2025 RI H2Chem Prelims P4 Answers

-1- 2025 RI H2 Chemistry Paper 4 – Suggested Solutions 1(a)(i) tests observations Test FA 1 with Universal Indicator paper. Universal Indicator paper turns orange. pH is 3 Add 1 cm depth of FA 1 into a test- tube. Add aqueous sodium hydroxide, slowly with shaking, until no further change is seen. Filter the mixture into a clean test-tube. To the filtrate, add dilute nitric acid drop-wise until in excess. Off-white/white ppt formed, insoluble in excess NaOH. Off-white ppt turned brown on contact with air. Off-white/brown residue. Colourless filtrate. White ppt formed, soluble in excess dilute nitric acid to give a colourless solution. Add 1 cm depth of FA 2 into a test- tube. Add 1 cm depth of dilute nitric acid. Effervescence. CO2 gas evolved gave a white ppt with limewater. 1(a)(ii) identity evidence cation in FA 1 Mn2+ In test 2 , FA 1 reacted with NaOH(aq) to give an off-white ppt of Mn(OH) 2 which was insoluble in excess NaOH(aq). On contact with air, Mn(OH)2 was oxidised to brown Mn(OH)3. cation in FA 1 Al3+ In test 2, FA 1 reacted with excess NaOH(aq) to form a colourless filtrate after filtration. On adding dilute nitric acid, a white ppt of Al(OH)3 is formed. Also, in test 1, FA 1 is acidic. anion in FA 2 CO32– In test 3 , FA 2 reacted with dilute nitric acid to give effervescence of CO2 gas which gave a white ppt with limewater.

-2- 1(b) test observations with FA 3 observations with FA 4 Add 1 cm depth of FA 3 to a test-tube. Add all of the magnesium turnings provided in the vial to this test-tube. Effervescence. Colourless H2 gas is evolved and ‘pops’ with a lighted splint. no observable change Add about 1 cm depth of FA solution to a test -tube. To this test-tube, add 8 drops of sodium hydroxide solution followed by iodine solution, dropwise, until a permanent orange/red colour is present. Solution FA 3 turns orange OR no yellow ppt Solution FA 4 turns orange. Yellow ppt forms. Add 1 cm depth of aqueous silver nitrate to a test -tube. Then slowly add 1 cm depth of aqueous sodium hydroxide. Add aqueous ammonia slowly, with shaking, until the precipitate just dissolves. You may use a clean glass rod to stir the mixture and help dissolve the precipitate. Add 1 cm depth of FA 4 to this mixture, shake the tube and place it in the test -tube rack to stand. no observable change Silver mirror formed 1(b)(iv) identity Y in FA 3 CH3COOH or ethanoic acid Z in FA 4 CH3CHO or ethanal 2(a) Total volume of FA 6 added, VFA 6 / cm3 Total volume of solution in the cup, Vtotal / cm3 Maximum temperature, T / oC T = T – T0 / oC (Vtotal × T) / cm3 oC 0.00 40.0 29.4 0.0 0.00 5.00 45.0 30.4 +1.0 45.0 10.00 50.0 31.0 +1.6 80.0 15.00 55.0 31.6 +2.2 121 20.00 60.0 32.2 +2.8 168 25.00 65.