Core practicals 15, 16: analysis of unknowns and preparation of aspirin

Q: Which tests are used to distinguish aldehydes from ketones?

Tollens' Reagent (forms a silver mirror with aldehydes) or Fehling's/Benedict's solution (forms a red precipitate with aldehydes). Ketones show no reaction.

Q: Why is ethanoic anhydride used instead of ethanoyl chloride in the synthesis of aspirin?

Ethanoic anhydride is safer , cheaper , and reacts less vigorously with water (less dangerous fumes) compared to ethanoyl chloride.

Q: What is the purpose of using reduced pressure (Buchner) filtration?

It is much faster than gravity filtration and results in a drier solid product.

Q: What are the key steps in the recrystallisation of a solid product like aspirin?

Dissolve in the minimum volume of hot solvent , filter hot, cool slowly to crystallise, filter under reduced pressure, and wash with ice-cold solvent .

Q: How does the melting point of a substance indicate its purity?

Pure substances have a sharp melting point at the literature value; impurities lower the melting point and cause the substance to melt over a wide range .

Chemistry (9CH0) · Pearson Edexcel A Level · 5 min read · Updated: 7 Sept 2026

Introduction: The Chemistry Detective and the Manufacturer

Welcome to the final stretch of your core practicals! In Core Practical 15, you become a chemical detective, using a series of logic-based tests to identify "unknown" substances. In Core Practical 16, you step into the shoes of a pharmaceutical chemist to synthesize and purify aspirin. These practicals are central to Paper 3, as they pull together everything you’ve learned about inorganic reactions, organic mechanisms, and analytical techniques.

Don't worry if the list of tests feels long; many of these are "old friends" from your earlier studies. Let's break them down into easy-to-follow steps!

Core Practical 15: Analysis of Unknowns

This practical is divided into two main categories: Inorganic Analysis (testing for ions) and Organic Analysis (testing for functional groups).

1. Inorganic Analysis: Identifying Ions

To identify an inorganic salt, you must test for both the cation (positive ion) and the anion (negative ion).

Testing for Cations (Metal Ions)

Flame Tests (Groups 1 and 2): Dip a nichrome wire in \(HCl\), then into the solid, and place it in a blue Bunsen flame.
- \(Li^+\): Red
- \(Na^+\): Orange/Yellow
- \(K^+\): Lilac
- \(Ca^{2+}\): Brick-red
- \(Sr^{2+}\): Crimson
- \(Ba^{2+}:\) Apple-green
Transition Metal Tests (Sodium Hydroxide, \(NaOH\)): Add \(NaOH_{(aq)}\) dropwise to a solution of the unknown.
- \(Cu^{2+}\): Blue precipitate
- \(Fe^{2+}\): Dirty green precipitate
- \(Fe^{3+}\): Orange/Brown precipitate
- \(Cr^{3+}\): Green precipitate (this dissolves in excess \(NaOH\) to form a dark green solution)
- \(Mn^{2+}\): Pale pink precipitate
The Ammonium Test: Add \(NaOH_{(aq)}\) and warm gently. If \(NH_4^+\) is present, ammonia gas (\(NH_3\)) is released. Test the gas with damp red litmus paper; it will turn blue.

Testing for Anions (Negative Ions)

Carbonates (\(CO_3^{2-}\)): Add dilute acid (like \(HCl\)). If it fizzes (effervescence), bubble the gas through limewater. If the limewater turns cloudy, it's \(CO_2\), confirming a carbonate.
Sulfates (\(SO_4^{2-}\)): Add dilute \(HCl\) (to remove carbonates), then add barium chloride (\(BaCl_2\)). A white precipitate of \(BaSO_4\) confirms the sulfate.
Halides (\(Cl^-\), \(Br^-\), \(I^-\)): Add dilute nitric acid (\(HNO_3\)), then silver nitrate (\(AgNO_3\)).
- \(Cl^-\): White precipitate (dissolves in dilute ammonia)
- \(Br^-\): Cream precipitate (dissolves in concentrated ammonia only)
- \(I^-\): Yellow precipitate (insoluble in ammonia)

2. Organic Analysis: Identifying Functional Groups

When given an unknown organic liquid, use these specific tests to find the functional group:

Alkenes: Shake with bromine water. The color changes from orange to colorless.
Alcohols: Add phosphorus(V) chloride (\(PCl_5\)). You will see steamy white fumes of \(HCl\) gas (turns damp blue litmus red).
- To distinguish primary/secondary from tertiary: Add acidified potassium dichromate(VI) (\(K_2Cr_2O_7\)). Primary and secondary alcohols turn the solution from orange to green.
Aldehydes vs. Ketones: Both react with 2,4-DNPH to form an orange precipitate. To tell them apart:
- Tollens' Reagent: Aldehydes form a silver mirror; ketones do nothing.
- Fehling's/Benedict's Solution: Aldehydes turn the blue solution to a red precipitate.
Carboxylic Acids: Add sodium hydrogencarbonate (\(NaHCO_3\)). It will fizz as \(CO_2\) is released.

Quick Review: Always perform the carbonate test before the sulfate test if you have a mixture, as carbonates also form precipitates with barium ions!

Core Practical 16: Preparation of Aspirin

Aspirin (acetylsalicylic acid) is made by reacting 2-hydroxybenzoic acid (salicylic acid) with ethanoic anhydride. We use ethanoic anhydride instead of ethanoyl chloride because it is safer, cheaper, and less reactive with water.

Step 1: Synthesis

1. Weigh out a known mass of 2-hydroxybenzoic acid in a conical flask.
2. Add a measured volume of ethanoic anhydride and a few drops of phosphoric(V) acid (\(H_3PO_4\)) as a catalyst.
3. Warm the mixture in a water bath for about 15 minutes.

Step 2: Isolation

1. Pour the warm mixture into cold water. The aspirin is relatively insoluble in cold water and will precipitate out as solid crystals.
2. Filter the mixture using reduced pressure filtration (a Buchner funnel and flask connected to a vacuum). This is much faster than standard gravity filtration and leaves the solid drier.

Step 3: Purification (Recrystallisation)

This is a vital skill for Paper 3 questions! The goal is to remove impurities trapped inside the crystals.

Dissolve: Dissolve the impure crystals in the minimum volume of hot solvent (usually an ethanol/water mix).
Filter (Hot): If there are insoluble impurities, filter the hot solution quickly.
Cool: Let the solution cool down slowly at room temperature, then place it in an ice bath. Aspirin crystals will reform (recrystallize), while soluble impurities stay dissolved in the solvent.
Filter (Cold): Filter again using a Buchner funnel.
Wash: Wash the crystals with a tiny amount of ice-cold solvent to remove any remaining liquid impurities without dissolving the product.
Dry: Leave the crystals in a warm place or a desiccator to dry.

Step 4: Checking Purity (Melting Point Determination)

To check how pure your aspirin is, you measure its melting point using a capillary tube and a melting point apparatus.

Pure substances: Have a sharp melting point that matches the data book value (Aspirin is approx. \(135^\circ C\)).
Impure substances: Will melt over a wide range of temperatures and at a lower temperature than the pure substance.

Calculations: Percentage Yield

You may be asked to calculate the yield using the formula:

\(Percentage Yield = \frac{Actual Yield}{Theoretical Yield} \times 100\)

Note: The theoretical yield is calculated using the moles of the limiting reagent (usually the 2-hydroxybenzoic acid).

Key Takeaway: Recrystallisation depends on the substance being very soluble in hot solvent but nearly insoluble in cold solvent. This is the "magic" that allows purification!

Common Hazards and Risks

Ethanoic anhydride: Corrosive and irritates the eyes/lungs. Use in a fume cupboard.
Phosphoric(V) acid: Corrosive. Wear gloves and eye protection.
Barium Chloride: Toxic if swallowed. Wash hands after use.
Silver Nitrate: Can cause dark stains on skin and clothes.

For more information on general laboratory safety and apparatus, please refer to the "Practical techniques, apparatus and safety" chapter.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the characteristic flame test colors for Group 1 and Group 2 metal cations?

The flame colors are: \(Li^+\) (Red), \(Na^+\) (Orange/Yellow), \(K^+\) (Lilac), \(Ca^{2+}\) (Brick-red), \(Sr^{2+}\) (Crimson), and \(Ba^{2+}\) (Apple-green).

How can you distinguish between \(Cu^{2+}\), \(Fe^{2+}\), \(Fe^{3+}\), and \(Cr^{3+}\) using sodium hydroxide?

\(Cu^{2+}\) forms a blue precipitate; \(Fe^{2+}\) forms a dirty green precipitate; \(Fe^{3+}\) forms an orange/brown precipitate; and \(Cr^{3+}\) forms a green precipitate that dissolves in excess \(NaOH\) to a dark green solution.

What is the test for the ammonium ion (\(NH_4^+\))?

Add aqueous \(NaOH\) and warm the mixture; it will release ammonia gas (\(NH_3\)), which turns damp red litmus paper blue.

How do you test for the presence of carbonate (\(CO_3^{2-}\)) and sulfate (\(SO_4^{2-}\)) ions?

Carbonates fizz with dilute acid (\(HCl\)) to produce \(CO_2\) (turns limewater cloudy). Sulfates produce a white precipitate when \(BaCl_2\) is added after acidification with \(HCl\).

How can you distinguish between chloride, bromide, and iodide ions using silver nitrate and ammonia?

\(Cl^-\) forms a white precipitate (dissolves in dilute \(NH_3\)); \(Br^-\) forms a cream precipitate (dissolves in conc. \(NH_3\)); \(I^-\) forms a yellow precipitate (insoluble in \(NH_3\)).

Which tests are used to distinguish aldehydes from ketones?

Tollens' Reagent (forms a silver mirror with aldehydes) or Fehling's/Benedict's solution (forms a red precipitate with aldehydes). Ketones show no reaction.

Why is ethanoic anhydride used instead of ethanoyl chloride in the synthesis of aspirin?

Ethanoic anhydride is safer, cheaper, and reacts less vigorously with water (less dangerous fumes) compared to ethanoyl chloride.

What is the purpose of using reduced pressure (Buchner) filtration?

It is much faster than gravity filtration and results in a drier solid product.

What are the key steps in the recrystallisation of a solid product like aspirin?

Dissolve in the minimum volume of hot solvent, filter hot, cool slowly to crystallise, filter under reduced pressure, and wash with ice-cold solvent.

How does the melting point of a substance indicate its purity?

Pure substances have a sharp melting point at the literature value; impurities lower the melting point and cause the substance to melt over a wide range.

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