Chromatography, Rf Values, Pure Substances & Common Gas Tests
In chemistry, a pure substance consists of a single element or compound not mixed with any other substance, which can be verified because pure substances melt and boil at sharp, specific temperatures rather than over a range. Paper chromatography separates mixtures of soluble substances (such as food colourings or inks) based on their different distributions between a mobile phase (the solvent) and a stationary phase (the chromatography paper). Examiners consistently test the paper chromatography required practical, Rf value calculations, formulation definitions, and the four essential laboratory gas tests (hydrogen, oxygen, carbon dioxide, and chlorine).
- Pure Substances vs Everyday Purity
- In everyday language, 'pure' means nothing has been added (e.g., pure orange juice or pure mineral water), which is still a chemical mixture. In chemistry, a pure substance is a single element or compound not mixed with any other substance. Pure substances melt and boil at specific, sharp temperatures; mixtures melt and boil over a range of temperatures.
- Formulations
- A formulation is a useful mixture that has been designed by following a precise recipe: each component is present in a carefully measured quantity so that it contributes a specific property to the final product. Examples include fuels, cleaning agents, paints, medicines (tablets), alloys, fertilisers, and foods.
- Paper Chromatography: Stationary & Mobile Phases
- Chromatography separates soluble mixtures based on differential solubility and attraction between two phases: 1. The stationary phase does not move (the absorbent chromatography paper). 2. The mobile phase moves (the liquid solvent, such as water or ethanol). A substance that is more soluble in the mobile phase and less attracted to the stationary phase travels further up the paper.
Chromatography, Rf Values, Pure Substances & Common Gas Tests: Reaction Profile (ΔH & E_a) + Le Chatelier Yield Simulator
Toggle a catalyst on the enthalpy profile diagram or adjust temperature and pressure to see how dynamic equilibrium shifts.
Modelled on N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = −92 kJ/mol) (4 moles gas → 2 moles gas):
Higher pressure shifts equilibrium to the side with fewer gas moles (right, 4 → 2). Lower temperature favours the exothermic forward reaction.
Key Concepts & Exam Definitions (6)
- Pure Substances vs Everyday Purity
- In everyday language, 'pure' means nothing has been added (e.g., pure orange juice or pure mineral water), which is still a chemical mixture. In chemistry, a pure substance is a single element or compound not mixed with any other substance. Pure substances melt and boil at specific, sharp temperatures; mixtures melt and boil over a range of temperatures.
- Formulations
- A formulation is a useful mixture that has been designed by following a precise recipe: each component is present in a carefully measured quantity so that it contributes a specific property to the final product. Examples include fuels, cleaning agents, paints, medicines (tablets), alloys, fertilisers, and foods.
- Paper Chromatography: Stationary & Mobile Phases
- Chromatography separates soluble mixtures based on differential solubility and attraction between two phases: 1. The stationary phase does not move (the absorbent chromatography paper). 2. The mobile phase moves (the liquid solvent, such as water or ethanol). A substance that is more soluble in the mobile phase and less attracted to the stationary phase travels further up the paper.
- Retention Factor (Rf) Calculation
- Rf = Distance moved by substance / Distance moved by solvent. Both distances must be measured from the pencil start line (baseline), measuring to the exact centre of the spot for the substance and to the solvent front for the solvent. A pure substance produces only a single spot on the chromatogram.
- Tests for Hydrogen (H₂) & Oxygen (O₂)
- Hydrogen test: Hold a burning (lighted) wooden splint at the open end of a test tube of gas; H₂ reacts explosively with oxygen in the air (2H₂ + O₂ -> 2H₂O), giving a 'squeaky pop'. Oxygen test: Insert a glowing wooden splint into a test tube of gas; the high concentration of O₂ relights the glowing splint into a flame.
- Tests for Carbon Dioxide (CO₂) & Chlorine (Cl₂)
- Carbon dioxide test: Bubble the gas through an aqueous solution of calcium hydroxide (limewater); CO₂ reacts to form an insoluble white precipitate of calcium carbonate (Ca(OH)₂(aq) + CO₂(g) -> CaCO₃(s) + H₂O(l)), turning the limewater cloudy/milky. Chlorine test: Hold damp litmus paper in the gas; chlorine dissolves in the water to form acidic bleaching solution, turning blue litmus red and then bleaching it white.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
How does adding an impurity affect both the melting point and the boiling point of a pure substance?
What is a formulation? Give three examples.
Why does dye A travel further up the chromatography paper than dye B?
On a chromatogram, the solvent front travels 8.0 cm from the baseline and a blue dye spot travels 5.2 cm from the baseline. Calculate the Rf value of the blue dye.
Why must the litmus paper be damp when testing for chlorine gas?
What is the chemical name of limewater, and what is the white precipitate formed when CO₂ is bubbled through it?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Describe how a student should set up and carry out a paper chromatography experiment to determine whether a black felt-tip pen ink contains three known pure food dyes (A, B, and C). [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Use a ruler and a graphite pencil to draw a horizontal baseline ~2 cm from the bottom of a piece of chromatography paper [1 mark]. 2. Use capillary tubes to place a small spot of the black ink and separate spots of pure dyes A, B, and C along the pencil baseline, allowing them to dry [1 mark]. 3. Pour a shallow layer of solvent (water) into a beaker so the liquid level is below the pencil baseline, and suspend the paper vertically in the beaker [1 mark]. 4. Place a lid on the beaker (to reduce solvent evaporation) and wait until the solvent rises near the top of the paper, then remove the paper and immediately mark the solvent front with pencil [1 mark]. 5. Compare the vertical heights (and calculate Rf values) of the spots separated from the black ink against the reference spots of pure dyes A, B, and C [1 mark].
- Q2
Explain in terms of the stationary phase and mobile phase why a green food colouring separates into a yellow spot with Rf = 0.82 and a blue spot with Rf = 0.35 during paper chromatography. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The green food colouring is a mixture of two different dyes (yellow and blue), where the chromatography paper is the stationary phase and the liquid solvent is the mobile phase [1 mark]. 2. Each dye has a different balance between its solubility in the mobile phase and its attraction (adsorption) to the stationary phase [1 mark]. 3. The yellow dye (Rf = 0.82) is much more soluble in the mobile phase and less strongly attracted to the stationary phase, so it moves faster and further up the paper [1 mark]. 4. The blue dye (Rf = 0.35) is less soluble in the mobile phase and more strongly attracted to the stationary phase, so it travels a shorter distance [1 mark].
- Q3
A medicinal painkiller tablet contains 500 mg of paracetamol, 50 mg of starch binder, 10 mg of magnesium stearate lubricant, and 5 mg of coating dye. State what type of mixture this tablet is, explain why it is not classified as a pure substance in chemistry, and explain how a chemist could test if a sample of isolated paracetamol crystals is pure. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The tablet is a formulation (a mixture designed with measured quantities of components for a specific purpose) [1 mark]. 2. In chemistry, a pure substance consists of a single element or compound not mixed with anything else; because the tablet contains paracetamol mixed with starch, lubricant, and dye, it is a mixture [1 mark]. 3. To test if the isolated paracetamol crystals are pure, heat a sample in a melting point apparatus and measure the temperature at which it melts [1 mark]. 4. Pure paracetamol will melt sharply at its exact databook melting point (169°C), whereas an impure sample will melt at a lower temperature and over a range of several degrees [1 mark].
- Q4
During the electrolysis of aqueous sodium chloride (brine), two different gases are collected at the electrodes, and during the reaction of calcium carbonate with acid, a third gas is collected. Describe the chemical tests and positive results used to identify Hydrogen, Chlorine, and Carbon Dioxide. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. Hydrogen: Hold a lighted splint to the mouth of the test tube; the gas burns with a squeaky pop [1 mark]. 2. Chlorine: Hold a piece of damp blue litmus paper in the gas; the paper turns red briefly and is then bleached white [1 mark]. 3. Carbon dioxide: Bubble the gas through limewater (aqueous calcium hydroxide); the clear solution turns cloudy/milky white [1 mark].
Common Exam Questions (4)
Why must the baseline in paper chromatography be drawn in pencil rather than ink, and sit above the solvent level?
Pencil lead is made of graphite, which is insoluble in the solvent so it will not dissolve or move up the paper and contaminate the chromatogram (whereas pen ink would dissolve and run). The baseline must sit above the solvent level in the beaker so the sample spots travel up the paper with the mobile phase rather than dissolving straight into the pool of solvent at the bottom.
How do you calculate the Rf value in paper chromatography?
Use the formula: Rf = Distance moved by substance (measured from the pencil baseline to the centre of the spot) / Distance moved by solvent (measured from the pencil baseline to the solvent front). An Rf value has no units and is always a decimal number between 0 and 1.
How do you use melting and boiling points to test if a sample of water is pure?
A chemically pure substance melts and boils at a single, sharp temperature (pure water melts at exactly 0°C and boils at 100°C). An impure substance (a mixture) melts and boils over a range of temperatures: impurities lower the melting point (e.g., salty ice melts at -3°C) and RAISE the boiling point (e.g., salty water boils at 102°C).
What are the 4 standard GCSE Chemistry gas tests and their positive results?
1. Hydrogen (H₂): Hold a lighted splint to the mouth of the tube; burns with a squeaky pop. 2. Oxygen (O₂): Insert a glowing splint into the tube; relights the glowing splint. 3. Carbon dioxide (CO₂): Bubble the gas through limewater (aqueous calcium hydroxide); turns clear limewater cloudy/milky white. 4. Chlorine (Cl₂): Hold damp blue litmus paper in the gas; bleaches the litmus paper white (may turn red briefly first).
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