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Module 1: Practical chemistry for exams›PAG 4.2 - Identifying unknowns 2

PAG 4.2 - Identifying unknowns 2

An overview of PAG 4.2 - Identifying unknowns 2

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In OCR Chemistry Required Practical 4 (PAG 4.2), the aim is to identify four unknown solutions, labelled A, B, C, and D, based on the results of qualitative chemical tests.

In OCR PAG 4.2: Identifying unknowns 2, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Unknown solutions should always be treated as irritants.
Silver nitrate can stain skin and surfaces – take care when handling.
Immediately clean up any spills of acid or alkali.

In OCR Chemistry Required Practical 4 (PAG 4.2), the experimental design is a key part of the assessment.
The identities of the solutions A–D are listed below in no particular order:

aqueous ammonium chloride,
aqueous sodium chloride,
aqueous sodium carbonate,
aqueous sodium sulfate,

Plan a series of tests that would allow each solution A–D to be identified.

Consider the four unknown solutions and devise a testing procedure to confirm their identities. You should carry out the minimum number of tests required.

A flowchart showing chemical tests for Na2SO4, Na2CO3, NH2Cl, and NaCl. Test 1 involves adding dilute nitric acid, leading to effervescence or no effervescence. If effervescence occurs, Na2CO3 is indicated. If no effervescence occurs, Test 2 involves adding BaCl2(aq), resulting in either a white precipitate (indicating Na2SO4) or no precipitate (indicating NH2Cl and NaCl). If there is no change, Test 3 involves warming NaOH(aq) and testing with damp pH paper, which can either show no change (indicating NaCl) or turn blue (indicating NH4Cl).

Note that the chloride ion could be positively identified through testing with acidified silver nitrate. The white precipitate formed dissolves in dilute ammonia.

Experimental method for OCR Chemistry Required Practical 4 (PAG 4.2)

EXPERIMENTAL METHOD OCR PAG 4.2. 1 Prepare a tests and observations table for the exercise. Give detail of the tests carried out and space for observations. Test A B C D Dilute nitric acid Barium chloride (aq) Warm dilute NaOH Test gas with pH paper. 2 Test for carbonates with dilute nitric acid. Pour 1 cm³ of solutions A, B, C, and D into separate test tubes. Label test tubes. Add a few drops of dilute nitric acid to each test tube in turn and observe for effervescence. Record observations in a results table, and confirm the identity of the carbonate. 3 Test remaining solutions for sulfates with barium chloride solution. Add a few drops of barium chloride solution to each of the remaining test tubes after test 1. Look for a white precipitate. Record observations and confirm the identity of the sulfate. 4 Test the remaining two solutions for ammonium ions with warm NaOH(aq). Pour a fresh 1 cm³ of the remaining two solutions into separate test tubes. Add 0.5 cm³ of NaOH(aq) and gently warm the tubes over a low blue Bunsen flame. Place a piece of damp indicator paper in the mouth of the tube and look for a colour change. Record the observations and confirm the ammonium compound. 5 Confirm the identity of NaCl. Add 1 cm³ of the suspected NaCl solution to a test tube and add an equal volume of dilute nitric acid and a few drops of silver nitrate solution. A white precipitate of silver chloride confirms the identity. This will dissolve in dilute ammonia.

Wearing the appropriate personal protective equipment is essential to avoid direct contact with harmful chemicals and to ensure safety throughout the experiment.

A simple illustration of a pair of goggles and gloves.

Do

Wear goggles and gloves when handling any chemicals, particularly concentrated acids, ammonia, and metal salts, as they can be corrosive or irritating to the skin and eyes.

A person wearing no goggles or gloves whilst handling chemicals.

Don't

Skip protective equipment just because ‘it’s only a few drops’.

It is crucial to use clean and dry glassware when performing analytical tests.

Five test tubes arranged in a row, with varying heights and shapes.

Do

Ensure all glassware is clean and dry before use to avoid contamination and ensure accurate results.

A test tube with a white body and brownish stains or markings on its surface.

Don't

Reuse glassware that has not been properly cleaned and dried, as this could lead to inaccurate readings or unintended chemical reactions.

Proper disposal of chemicals is essential to avoid environmental damage and ensure safe laboratory practices. Always follow disposal guidelines.

A diagram showing a chemically compatible container with screw-on cap labeled 'HAZARDOUS WASTE' placed inside a secondary container. There are labels indicating 'Labelling according to the local and national regulations'.

Do

Dispose of chemicals safely after testing by placing them in the designated waste containers. Always follow your teacher’s instructions for disposal.

A funnel pouring blue liquid into a drain.

Don't

Pour chemicals down the drain unless specifically directed to do so, as some chemicals may cause harmful reactions in the plumbing or be harmful to aquatic organisms.

Below is a summary of results for the cations from this PAG:

A table with three columns labeled 'Ion', 'Test', and 'Observation'. Four rows detail tests and observations for different ions: 1. Ammonium, NH4⁺: Test involves adding aqueous sodium hydroxide and warming, then testing the gas with damp indicator paper. Observation: Ammonia gas is produced. Damp red litmus paper turns blue. 2. Chloride, Cl⁻: Test involves adding aqueous nitric acid followed by aqueous silver nitrate. Observation: A white precipitate is formed. 3. Sulfate, SO4²⁻: Test involves adding aqueous barium chloride. Observation: A white precipitate is formed. 4. Carbonate, CO3²⁻: Test involves adding aqueous nitric acid. Observation: Effervescence is observed as carbon dioxide gas is produced.