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Chemical tests (Gas tests (Test for hydrogen (Test: Light a wooden splint…
Chemical tests
Gas tests
Test for hydrogen
Test: Light a wooden splint and insert it in a test tube filled with the gas sample. If the test is positive, hydrogen will make a 'pop' sound.
Observation: Hydrogen makes a squeaky 'pop' sound when it burns rapidly with oxygen in the air to form H2O.
Application: It is important to test for hydrogen because it's flammable and can cause explosions in reactions.
Reagents: hydrogen, oxygen
Test for Ammonia
Test: Insert damp red litmus paper into the test tube with the gas sample and close it with a bung so that the gas doesn't escape.
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Observations: If the test is positive, damp red litmus paper will turn blue since ammonia is alkaline.
Application: Ammonia is toxic to humans and may be fatal if inhaled, so it is important to test for it.
Test for carbon dioxide
Observation: If the test is positive, carbon dioxide makes limewater go cloudy.
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Test for oxygen
Test: Light a splint and allow it to burn for a few seconds before blowing it out, so that it is still glowing. Insert the glowing splint into the test tube with the gas sample.
Application: Oxygen is used in respiration and tanks of oxygen can be used for people with breathing problems.
Observation: If the test is positive, the glowing splint relights.
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Test for Chlorine
Observations: Chlorine turns damp blue litmus paper red first, as it is acidic, and then bleaches it (turns it white).
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Test: Insert damp blue litmus paper into the test tube with the gas sample and close it with a bung so that the gas doesn't escape.
Tests for anions
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Nitric acid and silver nitrate can be used to test for halides ions(halogens are the group 7 elements).
Test: Add dilute nitric acid (HNO3) to get rid of sulfite or carbonate ions. Then add silver nitrate solution (AgNO3). Different coloured halide precipitates will form.
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Tests for cations
Cation wet tests
Test: Add a few drops of sodium hydroxide solution to the solution containing containing the cations, to form a metal hydroxide. Transition metal hydroxides are insoluble, so they form solid precipitates. Observe the colour of the precipitate.
Aluminium (Al3+) forms a white precipitate, soluble in excess.
If you get a white precipitate, add excess sodium hydroxide, and the aluminium hydroxide will redissolve. This allows us to tell whether it's aluminium or not.
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Calcium (Ca2+) forms white precipitate, insoluble in excess.
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Magnesium (Mg2+) also forms a white precipitate, insoluble in excess.
Copper (Cu2+) forms a light blue precipitate, insoluble in excess.
Iron (II) (Fe2+) forms a green precipitate, insoluble in excess.
Iron (III) (Fe3+) forms a red-brown precipitate (rust), insoluble in excess.
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Flame tests
Test: Clean a nichrome wire loop by dipping it into hydrochloric acid. Clean it until the blue Bunsen burner flame shows no change of colour when you hold the wire in it, to make sure that there is nothing left on the wire loop that would contaminate the results. When the wire is clean, moisten it again with some of the acid and then dip it into the solid you're testing. A small amount of metal chloride will form on the wire. Place the wire in the flame again. Observe how the colour of the flame changes.
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As you 'excite' an ion by heating it, the electrons move to higher energy levels and energy is released in the form of light, so a colour is produced.
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Reagents: Hydrochloric acid, the solid that is being tested
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