Revision notes – Test for gases and ions - Sample Essay

Hydrogen H2 Ignite gas. Squeaky pop is heard. Oxygen O2 Place a glowing splint in a sample of the gas. The glowing splint relights. Carbon dioxide CO2 Bubble gas through limewater (saturated solution of calcium hydroxide) A solution turns from colourless to cloudy. A white (milky) precipitate of calcium carbonate forms which is sparingly soluble. Ammonia NH3 Test for gas using damp red litmus paper. Litmus paper turns blue. Chlorine Cl2 Test 1 Test for gas using damp litmus paper (red or blue) Chlorine bleaches the litmus paper very quickly.

Test 2 Test for gas using moist starch-iodide paper. The paper turns blue-black. Test 3 Pass gas through a solution of a bromide. The solution turns from colourless to orange. Test 4 Pass gas through a solution of an iodide. The solution turns from colourless to brown (possibly with a black precipitate, iodine). Nitrogen dioxide NO2 Not many tests for this gas. The gas is brown. Sulphur dioxide SO2 Test 1 Bubble gas through a solution of potassium dichromate (VI) dissolved in sulphuric acid. The solution turns from orange to green. Test 2.

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Bubble gas through a solution of potassium manganate (VII) dissolved in sulphuric acid. The solution turns from purple to colourless. Tests for Ions Ion Formula Test Observations Carbonate CO32- Test 1 Heat the solid in a test tube with a Bunsen burner. It should decompose producing the oxide and carbon dioxide. E. g. Test for the gas using limewater solution. Limewater should turn from colourless to cloudy in the presence of carbon dioxide due to precipitation of calcium carbonate. Test 2 Add dilute HCl to the solid. Test for the gas evolved using limewater solution.

Vigorous effervescence. Limewater should turn from colourless to cloudy in the presence of carbon dioxide due to precipitation of calcium carbonate. Hydrogencarbonate HCO3- Test Add calcium chloride to a hydrogencarbonate solution. No precipitate forms since calcium hydrogencarbonate is soluble. Sulphate (VI) SO42- Test Add barium chloride solution acidified with dilute HCl to the test solution. White precipitate of barium sulphate forms. Sulphite SO32- Test Warm the sulphite with dilute HCl. Test for gases using acidified potassium dichromate(VI) solution (or paper)

The solution turns green. Chloride Cl- Test 1 Add concentrated sulphuric acid to the solid chloride. White steamy acidic fumes are seen – HCl fumes. Test 2 Add dilute nitric acid to a solution of a chloride to acidify the solution. This eliminates any carbonates or sulphites. Add silver chloride to the solution. Add dilute ammonia solution. White precipitate of AgCl forms. Solid dissolves. Bromide Br- Test 1 Add concentrated sulphuric acid to the solid bromide. Steamy brownish acidic fumes are seen. Test 2′ Add dilute nitric acid to a solution of a bromide to acidify the solution.

This eliminates any carbonates or sulphites. Add silver nitrate to the solution. Add concentrated ammonia solution. Cream precipitate of AgBr forms. Solid dissolves. Iodide I- Test 1 Add concentrated sulphuric acid to the solid iodide. Purple acidic fumes are seen. The mixture turns to a brown slurry. Test 2 Add dilute nitric acid to a solution of a iodide to acidify the solution. This eliminates any carbonates or sulphites. Add silver nitrate to the solution. Add concentrated ammonia solution. Yellow precipitate of AgI forms. Solid is insoluble.

Nitrate NO3- Test 1 Heat solid nitrate. If group 1 solid (not Li) then will decompose to give the nitrite and oxygen. All other solid nitrates decompose to give the metal oxide, nitrogen dioxide and oxygen. Oxygen gas is evolved that will relight a glowing splint. Brown gas is seen (NO2). Oxygen gas is also evolved and will relight a glowing splint. Test 2 Boil nitrate solution with aluminium/Devarda’s alloy, in sodium hydroxide solution. Test vapour with red litmus paper. Litmus paper turns blue in the presence of ammonia. Ammonium NH4+ Test.

Warm ammonium compound with NaOH. Test vapours immediately using damp red litmus paper. NH 3 turns the litmus paper blue. Lithium Li+ Test Dip nichrome wire in HCl. Dip wire in solid. Heat wire in centre of flame. Observe colour of flame. A carmine red flame is seen. Sodium Na+ Test Dip nichrome wire in HCl. Dip wire in solid. Heat wire in centre of flame. Observe colour of flame. A yellow flame is seen. Potassium K+ Test Dip nichrome wire in HCl. Dip wire in solid. Heat wire in centre of flame. Observe colour of flame. A lilac flame is seen. Magnesium Mg2+.

Test Add NaOH solution to the magnesium solid. A white solid forms which is insoluble in excess NaOH(aq). This is Mg(OH)2(s) Calcium Ca2+ Test Dip nichrome wire in HCl. Dip wire in solid. Heat wire in centre of flame. Observe colour of flame. A brick red flame is seen. Strontium Sr2+ Test Dip nichrome wire in HCl. Dip wire in solid. Heat wire in centre of flame. Observe colour of flame. A crimson red flame is seen. Barium Ba2+ Test Dip nichrome wire in HCl. Dip wire in solid. Heat wire in centre of flame. Observe colour of flame. A apple green flame is seen.

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