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CHEMICAL TESTS (Testing for gases (For hydrogen (How it is carried out:…
CHEMICAL TESTS
Testing for gases
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For oxygen
How it is carried out: Hold a test tube of oxygen upside down and place a glowing (extinguished) splint into it.
Observations: If oxygen is present, the splint should reignite.
For carbon dioxide
How it is carried out: Bubble carbon dioxide through limewater. This forms a solid precipitate of calcium carbonate.
Observations: As calcium carbonate is insoluble in water, the limewater will turn milky - or cloudy - when carbon dioxide is present.
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Tests for anions
For sulphate anions
How it is carried out: Add a few drops of dilute hydrochloric acid to the sample before then adding a few drops of dilute barium chloride solution.
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For carbonate anions
How it is carried out: Add any acid (but usually dilute hydrochloric acid is used) to the test compound. Then use limewater to confirm that the gas in any bubbles given off is carbon dioxide.
Observations: Bubbles are produced when an acid is added to the test compound and later, if the limewater turns milky, that means that there is carbon dioxide bubbling through it.
For halide ions
How it is carried out: First, add a few drops of dilute nitric acid to the sample and then add a few drops of dilute silver nitrate.
Observations(precipitate colours): Cl- = white, Br- = a creamy colour, I- = yellow
Tests for cations
Using sodium hydroxide
How it is carried out: sodium hydroxide is added to the solution being tested and the colour of the precipitation formed allows for identification of the compound
Observations: Fe2+ = pale green, Fe3+ = Red-brown, Cu2+ = pale blue
Flame tests
How it is carried out: First, dip the nichrome wire loop into concentrated hydrochloric acid to remove any ions that would affect the result and place in a blue (non-luminous) Bunsen flame to check it is clean. Next, dip it into the HCl again and then the compound you are going to test. Finally, place the wire into the edge of the blue Bunsen flame and record the colour that appears.
Observations: Li+ = red, Na+ = yellow, K= = lilac, Ca2+ = brick red, Cu2+ = blue-green