SASS Chem Data of Ionic Cmpds Summary

Page 1 ST ANDREW’S SECONDARY SCHOOL NAME CLASS CHEMISTRY 6092 Chemical Data of Common Ionic Compounds: A Summary Solubility, colour, chemical analysis General advice • It is essential that you follow the instructions and read the information carefully before you carry out the experiment(s). Apart from testing of gases, you should not use additional reagents for any test in the experiment. • Amount to use: • Reagents selected for use in tests must be included in your answers with their full chemical name/ correct formulae. E.g: Dilute hydrochloric acid : HCl (aq) [NOT H+ / dilute acid] Dilute silver nitrate solution : AgNO3 (aq) [NOT Ag+ / nitrate solution / Ag2NO3] • Check for contradictions within your answers. E.g: When recording the observation for Zn2+ in excess NaOH (aq): “White insoluble precipitate dissolves” does not make sense! You should record it as: “White precipitate formed at first, soluble in excess NaOH (aq) forming a colourless solution” • When you are asked to test the pH, you should give the pH number and NOT acidic / alkaline. pH is determined using Universal Indicator; litmus paper can only determine if it is acidic / alkaline. E.g: Turns blue litmus paper red = the solution is acidic Blue litmus paper remains unchanged = the solution is NOT acidic (either alkaline or neutral) • When heating a solid, heat gently first then strongly. When heating a liquid or solution, move the test tube in and out of the flame every now and then. Skills Build-up These are some important skills in Chemistry: Manipulation and Observation • Demonstrating experimental techniques and skills • Making observations and recording readings or measurements • Presenting data Analysis • Processing data • Drawing conclusions Planning • Planning an investigation • Producing a procedure for an investigation • Evaluating the plan and procedure (after carrying out the plan) The next few sections list some guidelines for you to follow when carrying out Chemistry experiments. 1 Manipulation and Observation 1.1 Demonstrating Experimental Techniques and Skills A Set up an apparatus from a diagram or from written instructions. B Follow a sequence of instructions to perform an experiment. C Use clean apparatus when doing experiments. D Use the appropriate amount of chemicals. Unless otherwise specifi ed, • use suffi cient amount of the solid to fi ll about 1 cm depth of a test tube (Figure 1); • add liquids or solutions to a depth of about 2 cm in a test tube (Figure 2) or about one-third height in a boiling tube (Figure 3). Figure 3 Boiling tube with liquid Figure 1 Test tube with solid Figure 2 Test tube with liquid 1–3 of boiling tube liquid solid1 cm { 2 cm liquid © 2013 Marshall Cavendish International (Singapore) Private Limited 1Skills Build-up (S)CMPB_Skills-Buildup.indd 1(S)CMPB_Skills-Buildup.indd 1 8/22/12 12:06 PM8/22/12 12:06 PM

Page 2 Use a test tube holder! Recording observations You will be asked to carry out experiment(s) to investigate an unknown substance or substance(s). Follow the instructions for each test and record your observation. • The observations must be recorded in the same order as they are made. E.g: Test A ® B ® C Observation A ® B ® C • When you are asked to describe the appearance of a substance, you should provide its colour and physical state. E.g: Black solid : C Colourless solution (aq) / liquid (l) Brown gas : NO2 • Observations are what you can see, hear, smell or feel by touch in each test. Examples include: o colour of solids/ precipitates (white, black, brown, etc) o colour of solutions – use colourless if the solution has no colour and NOT clear o effervescence, bubbles or fizzing o pop sound o test tube feels warm o pungent odour of a gas (usually NH3) Proper method to observe the odour (ii) Heating solids • Use a test tube holder when heating solids in a test tube (Figure 5). • Use a clean, dry hard-glass test tube or crucible for heating solids. • Heat gently fi rst, then strongly until the glass is glowing red hot. • Use a pair of tongs when handling a heated crucible (Figure 6). Figure 5 Using a test tube holder Figure 6 Using a pair of tongs (iii) Heating liquids • Use a test tube holder when heating liquids in a test tube. • Swirl/shake the contents in the test tube gently to ensure uniform heating. • Remove the test tube from the fl ame every now and then. This is to prevent sputtering of the liquid. • Use a water bath for fl ammable liquids. Do not heat fl ammable liquids over an open fl ame. (b) Transferring substances Ensure that there is no loss of chemicals (no spilling) when transferring or pouring substances. (i) Transferring solids Use a spatula to scoop a small quantity of solid into a test tube or beaker (Figure 7). Figure 7 Transferring a solid with a spatula © 2013 Marshall Cavendish International (Singapore) Private Limited 3Skills Build-up (S)CMPB_Skills-Buildup.indd 3(S)CMPB_Skills-Buildup.indd 3 8/22/12 12:06 PM8/22/12 12:06 PM (ii) Test the gas using litmus paper. • Hold the moist litmus paper near the mouth of the test tube. • Make sure the litmus paper does not touch the sides of the test tube or drop into the test tube. The solution in the test tube might react with the litmus paper, leading to inaccurate observations. (iii) Carry out a specifi c test to identify the gas. • If the test involves passing a gas into a reagent via a delivery tube (e.g. test for carbon dioxide), do not use excessive amounts of reagent (see page 99). 2. Whether a precipitate is formed (a) We say a precipitate is present when an insoluble substance is formed after mixing two solutions. Do not refer to all solids as precipitates. A precipitate is the solid product of mixing two solutions. (b) When testing for cations in an unknown solution, add aqueous sodium hydroxide or aqueous ammonia dropwise to about 2 cm3 of the unknown solution. Shake the test tube well and observe if a precipitate is formed each time a drop of the reagent is added to the solution. After this has been done, add an excess of the reagent until there is no further change. If a large volume of a reagent is added to the unknown solution at once, the precipitate formed may dissolve immediately in the reagent before its formation can be observed. (b) Test for any gas that is evolved. • Test for gases only if you observe effervescence. • Your results should be based on what you have observed, not what you have memorised. For example, when you heat a mixture of sodium hydroxide and aluminium foil, do not assume that the gas evolved is ammonia. The gas could also be hydrogen. (i) Observe the colour and odour of the gas evolved. • Hold the test tube a few centimetres away and fan the gas towards yourself (Figure 1). • Do not inhale the gas directly. Figure 1 How to smell a gas © 2013 Marshall Cavendish International (Singapore) Private Limited92Introduction to Qualitative Analysis (S)CMPB_IntroQua.indd 92(S)CMPB_IntroQua.indd 92 8/22/12 12:05 PM8/22/12 12:05 PM

Page 3 • Do not give conclusions when recording observations. E.g: “A gas is given off/ evolved” is a conclusion; you should record as “Effervescence observed. The gas produced extinguishes lighted splint with a pop sound”. except for ammonia Must include “gas produced” for the observation, as it turns damp red litmus paper blue. • Always use the same language as the Qualitative Analysis Notes provided. E.g: Light blue precipitate formed at first, soluble in excess aqueous ammonia solution giving a dark blue solution. In rare cases where you cannot find similar observations in the notes, then record what you observed. E.g: Turns white solid (anhydrous