Chem Practical Notes

NOTESFORUSEIN QUALITATIVE ANALYSIS TESTS FOR ANIONS anion test test result carbonate (CO;) |add dilute acid effervescence, carbon dioxide produced chloride (CI) [in solution] acidify with dilute nitric acid, then add whiteppt. aqueous silver nitrate add aqueous sodium hydroxide, then | ammonia produced aluminium foil; warm carefully nitrate (NO;) in solution] sulphate(SO;) in solution] acidify with dilute nitric acid, then add white ppt. aqueous barium nitrate TESTS FOR AQUEOUS CATIONS effect of aqueous sodium hydroxide cation effect of aqueous ammonia ammonium (NH,) ammonia produced on warming calcium (Ca*) white ppt., insoluble in excess no ppt. copper(II) (Cu*") light blue ppt., insoluble in excess light blue ppt., soluble in excess giving a dark blue solution iron(11) (Fe*") green ppt., insoluble in excess green ppt., insoluble in excess red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess iron(11) (Fec) lead(II) (Pb*") white ppt., soluble in excess giving | white ppt., insolublein excess a colourlesssolution zinc (Zn) white ppt., soluble in excess giving | white ppt., soluble in excess giving a a colourless solution colourless solution [Lead(lI) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.] TESTS FOR GASES test and result gas ammonia (NH,) turns damp red litmus paper blue given white ppt. with limewater (ppt. dissolves with excess CO;) carbon dioxide (CO:) chlorine (C,) bleaches damp litmus paper hydrogen (H) "pops with a lighted splint relights a glowing splint sulphur dioxide (SO,) turns aqueous potassium dichromate (VI) from orange to green oxygen (O:)

COLOURS OF SOME COMMON METAL HYDROXIDES Calcium hydroxide white Copper(11) hydroxide light blue Iron(11) hydroxide green Iron(111) hydroxide red-brown Lead(11) hydroxide white Zinc hydroxide white 2

NOTES ON QUALITATIVE ANALYSIS In the practical examination, candidates are usually provided with unknown substance(s) and the necessary experimental procedures. They are expected to carry out testsaccording to the given procedures, record all observations and deduce the nature of the unknown substance(s). The following are some notes that could be of great help to candidates when performing a qualitative experiment. 1. If candidates are given the following instruction: (a) Carry out the following experiments and record all your observations and deductions of the unknown substance W." Such instruction implies that candidates are expected to identify the unknown with the results of the experiments. For example: W is copper(11) nitrate. (b) You are not required to identify the unknown substance W but you must describe all that you observe, identify any gas evolved and state any conclusions you draw from the experiments." In this case, candidates are expected to state the action of the unknown substance W with other reagents or certain identifiable products produced by W. For example: W is an acid or alkali in nature or W is reducing or oxidizing in nature. (c)You are required to identify the anion of the sodium salt and the metal of the oxide by carrying out tests of your own in addition to those described. Positive tests only are required."In this case, candidates are required to design and perform confirmatory tests to identify the substance given. The details of the tests should be clearly described. 2. Candidates are advised to record all observations immediately after performing a test. Recordings should be brief but complete. Phrases should be used instead of sentences for recording changes observed. Negative results should also be recorded as deduction may sometimes be drawn from them. Chemical formulae could be used instead of speling out the chemical name in full. "Observations" refer to changes that are seen by candidates during experiments. Some of the 3. common visible changes include: (a) changes in colour, (b) formation of a precipitate, (c) evolution of gases, (d) absorbing or giving out heat. Candidates should bear in mind that if there is no visible change observed after performing a test, the phrase "No visible reaction" should be recorded under the Observation Column rather than leaving it blank. 3

Gases that are produced due to the addition of reagents must not be recorded as observations. For 4. example, the use of aqueous ammonia as reagent is always associated with evolution of ammonia gas. This is not a change, hence should not be regarded as an observation. 5. It will be easy to distinguish and identify gases if candidates learn in advance their colours, odours and effects on moist litmus paper. The flowchart below illustrates the basic approach for distinguishing and identifying gases. NO ANY YES COLOUR? YELLOWISH BROWN ODOUR? GREEN YES NO NO PUNGENT SMELL TURNS TURNS LITMUS LITMUS PAPER MOIST RED PAPER MOIST BLUE LITMUS LITMUS TO BLUE TO RED NEUTRAL ACIDIC ALKALINE ACIDIC O2 CO (NH,) HC SO 6. Candidates should bear in mind the following when making a deduction: (a) A deduction must be specific. Statement, such as "W contains Cu++", should be written instead of using statement like "Copper or Cut+". b) The identity of a gas should not be given as a deduction. Such information should be regarded as an observation. Instead, the identity of the substance that is responsible for the gas to give off should be regarded as part of the deductions. (c)The recording of a deduction should be made as soon as an observation is recorded for such deduction will serve to provide an immediate source of information for subsequent tests. In experiments where conclusionsabout the identity or the nature of the unknowns need to be drawn, candidatesmay simply combine all deductions recorded during the experiment. For instance, if substance W was found containing Copper(I) ions from one test and sulphate(V1) ions from another, the conclusion may be made as: "W is copper(II) sulphate". 7. 4

ANALYSIS TABLES TABLE I cOLOUR OF SUBSTANCES Colour of Unknown Substance Deductioon Oxides of copper(II), iron(II) or manganese(VI), iodine crystals copper() salt iron(III) salt, copper metal Black Blue Reddish Brown Green light green iron(1I) salt, copper(II) salt Grey Powder form of metals like zinc, iron or aluminium White Absence of coloured salts like iron(1), iron(III), copper(11) and other transition elements Dilute acids or alkalis or hydrogen peroxide Colourless solution Purple potassium manganate(VII) Orange potassium dichromate(VI) cobalt(11) salts iron(III) chloride solution Pink Yellow TABLE 2 SOLUBILITY OF SALTs Type of Salt Solubility Carbonate All are insoluble except those of Na, K and NH Chloride All are soluble except PbCI, and AgCl lodide All are soluble except Pbl, and Agl Nitrate All are soluble Sulphate All are soluble except PbSO, BaSO, and CaSO, Hydroxide All are insoluble except those of Na", K' and NH - Sodium, Potassium, Ammonium All are soluble

TABLE 3 TEST FOR GASES No. GAS COLOUR LITMUS CONFIRMATORY ODOUR TEST TEST 1. Oxygen (0,) Colourless |Odourless Neutral Glowing splinter glows brighter or rekindled 2. Hydrogen (H,) Colourless Odourless Neutral Pop' sound when lighted splinter applied 3. Carbon dioxide Colourless (CO) Odourless Blue to red Lime water turned chalky 4. Ammonia (NH,) Colourless Pungent Red to blue Dense white fumes when smell of exposed to concentrated hydrochloric acid ammonia 5. Chlorine (C,) Greenish-yellow Choking Blue to red Potassium iodide smell and finally bleached solution turned brownish 6. Nitrogen dioxide (NO) Brown (often accompanied by O,) |smell Choking Blue to red 7. Water vapour (H,0) Colourless Odourless Neutral Blue cobalt chloride paper turned pink 6

TABLE 4 HEATING OFAN UNKNOWN SUBSTANCE A White sublimates (It infers that NH; may be present) Substance X at the cooler parts of the test-tube A Same white colour (It infers that Ca* or AP may be present) Gas Y + Residue Z- as original X Yellow when hot (It infers that Zn* may be present) white when co