samples dissolve into the solvent in the beaker.

Question 3. A spot moves 3.9 cm and the solvent moves 6.5 cm from the baseline. Calculate the R_f value.

Answer: 0.6

Working: 3.9 ÷ 6.5 = 0.60

Related lessons

• Previous lesson: 12.6 Instrumental Analysis
• Next lesson: 13.1 History of Our Atmosphere
• In this chapter: 12.1 Pure Substances and Mixtures
• In this chapter: 12.2 Analysing Chromatograms
• In this chapter: 12.3 Testing for Gases

Continue in the full course

This lesson is section 12.7 (ARPCH1207) of our free GCSE Chemistry 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 Chemistry course.