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Experiment 2 Identification of Compounds through Physical and Chemical Changes OBJECTIVE To identify compounds on the basis of physical and chemical changes and design a procedure for determining the presence of a particular compound. EQUIPMENT Fifteen 12 x 75-mm test tubes, test tube rack, test tube holder, permanent marker (to label test tubes), three medicine droppers. If test tubes are not available, a 24-well plate may be used. REAGENTS Part A: Approximately 1 mL (~5-10 drops) each of aqueous known solutions of NaCl, Na2CO3, MgSO4, NH4Cl, and H2O (this step is repeated for the three test reagents); 5-10 drops each of aqueous test reagents of AgNO3, NaOH, and HCl. Part B:
Approximately 5-10 drops each of solutions labeled 1 – 5; 5-10 drops of test reagents labeled A, B, and C.
SAFETY AND DISPOSAL Refer to the MSDS information available online when working with NaCl,1 Na2CO3,2 MgSO4,3 NH4Cl,4 AgNO3,5 NaOH,6 and HCl.7 Contact with silver nitrate solutions can lead to staining of the skin, dermatitis, and painful burns.5 Disposal should be in accordance with local, state and federal regulations. Disposal for NaCl, Na2CO3, MgSO4, NaOH, and HCl should be into a labeled laboratory waste container for inorganic chemicals. All waste silver salts, especially those that result from the AgNO3 reagent observations found in this experiment, should be recovered in a specially labeled waste container for silver salts. INTRODUCTION A goal of a research chemist is, for example, to separate the substances of a reaction mixture and attempt to identify each substance through a systematic study of their chemical and physical properties.1 Any change in the appearance of a substance can be classified as either a physical or chemical change. In a physical change, the same substance remains after the change (melting or boiling a liquid, cutting a piece of wood, tearing paper, dissolving sugar in water, and pouring a liquid from one container to another are some examples). These changes do not change the chemical character of the substance. In a chemical change, the substance undergoes a change so that one or more new substances with different characteristics are formed (burning, digestion, and fermenting are some examples). Let us take a specific example. Sodium is a silvery, soft metal that reacts vigorously with water. Chlorine is a yellowgreen gas that is highly corrosive and poisonous. However, if these two elements are combined, they produce a different substance, a white crystalline solid. This new substance is common table salt, sodium chloride, which is neither reactive with water nor poisonous! Experiment 2 Physical and Chemical Changes 41
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As a rule of thumb, if there is gas formation, color change, precipitation, a reaction that generates or removes energy, or odor change occurs, a chemical change has taken place. A gas forms. This evolution may be quite rapid, or it may be a “fizzing” sound. For example, the bubbles of gas observed when antacid is dropped into water. A color change occurs. A substance added to the system may cause a color change. In describing a colored liquid, name the color. In describing a liquid that has no color, call it colorless. For example, milk would be described as a cloudy white liquid. If it is a transparent, colorless liquid, describe the liquid as clear and colorless. A precipitate appears (or disappears). The nature of the precipitate is important. It may be crystalline, it may have color or it may merely cloud the solution. An example is shown in (Fig. 2-1). An energy change (heat or absence of heat) occurs. Heat may be evolved or absorbed. The reaction vessel becomes warm if it releases heat (exothermic) or cools if it absorbs heat (endothermic). An odor change is detected. The odor of a substance may appear, disappear, or become more intense. Part A of this experiment asks you to observe chemical reactions of various compounds and identify these compounds based on their chemical properties.
Fig. 2-1. Precipitate formation
In Part B you will be given an unknown compound (one that you have previously investigated in Part A) to identify. The interpretations of the collected data (from Part A) will help you in identifying your unknown. The chemical properties of the following known compounds, dissolved in water, will be investigated in Part A: sodium chloride sodium carbonate magnesium sulfate
NaCl (aq) Na2CO3 (aq) MgSO4 (aq)
ammonium chloride water
NH4Cl (aq) H2O (l)
The following reagents will be used to identify and characterize the above compounds: silver nitrate AgNO3 (aq) sodium hydroxide NaOH (aq) hydrochloric acid HCl (aq) Note: You should discuss your findings and interpretations with a partner, but each of you should analyze your own unknown. Remember, you will be conducting a test on each known compound and your unknown with a single test reagent. To organize your work, there is a “reaction matrix” provided for you to describe your observations. Because the space is limited, you may want to use the following codes: pc – precipitate + color g – gas, no odor cc – cloudy + color go – gas, odor nr – no reaction
Experiment 2 Physical and Chemical Changes
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PROCEDURE Wear your safety goggles at all times in the laboratory. Part A 1. Silver nitrate (AgNO3) reagent observations. Label five small, clean test tubes with a permanent marker (Fig. 2-2). Alternatively, you can use a clean 24-well plate (Fig. 2-3). You should ask your lab instructor which setup to use. Place 5-10 drops of each of the five “known” solutions into your labeled test tubes (or wells A1-A5, Fig. 2-3). Use a dropper bottle (or a dropper pipet) to deliver 3-5 drops of AgNO3 solution. (Caution: AgNO3 forms black stains on skin that can be painful. Thoroughly rinse your hands after working with silver salts.) If you observe a chemical change, add 5-10 more drops to see if there are any additional changes. Record your observations in the “reaction matrix”. SAVE YOUR TEST SOLUTIONS for part A. 4 of your Fig. 2-2. Arrangement of test tubes for report. You will need to write the formula for each AgNO3 test precipitate that forms. For example, as shown in Fig. 2-1, a precipitate of AgCl(s) will form when you mix solutions of NaCl(aq) and AgNO3(aq). You may ask your lab instructor for assistance. You may also refer to a solubility table if needed. 2. Sodium hydroxide (NaOH) reagent observations. Follow the same procedure as in part A.1.a. above for your second set of test tubes (or wells B1-B5, Fig. 2-3). Fig. 2-3. Arrangement of 24-well plate for all To each test tube, slowly reagents add 5-10 drops of NaOH; make observations as you add the NaOH. In particular, observe if any gas evolves in any of the tests. Check for odor.
SAVE YOUR TEST SOLUTIONS for part A. 4 of your report. You will need to write the formula for each precipitate that forms. 3. Hydrochloric acid (HCl) reagent observations. Follow the same procedure as above for your third set of test tubes (or wells C1-C5, Fig. 2-3). To each test tube, slowly add 5-10 drops of HCl; make observations as you did for the NaOH. In particular, observe if any gas evolves in any of the tests. Check for odor. SAVE YOUR TEST SOLUTIONS for part A. 4 of your report. You will need to write the formula for each precipitate that forms. 4. Identification of unknown. Obtain an unknown for Part A from your instructor. Perform the three reagent tests on your unknown. Identify the unknown compound based on the data from the ‘known’ solutions collected in your “reaction matrix”. Experiment 2 Physical and Chemical Changes 45
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Part B The Part B experiment design is similar to that of Part A. Thus, you will need either 15 clean test tubes or a clean 24-well plate. 1. Prepare solutions Place ~1 mL of each test solution (there are five solutions labeled 1 through 5) in the clean test tubes or in the well plate. 2. Test solutions Test each of the five solutions with reagent A. If, after adding several drops, you observe a chemical change, add 5-10 drops more to see if there are any additional changes. Record your observations in the “reaction matrix”. With a fresh set of solutions 1-5 in clean test tubes (or wells), test each with reagent B. Repeat with reagent C. 3.
Identification of unknown Obtain an unknown for Part B from your instructor. Perform the three reagent tests on your unknown. Identify the unknown as one of the five solutions from Part B. 1 above. Record your observations.
FURTHER READING 1. http://fscimage.fishersci.com/msds/21105.htm (NaCl, last accessed 08/2013) 2. http://www.flinnsci.com/Documents/MSDS/S/SodiumCarbonate.pdf (Na2CO3, last accessed 08/2013) 3. http://www.sciencelab.com/msds.php?msdsId=9927218 (MgSO4, last accessed 08/2013) 4. http://www.sciencelab.com/msds.php?msdsId=9927431 (NH4Cl, last accessed 08/2013) 5. http://www.sciencelab.com/msds.php?msdsId=9927411 (AgNO3, last accessed 08/2013) 6. http://www.sciencelab.com/msds.php?msdsId=9924997 (NaOH, last accessed 08/2013) 7. http://www.sciencelab.com/msds.php?msdsId=9924285 (HCl, last accessed 08/2013) 8. Selco, J., M. Bruno, and S. Chan, “Discovering Periodicity: Hands-On, Minds-On Organization of the Periodic Table by Visualizing the Unseen.” J. Chem. Educ., 90 (2013) 995-1002. 9. DeMeo, S., “Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical Change in the Introductory Laboratory.” J. Chem. Educ., 72 (1995) 836.
Experiment 2 Physical and Chemical Changes
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Name _____________________________________ Lab Section_______________Date__________ REPORT ON EXPERIMENT 2 Identification of Compounds through Physical and Chemical Changes Part A Reaction matrix Test
NaCl (aq)
Na2CO3 (aq)
MgSO4 (aq)
NH4Cl (aq)
H2O (l)
Unknown
H2O (l)
Unknown
5
Unknown
AgNO3 (aq) NaOH (aq) HCl (aq)
Write formulas for the precipitates that formed in the reaction matrix above. Test
NaCl (aq)
Na2CO3 (aq)
MgSO4 (aq)
NH4Cl (aq)
AgNO3 (aq) NaOH (aq) HCl (aq)
Sample number of unknown for Part A: ____________________ Compound in the unknown solution: ______________________ Part B Reaction matrix Solution
1
2
3
4
Reagent A Reagent B Reagent C Sample number of unknown for Part B:
____________________
Compound in the unknown is the same as solution number: ____________________ Experiment 2 Physical and Chemical Changes 47
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QUESTIONS (Submit your answers on a separate sheet if necessary) 1. Predict what would be observed (and explain why) when aqueous solutions of the following are mixed: a. K2CO3 and HCl
_____________________________
b. KCl and AgNO3
_____________________________
c. MgCl2 and NaOH
_____________________________
d. NH4NO3 and NaOH _____________________________ 2. Physical changes are often used to separate two or more substances in a mixture. What physical change and what property do you think could be used to separate each of the following pairs? a. Salt and iron filings _____________________________ b. Iron filings and aluminum filings _____________________________ c. Sand and water _____________________________ d. Rubies and emeralds _____________________________ 3. Classify each of the following as a physical change or a chemical change. a. Liquid water freezes at 0 ˚C
_____________________________
b. A sheet of paper caught on fire
_____________________________
c. A copper penny is oxidized (turns black)
_____________________________
d. Ice melts to form water
_____________________________
4. Imagine a lovely beach scene –something majestic like sand, surf, and some salt crusting the sheer black cliffs that rise up from the water. Which of the substances in that image are water soluble? a. Sand _____________________
c. Salt ______________________
b. Rocks ____________________
d. Shells- ____________________
5. The Wicked Witch of the West in "The Wizard of Oz." is about to do something very, very wicked to Dorothy and her friends. As any well-meaning Kansas girl might do, Dorothy dumps a big ol’ bucket of water on the Wicked Witch. “I'm melting! Meeeeeellllttiiiinnggg!” cries the witch as she turns into a puddle of goo. Is the Wicked Witch of the West water-soluble? Is she really melting? 6. Calcium carbonate is an extremely common substance. You can find it in the following: minerals calcite and aragonite, the shell of an egg or an oyster, a stately marble floor, or that nasty
Experiment 2 Physical and Chemical Changes
49 whitish crud around your faucet. Is calcium carbonate (CaCO3) easily water-soluble? Explain your answer.
Experiment 2 Physical and Chemical Changes 49
