Separation and purification

1. Overview

In chemistry, substances are rarely found in a pure state. This topic explores the physical methods used to separate mixtures into their individual components based on their physical properties, such as solubility and boiling points. These techniques are vital for industrial processes, medical applications, and ensuring the quality of chemical products.

Key Definitions

• Solute: The substance (usually a solid) that is dissolved in a solvent.
• Solvent: The liquid in which the solute dissolves to form a solution.
• Solution: A mixture formed when a solute dissolves in a solvent.
• Saturated Solution: A solution containing the maximum amount of solute that can be dissolved at a specific temperature.
• Residue: The solid trapped in the filter paper during filtration.
• Filtrate: The liquid that passes through the filter paper during filtration.
• Distillate: The liquid collected after it has been condensed during distillation.
• Pure Substance: A substance consisting of only one element or compound with fixed melting and boiling points.

Core Content

(a) Suitable Solvents

Separation often depends on choosing a solvent that dissolves only one component of a mixture.

• Water: Used for most ionic compounds (e.g., Sodium Chloride) and some covalent ones (e.g., Sugar).
• Organic Solvents: Ethanol, propanone, or cyclohexane are used for substances that do not dissolve in water (e.g., fats, oils, or sulfur).

(b) Filtration

Used to separate an insoluble solid from a liquid (e.g., sand from water).

• The mixture is poured through filter paper in a funnel.
• The solid remains on the paper (residue).
• The liquid passes through into a flask (filtrate).
• 📊A glass funnel lined with filter paper sitting in a conical flask, with solid residue caught in the paper and liquid filtrate in the flask.

(c) Crystallisation

Used to separate a soluble solid from its solution (e.g., Copper(II) sulfate crystals from a solution).

1. Heat the solution in an evaporating dish to remove some solvent until the "saturation point" is reached.
2. Test for saturation by dipping a cold glass rod; crystals will form on the rod.
3. Allow the solution to cool slowly. Solubility decreases as temperature drops, causing crystals to grow.
4. Filter the crystals, wash with a small amount of cold distilled water, and dry with filter paper.

Example: Formation of Hydrated Copper(II) Sulfate

• Word Equation: Copper(II) oxide(s) + Sulfuric acid(aq) → Copper(II) sulfate(aq) + Water(l)
• Symbol Equation: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

(d) Simple Distillation

Used to separate a solvent from a solution (e.g., obtaining pure water from sea water).

• The solution is heated until the solvent boils.
• The vapor rises, enters a condenser (cooled by a water jacket), and turns back into a liquid.
• The solute remains in the original flask.
• 📊A distillation flask connected to a Liebig condenser with a thermometer at the junction. A receiving flask collects the distillate.

(e) Fractional Distillation

Used to separate miscible liquids with different boiling points (e.g., ethanol and water, or crude oil).

• A fractionating column (filled with glass beads to increase surface area) is placed above the flask.
• The liquid with the lower boiling point evaporates first and reaches the top of the column to be condensed.
• Liquids with higher boiling points condense on the beads and fall back into the flask until their boiling point temperature is reached at the top of the column.

Suggesting Techniques

• Solid + Solid (one soluble): Dissolve in solvent → Filter → Crystallise filtrate.
• Insoluble solid + Liquid: Filtration.
• Soluble solid + Liquid (want the solid): Crystallisation.
• Soluble solid + Liquid (want the liquid): Simple distillation.
• Two or more liquids: Fractional distillation.

Assessing Purity

• Pure substances: Melt and boil at specific, "sharp" temperatures (e.g., pure water boils at exactly 100°C).
• Impure substances:
  1. Melting Point: Is lowered and occurs over a range of temperatures.
  2. Boiling Point: Is raised and occurs over a range of temperatures.

Extended Content (Extended Only)

There are no specific Supplement-only learning objectives for this sub-topic in the current IGCSE syllabus.

Key Equations

While separation is largely physical, understanding the dehydration of crystals is often tested in this context:

Dehydration of Copper(II) Sulfate Crystals:

• Word Equation: Hydrated copper(II) sulfate(s) ⇌ Anhydrous copper(II) sulfate(s) + Water(g)

• Symbol Equation: CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)

• Symbols:

  • CuSO₄·5H₂O: Blue crystals (Hydrated)
  • CuSO₄: White powder (Anhydrous)
  • ⇌: Reversible reaction

Common Mistakes to Avoid

• ❌ Wrong: Using "evaporation to dryness" to get large crystals. ✓ Right: Heat to the point of crystallisation and then cool slowly to allow large, well-defined crystals to form.
• ❌ Wrong: Putting the thermometer bulb into the liquid during distillation. ✓ Right: Place the thermometer bulb at the entrance to the condenser to measure the temperature of the vapor.
• ❌ Wrong: Confusing the terms filtrate and distillate. ✓ Right: Filtrate is for filtration; Distillate is for distillation.

Exam Tips

• Command Word: "State": If asked to "state" a method, a single word or short phrase like "Fractional distillation" is sufficient.
• Command Word: "Suggest": Look at the properties provided (solubility and boiling points). If boiling points are close together (e.g., 78°C and 100°C), you must suggest fractional distillation, not simple.
• Purity Questions: If an exam table shows a substance melting at 122°C–126°C, it is impure because it melts over a range rather than at a single point.
• Real-world contexts: Expect questions on separating gases from the air (fractional distillation of liquid air) or separating crude oil. Remember that nitrogen ($N_2$) has a lower boiling point than oxygen ($O_2$).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student is given a mixture of sand and sodium chloride. They want to obtain pure sodium chloride crystals.

(a) Describe the steps the student should take to separate the sand from the sodium chloride. [3]

(b) Name the process used to obtain pure sodium chloride crystals from the sodium chloride solution. [1]

(c) State one method the student could use to check the purity of the sodium chloride crystals obtained. [1]

Worked Solution:

(a)

1. Add water to the mixture and stir to dissolve the sodium chloride. Sodium chloride is soluble in water, sand is not.
2. Filter the mixture. The sand will remain as a residue on the filter paper.
3. The sodium chloride solution will pass through as the filtrate. This separates the dissolved salt from the solid sand.

How to earn full marks:

• Mention adding water and stirring to dissolve the salt.
• State that filtration is used.
• Correctly identify the sand as the residue/remaining on the filter paper, and the salt solution as the filtrate.

(b)

1. Crystallisation. This process allows the salt to form crystals.

How to earn full marks:

• State the correct name of the process.

(c)

1. Determine the melting point of the crystals. Pure substances have sharp, well-defined melting points.

How to earn full marks:

• Mention measuring the melting point.
• Relate the melting point to the purity of the sample.

Common Pitfall: Make sure you understand the difference between the terms 'residue' and 'filtrate' in filtration. Also, remember that a pure substance has a very sharp melting point, while an impure substance will melt over a range of temperatures.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

Crude oil is a mixture of hydrocarbons.

(a) Define the term 'hydrocarbon'. [1]

(b) Name the process used to separate crude oil into fractions. [1]

(c) Describe how this process separates the crude oil into fractions. [4]

Worked Solution:

(a)

1. A compound containing carbon and hydrogen only. This is the definition of a hydrocarbon.

How to earn full marks:

• Correctly state that the compound must contain carbon and hydrogen.
• State that the compound contains only carbon and hydrogen.

(b)

1. Fractional distillation. This is the method used to separate crude oil.

How to earn full marks:

• State the correct name of the process.

(c)

1. The crude oil is heated and vaporised. Heat is required to change the crude oil into a gaseous state.
2. The vapours enter a fractionating column that is hot at the bottom and cool at the top. This temperature gradient is essential for separation.
3. Different fractions condense at different temperatures. The different boiling points of the fractions determine where they condense.
4. Fractions with lower boiling points rise higher up the column before condensing. This separates the fractions by their boiling points.

How to earn full marks:

• Mention heating and vaporising the crude oil.
• Describe the temperature gradient in the fractionating column.
• State that different fractions condense at different temperatures/heights.
• Relate boiling point to the height the fraction rises in the column.

Common Pitfall: Be sure to mention the temperature gradient in the fractionating column. Many students forget that the column is hotter at the bottom and cooler at the top, which is crucial for the separation process.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

A student investigates the solubility of potassium nitrate in water at different temperatures. The student adds potassium nitrate to 25.0 cm³ of water at various temperatures until no more dissolves. The mass of potassium nitrate that dissolves is recorded. The results are shown in the table below.

Temperature (°C)	Mass of Potassium Nitrate Dissolved (g)
20	9.0
40	16.0
60	25.0
80	35.0
100	46.0

(a) Plot a graph of the mass of potassium nitrate dissolved (y-axis) against temperature (x-axis). Draw a best fit curve. [4]

(b) Use your graph to estimate the mass of potassium nitrate that would dissolve in 25.0 cm³ of water at 50 °C. [1]

(c) The student then heats the solution from the experiment at 100°C to evaporate some of the water. Describe how the student could obtain pure, dry crystals of potassium nitrate from the remaining solution. [3]

(d) State why it is important to stir the solution when dissolving the potassium nitrate. [1]

Worked Solution:

(a)

1. 📊A graph with temperature on the x-axis (ranging from 0 to 100°C) and mass of potassium nitrate dissolved on the y-axis (ranging from 0 to 50g). Plot the points from the table accurately. Draw a smooth curve of best fit through the points.

How to earn full marks:

• Correctly label both axes with units.
• Use a suitable scale to plot the points accurately.
• Plot all points correctly.
• Draw a smooth curve of best fit (not a straight line connecting the points).

(b)

1. Approximately 20.5 g. Read the value from the graph at 50°C. $\boxed{20.5 \text{ g}}$

How to earn full marks:

• Correctly read the value from the graph, showing working on the graph. Allow a range of +/- 0.5g.
• Include units in the final answer.

(c)

1. Allow the solution to cool slowly. This allows crystals to form.
2. Filter the crystals from the remaining solution. Filtration separates the solid crystals from the liquid.
3. Wash the crystals with a small amount of cold, distilled water. This removes any remaining impurities.
4. Dry the crystals between filter paper. This removes the water from the crystals.

How to earn full marks:

• State that the solution should be cooled slowly.
• Mention filtering the crystals.
• State that the crystals should be washed with cold distilled water.
• State that the crystals should be dried.

(d)

1. To ensure the potassium nitrate dissolves evenly/completely. Stirring increases the rate of dissolving.

How to earn full marks:

• State that stirring ensures the potassium nitrate dissolves evenly or completely.

Common Pitfall: When plotting graphs, always double-check that your axes are labeled correctly with units, and that you've chosen a scale that makes good use of the graph paper. Also, remember to draw a smooth curve of best fit, not just connect the dots with straight lines.

Exam-Style Question 4 — Extended Response [8 marks]

Question:

A chemist is investigating a sample of seawater suspected of containing a pollutant. The pollutant is known to be an organic compound soluble in hexane but not in water.

(a) Describe how the chemist could use a suitable solvent to extract the organic compound from the seawater sample. [3]

(b) Name a technique the chemist could use to separate the extracted organic compound from the hexane. [1]

(c) Describe the process named in (b). [3]

(d) Suggest how the chemist could use thin-layer chromatography (TLC) to determine if the extracted organic compound is a single pure substance or a mixture of different compounds. [1]

Worked Solution:

(a)

1. Add hexane to the seawater sample. Hexane will dissolve the organic compound.
2. Shake/stir the mixture thoroughly. This will help the organic compound dissolve.
3. Allow the mixture to settle and decant/separate the hexane layer. This separates the hexane containing the organic compound from the water.

How to earn full marks:

• Mention adding hexane to the seawater sample.
• State that the mixture should be shaken or stirred.
• State that the hexane layer containing the organic compound is decanted or separated.

(b)

1. Simple distillation. This will separate the hexane (lower boiling point) from the organic compound.

How to earn full marks:

• State the correct name of the process.

(c)

1. The solution is heated. Heat is required to boil the liquid.
2. The hexane evaporates and the vapour passes into a condenser. Evaporation and condensation are the key processes.
3. The condenser cools the hexane vapour, causing it to condense back into a liquid. Cooling condenses the vapour.
4. The liquid hexane is collected. This separates the hexane from the organic compound (which remains in the original flask).

How to earn full marks:

• Mention heating the solution.
• State that the hexane evaporates and passes into a condenser.
• State that the condenser cools the vapour, causing it to condense.
• The final mark is for stating that the liquid hexane is collected, and that the organic compound remains behind.

(d)

1. Run a TLC chromatogram of the extracted organic compound. If it produces only one spot, it is a pure substance; if it produces multiple spots, it is a mixture. This identifies the composition of the organic compound.

How to earn full marks:

• State that a TLC chromatogram should be run.
• Relate the number of spots to the purity of the substance (1 spot = pure, multiple spots = mixture).

Common Pitfall: When describing distillation, remember to mention both evaporation and condensation. Also, be clear about which substance is being collected and which remains behind in the flask. In chromatography, the number of spots directly indicates the number of substances present.