(h) Chemical tests
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
2.44Gas testsDescribe tests for these gases:• hydrogen• oxygen• carbon dioxide• ammonia• chlorine.2.45Flame-test methodDescribe how to carry out a flame test2.46Flame-test coloursKnow the colours formed in flame tests for these cations:• Li+ is red• Na+ is yellow• K+ is lilac• Ca2+ is orange-red• Cu2+ is blue-green.2.47Cation testsDescribe tests for these cations:• NH4+ using sodium hydroxide solution and identifying the gas evolved• Cu2+, Fe2+ and Fe3+ using sodium hydroxide solution.2.48Anion testsDescribe tests for Cl⁻, Br⁻ and I⁻ using acidified silver nitrate; SO₄²⁻ using acidified barium chloride; and CO₃²⁻ using hydrochloric acid and identification of the evolved gas.2.49Test for waterDescribe a test for the presence of water using anhydrous copper(II) sulfate2.50Water purity testDescribe a physical test to show whether a sample of water is pure
Identify five gases with decisive tests
A gas test is reliable only when both the test procedure and its distinctive positive result are stated. Use a fresh sample and expose it to the reagent or splint as described; do not identify a gas from colour or smell.
| Gas | Test | Positive result |
|---|---|---|
| hydrogen | place a lighted splint at the mouth of the container | burns with a squeaky pop |
| oxygen | insert a glowing splint | the glowing splint relights |
| carbon dioxide | bubble the gas through limewater | limewater turns milky or cloudy |
| ammonia | hold damp red litmus paper in the gas | the paper turns blue |
| chlorine | hold damp blue litmus paper in the gas | it turns red, then is bleached white |
Test only a small quantity. Chlorine and ammonia are harmful to inhale, so keep the gas contained and use suitable ventilation; never use smell as the test.
A lighted splint tests hydrogen, whereas a glowing splint tests oxygen. Litmus must be damp for ammonia and chlorine because the gases must dissolve before affecting the indicator.
Carry out a clean flame test
A flame test identifies certain metal ions from the colour they produce in a non-luminous Bunsen flame. A clean wire and uncontaminated sample are essential because traces of another ion can mask the colour.
| Stage | Action and purpose |
|---|---|
| 1 | Dip a platinum or nichrome wire loop in hydrochloric acid, then heat it until no flame colour is seen; this removes contamination. |
| 2 | Dip the clean loop into the sample, using a little hydrochloric acid to help a solid sample adhere if needed. |
| 3 | Place the loop at the edge of a non-luminous blue Bunsen flame. |
| 4 | Observe and record the flame colour, then clean the loop before testing another sample. |
Wear eye protection, keep the acid away from skin and point the wire away from people. Use a blue flame: a yellow safety flame would hide the test colour.
Cleaning is complete only when heating the loop produces no colour. Reusing an unclean loop can give a false result, especially because sodium contamination produces an intense yellow flame.
Match five metal ions to flame colours
After carrying out a clean flame test, compare the observed colour with the required reference colours. The colour identifies the metal ion, not its accompanying negative ion.
| Metal ion | Flame colour |
|---|---|
| lithium, LiX+ | red |
| sodium, NaX+ | yellow |
| potassium, KX+ | lilac |
| calcium, CaX2+ | orange-red |
| copper(II), CuX2+ | blue-green |
For example, an orange-red flame supports the presence of calcium ions, while a lilac flame supports potassium ions. Record the specified colour precisely rather than only calling it red or blue.
Do not confuse lithium red with calcium orange-red, or potassium lilac with copper(II) blue-green. A yellow result may be caused by sodium contamination, so repeat the test with a freshly cleaned loop if the result is unexpected.
Identify four cations with sodium hydroxide
Aqueous sodium hydroxide can identify ammonium ions by releasing ammonia and can identify three metal ions by the colour of the hydroxide precipitate formed. Use a fresh sample for each test.
| Ion | Procedure | Positive result |
|---|---|---|
| ammonium, NHX4X+ | add aqueous sodium hydroxide and warm gently; test any gas with damp red litmus | ammonia is released and turns the paper blue |
| copper(II), CuX2+ | add aqueous sodium hydroxide | blue precipitate |
| iron(II), FeX2+ | add aqueous sodium hydroxide | green precipitate |
| iron(III), FeX3+ | add aqueous sodium hydroxide | brown or red-brown precipitate |
For ammonium ions, warming drives the reaction that releases ammonia; the gas test completes the identification. For the metal ions, the named precipitate colour is the identifying observation.
Do not report only that a precipitate forms: copper(II), iron(II) and iron(III) are distinguished by blue, green and brown respectively. Do not try to identify ammonia by smell.
Identify halide, sulfate and carbonate ions
An anion test requires the correct reagents in the correct order and the expected observation. Acidification removes interfering ions, but the acid must not introduce the ion being tested.
| Ion | Procedure | Positive result |
|---|---|---|
| chloride, ClX− | acidify with dilute nitric acid, then add aqueous silver nitrate | white precipitate |
| bromide, BrX− | acidify with dilute nitric acid, then add aqueous silver nitrate | cream precipitate |
| iodide, IX− | acidify with dilute nitric acid, then add aqueous silver nitrate | pale-yellow precipitate |
| sulfate, SOX4X2− | acidify with dilute hydrochloric acid, then add aqueous barium chloride | white precipitate |
| carbonate, COX3X2− | add dilute hydrochloric acid and pass the gas through limewater | effervescence; the carbon dioxide turns limewater milky |
For halides, nitric acid is used before silver nitrate because hydrochloric acid would add chloride ions and could create a false white precipitate. A carbonate result is completed by identifying the evolved gas as carbon dioxide.
A white precipitate alone is not enough to distinguish chloride from sulfate: the reagent sequence identifies which test was performed. Preserve the cream result for bromide and pale-yellow result for iodide.
Test a substance for the presence of water
Anhydrous copper(II) sulfate tests whether water is present. The anhydrous solid is white and becomes blue when it is hydrated by water.
| Stage | Action or observation |
|---|---|
| 1 | Place a small amount of white anhydrous copper(II) sulfate on a dry surface. |
| 2 | Add the liquid being tested. |
| 3 | A change from white to blue is a positive result for water. |
Keep the reagent dry before use because moisture from the air can turn it blue and spoil the test. A known dry sample can be used to confirm the starting colour.
This test shows that water is present; it does not show that the liquid is pure water. An aqueous solution containing dissolved impurities would also turn anhydrous copper(II) sulfate blue.
Use a physical constant to test water purity
A pure substance changes state at a sharp, fixed temperature. At standard atmospheric pressure, pure water boils at 100∘C and freezes at 0∘C.
| Stage | Action and interpretation |
|---|---|
| 1 | Measure the atmospheric conditions and choose either boiling point or freezing point. |
| 2 | Heat or cool the sample while measuring temperature with a suitable thermometer. |
| 3 | A sharp boiling point of 100∘C or freezing point of 0∘C at standard pressure supports that the sample is pure water. |
| 4 | A shifted temperature or a change over a range indicates dissolved impurities. |
Boiling point depends on atmospheric pressure, so compare the measurement with the expected value for the conditions. Take repeated readings and avoid reading the thermometer while its bulb touches the container.
The anhydrous copper(II) sulfate test cannot establish purity because it responds to water in both pure water and solutions. Purity requires a physical-property measurement with a sharp value.