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IDENTIFICATION OF HOUSEHOLD CHEMICALS TEST EXERCISE – 105 Points A PUZZLE
OBJECTIVES YOU determineand describe 7 properties of a substance sufficiently accurately to permit: other investigators to tell if they have the same substance from their determination of the same properties YOU, with those other investigators, Infer the identityof the substance by comparison of its 7 properties with those of a list of 9 possible substances. Confirm the identityof the substance through an additional confirmatory test
Concepts: • Acid Ammonia • Base Confirmatory Test • Flame Test High Temperature Behavior • Isopropanol Known Substance Results • Known Substances Melting Point • Other’s Unknowns Other’s Results • Partners pH of Aqueous Solution • Posted Reaction with Iodine • Solubility Tests • VinegarWater • Your Results Your Unknown • Oxidation / Reduction
Techniques: Solubility determination Reporting Observations Branched Procedures Apparatus: Test Tubes pH Paper Bunsen Burner Melting Point Apparatus Stirring Rod
WHAT ARE THE SEVEN TESTS? • 4 Solubility / Reactivity tests • 1. Solubility in / Reactivity with Water • 2. " Ammonia • 3. " Vinegar • 4. " Rubbing alcohol • 3 other tests • 5. pH of aqueous solution of unknown • 6. Reactivity with tincture of iodine • 7. Behavior at high temperature Soluble: Forming a homogeneous liquid phase with 2 or more components (called a solution).
Chemical Pix The Solvents NH3 (aq) Rubbing Alcohol Vinegar Ammonia CH3 COOH(aq) (CH3)2CHOH H2O Water
SOLUBILITY / REACTIVITY TESTS • Why these four solvents? • Water - The most common terrestrial solvent • (Extensive data on solubility in water exist, • but hydrogen bonding makes it a complex ) • 2.Aqueous ammonia - A basic aqueous solvent • Likely to dissolve and react withacidic substances • 3. Vinegar (aqueous acetic acid) - An acidic aqueous solvent • Likely to dissolve and react withbasic • substances • 4. Rubbing alcohol (isopropanol) - A non-aqueous solvent • Likely to dissolve non-polar materials Rule of Thumb: Acids react with Bases & vice versa
HOW TO DO SOLUBILITY TESTS Operational definition of solubility: Solid is solublein solvent if ~20 mg (1/4 spatulaful) or more dissolves in 2mL of solvent ( 0.01g/1.00g = 1% solution ) @ ~20oC Use small test tubes Solubility can be accelerated by tapping test tube, or stirring (using a thin glass rod) using a thin glass rod Soluble: Forming a homogeneous liquid phase with 2 or more components (called a solution). Requires Patience!
How much is 20 mg? ¼ spatulaful 2 mL
OTHER TESTS 1. Behavior with tincture of Iodine:(I2 in alcohol) Looking for evidence of REACTION, - i.e., color change, - not solubility Reaction has occurred if mixing with an aqueous solution bleaches the I2 color Such substances are reducingagents Whether or not you have studied oxidation and reduction in detail, OPERATIONALLY, you are asked to determine whether or not an aqueous solution of a substance bleaches tincture of iodine. If it does, it is a reducing agent. bleaches I2 (alcohol) + 2e- 2I- colorless brown Reducing agents give up electrons: R R+ + e- Note: Strongly basic solutions also bleach iodine solutions, but this occurs slowly. e.g., vitamin C (ascorbic acid) , hypo
2. pH of aqueous solution • Looking for [acidic / neutral / basic] property of • aqueous solution of unknown. • DO NOT make new aqueous solution! • Transfer drop of aqueous solution to piece of • pH paper using clean stirring rod • Observe color change. • Compare with colors on pH paper container • (posted in lab) • Record and post value! One piece of pH paper can be used for multiple determinations. Please use it efficiently! Stirring rod pH = 8 pH paper
3. Behavior at elevated temperature • Heat solid sample in small test tube over • Bunsen flame ( In hood ) • Point test tube away from anyone! • Observe any and all changes: • Melting • Sublimation (Solid to vapor with no melting) • Water condensing at top of tube • Bubbling, Gas evolution (smell, etc.) • Color change • Charring, etc. As you complete each test, post your results (in summary form) on table at front of laboratory
Each student must perform & report all 7 tests on his/her unknown and anauthentic sample • True • False
Each student must perform & report all 7 tests on his/her unknown and an authentic sample A= True AND, in addition,you must report the results of the confirmatory test on both the unknown & authentic sample
SECTION 3-A Post results AS YOU GET THEM. Other students are waiting for your results. ..................... ..................... S = SOLUBLE (SS = SLIGHTLY SOLUBLE) I = INSOLUBLE C = COLOR CHANGE G = GAS EVOLVED NR = NO REACTION .............. OTHERS AS REQUIRED
Use collective results to deduce which students have the same substance Will require consultationon details that are basis for summary observations in table May require collaborative repetition of some tests, e.g., if all observations except one agree DO NOT PROCEED WITH IDENTIFICATION AND CONFIRMATION OF UNKNOWN UNTIL TA HAS VERIFIED THAT ALL MEMBERSOF YOUR GROUP ARE CORRECTLY IDENTIFIED Exchange unknown samples and repeat the test together
STEP 3: Determinetentative identityof unknown • Working as team, carry out measurements • that permit making tentative identification • of unknown. • HOW? • Compare results of 4 solubility and 3 other • tests from Part 1 on your unknown • with results of same tests on a • selected set of • authentic samples. CaCO3
Chemical Pix The Unknowns Aspirin Acetyl Salicylic Acid Urea Organic acids are characterized by the following grouping of atoms: K Cream of Tartar Potassium Hydrogen Tartrate Vitamin C Ascorbic Acid
Chemical Pix CaCO3 Limestone Sour Salt Citric Acid Washing Soda Na2CO3.10H2O Na2S2O3·5H2O Na2B4O7·10H2O Hypo Sodium Thiosulfate Borax Sodium Tetraborate
The 9 Unknowns – Critical Properties LimestoneCO3= + Ca+ (Base) Washing SodaCO3= + Na+ Base HypoReducing Agent + Na Cream of TartarWeakAcid + K Aspirin Weak Acid Ascorbic Acid Reducing Agent/Weak Acid Sour Salt Weak Acid Urea Weak Base Borax Na + Base
Carbonates and bicarbonates, e. g. CaCO3, Na2CO3, etc., are expected to generate which gas upon the addition of vinegar. • O2 • CO2 • H2 • N2
On what basis are you expected to know the answer to this question? By generalization of the epic poem: Johnny finding life a bore, Drank some H2SO4 Johnny's father, an M.D., Gave him CaCO3 Now Johnny's neutralized its true, But he's also full of CO2 (And now, on the basis of this lecture!)
Do you have to do 7 X 9 = 63 Tests? • Not if you approach this step logically • and use common sense. • Analyze the results of the 7 tests from Part 1 • e.g., IF - your unknown appears to be an acid • i.e., • dissolves in and reacts with NH3, • aqueous solution has low pH, • THEN - Start tests with known substances that are acids.
However, Consistency of test results is necessary, but not sufficient! E.g., if you claim the sample is NOT SOLUBLE in water and, therefore, do NOT perform the pH test, but the sample is actually soluble, BOTH results are incorrect! YOUR RESULTS MUST BE CONSISTENT, BUT THEY ALSO MUST BE CORRECT! A table of solubilities in water of the substances in this exercise is posted on the Web
Table of Solubilities in Water of Substances in SUSB-004 mg/mL A well prepared student will check to see how the tabulated solubilities relate to the solubility criterion that we have defined (~10 mg/mL) Acidic Hydrogens are indicated in Red
Look for properties that have high discrimination. I.e., those whose values vary most from substance to substance in posted table If authentic sample behaves completely differently from unknown in any test No reason to perform additional tests on that authentic sample If authentic behaves sameas unknown in a test Limited evidence that they may be the same Continue testing that known E.g., Unknown dissolves in ammonia but Authentic Sample X does not
Each group must perform and report all 7 tests on each authentic sample. • True • False
Each group must perform and report all 7 tests on eachauthentic sample. B= False If tests are performed carefully & results are analyzed logically, number of tests required to infer a presumed identity for unknown can be small.
BUT: • You must report results of all 7 testson an authentic sample of your reported (un)known. THEN -
STEP 4 – Confirmation • Aftertentative identification, confirm hypothesis with one additional comparative measurement, • This is confirmation for: • ASPIRIN, UREA, ASCORBIC ACID & SOUR SALT • 1. Melting point • For substances that showed clean melting behavior, melting point determinations are used. Measure and compare melting points of: • Unknown • Authentic sample TA’s will demonstrate use of Melting point apparatus
2. Flame test • Flame test confirms presence of • Alkali metal salts • Na – YELLOW K - VIOLET • Alkaline earth salts • Ca - BRICK RED Confirmation for: Limestone Washing Soda Hypo Cream of Tartar Borax
TIME MANAGEMENT FINAL EXERCISE 100 POINTS BASED ON: 1. Accuracy of identification 2. Quality and completeness of observations & appropriateness of methods of identification and confirmation Must report results of all 7 tests for both unknown AND authentic sample + results of confirmation test for both unknown AND authentic sample Exercise involves many potential operations. Grade will reflect ability to Organize activities, AND Do only those which are necessary
What the well organized student will do • Begin working immediately when you get to lab. • 1. Assemble required apparatus • Test tube block, • 6 clean SMALL test tubes, • Test tube holder • Label tubes or block • Once you get your unknown, • 2. Prepare five samples of unknown • Place samples in test tubes
3. Perform tests • Solubilility in H2O, NH3, acetic acid, alcohol, pH test, I2 test, Heat test • Measure 2 mL with cylinder – once • use level in test tube for other samples • Reminder:Sample used to test for water solubility should also be used for: • pH measurement, AND • reaction with Tincture of I2 4. Post results as you get them !!!!
5. Find Partners • Look for individuals who have posted results that are closest to agreeing with your own. • Once you have found your (3) partners: • 6. Do tests on authentic samples • Look for samples whose test results are closest to agreeing with your unknown. • Identify a tentative substance. (Be sure you have all 7 test results for that substance). 7. Do appropriate confirmatory test
A substance that bleaches tincture of Iodine immediately is a(n) • acid • base • oxidizing agent • reducing agent
A substance that bleaches iodine solution is a(n): D= Reducing Agent 1. Behavior with tincture of Iodine:(I2 in alcohol) Looking for evidence of REACTION, - i.e., color change, - not solubility Reaction has occurred if mixing with an aqueous solution bleaches the I2color immediately Such substances are reducingagents bleaches Note: Strongly basic solutions also bleach iodine solutions, but this occurs slowly.
Part 1 – Data collection on unknown - must be done ALONE and WITHOUT CONSULTATION Next Exercise Synthesis of Vanillyl Alcohol from Vanillin Read and do Pre-lab – SUSB-039