c21 qualitative analysis

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  • The colour tells us about the possible cations present in the substance.What is the colour of the substance?

    - Is it white, colourless, blue, yellow or some other colour?What is the nature of the substance?

    - Is it crystalline, metallic or in powder form? Preliminary Observations Qualitative AnalysisChapter 21

    Qualitative AnalysisChapter 21

    Qualitative AnalysisChapter 21 Preliminary Observations

    ColourPossible identity of substance White (colourless)Sodium (Na+), potassium(K+), ammonium (NH4+) BlackCopper(II) oxide (CuO), Iron(II) oxide (FeO) Blue/greenCopper(II), Cu2+ or Iron(II), Fe2+ Yellowish/brownIron(III), Fe3+ Greyish, metallicMetals: Zinc, Iron, Magnesium, Aluminium

  • Is it soluble in water? The solubility tells us about the possible anions present in the substance. InsolubleSolubleQualitative AnalysisChapter 21 Preliminary Observations

    Qualitative AnalysisChapter 21

    Table of SolubilityQualitative AnalysisChapter 21 Preliminary Observations

    SaltSolubility in waterCarbonatesInsoluble except those of sodium, potassium and ammoniumChloridesSoluble except those of lead(II) and silver.NitratesAll are soluble.SulphatesSoluble except those of lead(II) and Barium

  • Solid : 1 small spatula, just enough to cover the bottom of a test tube. Liquid : About 2 cm-depth in a test tube. Reagent : Pour slowly and carefully down the side of the test tube. Or use a dropper and drop it slowly into the test tube.

    Qualitative AnalysisChapter 21 Quantity of Substance

  • Using a Bunsen burnerHeating LiquidsUse a gentle non-luminous flame (air-hole half open) for heating a liquid in a test tube.Move the test tube up and down the flame and stop the heating as soon as the liquid starts to boil.Heating SolidsWhen heating a solid, place it in a dry, clean test tube. Heat gently at first, then strongly.For your own safety, wear protective goggles and point the test tube away from yourself and others when heating.Qualitative AnalysisChapter 21

  • Recording ResultsRecord all your observations immediately or you may forget.Record the change in colour before and after the test.

    Record the formation of a precipitate - its colour and whether it is soluble in excess (extra amount) of the reagent. BeforeAfterPpt. is solubleWhite ppt. formedQualitative AnalysisChapter 21

  • Recording Resultsformulae for names of chemicals ppt. for precipitate aq. for aqueous solution You may use phrases and short forms: Record the liberation of a gas by noting if any bubbles (effervescence) are seen. State the test you carried out to identify the gas, and the result you obtained.

    Qualitative AnalysisChapter 21

    Qualitative AnalysisChapter 21

    Quick Check 1State the colour of the following substances: (a) Sodium carbonate: __________ (b) Potassium sulphate: _________ (c) Calcium chloride: ___________ (d) Iron(II) sulphate: ___________ (e) Iron(III) sulphate: ___________ (f) Copper(II) sulphate: _________ (g) Copper(II) carbonate: ________ (h) Copper(II) oxide: ___________ SolutionQualitative AnalysisChapter 21

    Qualitative AnalysisChapter 21

    Quick Check 1 (contd)State whether the following substances are soluble or insoluble in water: (a) Sodium carbonate: ________________ (b) Magnesium carbonate: _____________ (c) Calcium chloride: _________________ (d) Zinc sulphate: ____________________ (e) Calcium carbonate: ________________ (f) Lead(II) chloride (g) Lead(II) nitrate: ___________________

    SolutionQualitative AnalysisChapter 21

    Qualitative AnalysisChapter 21

    State the colour of the following substances: (a) Sodium carbonate: white (b) Potassium sulphate: white (c) Calcium chloride: white (d) Iron(II) sulphate: greenish-yellow (e) Iron(III) sulphate: reddish-brown (f) Copper(II) sulphate: blue (g) Copper(II) carbonate: green (h) Copper(II) oxide: blackReturnQualitative AnalysisChapter 21Solution to Quick Check 1

    Qualitative AnalysisChapter 21

    2.State whether the following substances are soluble or insoluble in water: (a) Sodium carbonate: soluble (b) Magnesium carbonate: insoluble (c) Calcium chloride: soluble (d) Zinc sulphate: soluble (e) Calcium carbonate: insoluble (f) Lead(II) chloride: insoluble (g) Lead(II) nitrate: soluble

    ReturnQualitative AnalysisChapter 21Solution to Quick Check 1 (contd)

    Qualitative AnalysisChapter 21

    Detection of gases How do we tell if a gas is given off? (a) Effervescence - Bubbles of gas are seen in the solution. (b) Fumes - White fumes are seen when a substance is heated.Qualitative AnalysisChapter 21

  • Testing for gases Look hereNot here Note the smell of the gas by holding the mouth of the tube a few cm from the nose and fanning the gas towards you. Note the colour of the gas by looking through the side of the test tube above the substance or solution.

    Qualitative AnalysisChapter 21

  • Testing for gases Testing with litmus paperWet the litmus papers (one piece of blue and one piece of red) with distilled water, then hold it near the mouth of the test tube.Do not allow the litmus paper to touch the sides of the test tube or drop it into the reagent, as it will react with the reagent and give a false result.

    Qualitative AnalysisChapter 21

  • Table: Testing for gases Qualitative AnalysisChapter 21

    GasColourOdourLitmus paper (moist)OxygenColourlessOdourlessNo effectHydrogenColourlessOdourlessNo effectCarbon dioxideColourlessOdourlessBlue to red AmmoniaColourlessPungent (like urine)Red to blueChlorineLight greenish-yellowPungent (Like swimming pool water)Blue to red then whiteSulphur dioxideColourlessPungent (like burning match)Blue to red

  • Test for gases with lighted splintPOPPlace a lighted (burning) splint near the mouth of the test tube. (A) If the lighted splint extinguishes with a pop sound, then the gas is hydrogen. Qualitative AnalysisChapter 21

  • Test for gases with lighted splint (B) If the lighted splint burns more brightly, then the gas may be oxygen. In this case, you can confirm its identity by using a glowing splint. (D) If the burning splint is extinguished, then the gas may be carbon dioxide, water vapour, or some other gases.(C) Insert a glowing splint (no fire) into the mouth of the test tube. If the glowing splint is relighted (rekindled) and burns brightly, then the gas is oxygen. flame extinguishedQualitative AnalysisChapter 21

  • Testing for carbon dioxideNote The carbon dioxide should not be passed for too long, as the limewater will turn clear again (due to the formation of soluble calcium hydrogen carbonate), giving a false impression of a negative test. Pour limewater (calcium hydroxide solution) into a test tube until it reaches a depth of 2 cm from the bottom of the test tube. Stopper the reacting test tube with a delivery tube and pass the gas into the limewater. If a white ppt. is formed, the gas is carbon dioxide.Qualitative AnalysisChapter 21

    Qualitative AnalysisChapter 21

    Quick Check 2SolutionComplete the table below.Qualitative AnalysisChapter 21

    TestObservationInference1. Heat a little of solid X strongly in a test tube.Gas evolved extinguishes a burning splint. It turns limewater chalky.2. Heat the solution P gently and test for any gas evolved. A pungent gas was given off. It turned moist red litmus to blue.3. Heat solid Q strongly in a test tube. Test for any gas evolved.A colourless and odourless gas is given off. The gas relights a glowing splint.4. Add solid X to the solution Z and warm. Test for any gas evolved.Bubbles of gas were given off. The burning splint is extinguished with a pop sound.

    Qualitative AnalysisChapter 21

    ReturnQualitative AnalysisChapter 21 Solution to Quick Check 2

    TestObservationInference1. Heat a little of solid X strongly in a test tube.Gas evolved extinguished a burning splint. It turned limewater chalky.Gas produced is carbon dioxide. CO32- may be present.2. Heat the solution P gently and test for any gas evolved. A pungent gas was given off. It turned moist red litmus to blue.Gas evolved is NH3. NH4+ may be present.3. Heat solid Q strongly in a test tube. Test for any gas evolved.A colourless and odourless gas is given off. The gas relighted a glowing splint.Gas evolved is oxygen.4. Add solid X to the solution Z and warm. Test for any gas evolved.Bubbles of gas were given off. Gas extinguished a lighted splint with a pop sound.Gas evolved is hydrogen. Solid X may be a metal and solution Z may be an acid.

    Qualitative AnalysisChapter 21

    Identification of cations Aluminium, Al 3+Iron(II), Fe2+ Calcium, Ca2+Iron(III), Fe3+ Zinc, Zn2+Ammonium, NH4+ Copper(II), Cu2+Lead(II), Pb2+Qualitative AnalysisChapter 21 The cations can be identified using sodium hydroxide solution and aqueous ammonia (ammonium hydroxide).The cations react with the hydroxide ions in the alkali to form insoluble metal hydroxides. These hydroxides appear as precipitates.The cations can be identified from: (a) the colour of the precipitate, (b) the solubility of the precipitate in excess of the reagent.

  • Identification of cationsThe unknown substance must be in solution form. If not, a solution must be made as follows: - soluble salt : dissolve in distilled water. - insoluble salt : dissolve in a little dilute nitric acid.Test using sodium hydroxide solution: (a) Add sodium hydroxide solution drop by drop to the unknown solution. (b) Observe the precipitate formed and

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