Chem Exp 2
Experiment 2. Physical and Chemical Changes
Objectives
Learning Objective 1: To observe physical and chemical changes.
Learning Objective 2. To identify physical and chemical changes.
Introduction
A physical change is one in which the appearance of a substance changes but its composition or identity is unaffected. No new substances are formed. Physical changes are usually reversible.
Examples of physical changes include the boiling of water to produce steam, the filing of a metal to produce dust or filings, and the dissolving of table salt in water. In most cases, one or two simple processes are all that is needed to reverse the physical change. For example, salt that has been dissolved in water may be recovered water by evaporating the water from the solution.
A chemical change (Chemical reaction) is one that results in a change in the composition and identity of a substance. New substances are formed. Chemical changes are usually irreversible. Examples of chemical reactions include the burning of wood to form carbon dioxide and water, the rusting of iron to form iron oxide, and the heating of limestone to produce lime and carbon dioxide. Usually, reversing a chemical reaction requires one or more chemical reactions.
In a chemical reaction, one or more substances called reactants are necessary. The newly formed substance(s) is(are) called the product(s) and these product(s) always differ from the reactants in their physical and chemical properties. Physical properties are properties that do not involve the reaction of a substance with other substances. They include such things as color, state (solid, liquid, or gas), density, and melting and boiling points. Chemical properties are displayed when the substance reacts to produce products or undergoes a chemical change. A chemical reaction is indicated by any of the following observations.
1. Change of color.
2. Production of heat, light, or sound.
3. Evolution of gas.
4. Formation of a precipitate.
These observations are not infallible indications that a chemical reaction has occurred. For example, when ice forms from liquid water, a solid is present where none was before, but a physical change rather than a chemical reaction has occurred. In this case, one would classify the change as physical based on the ease of reversing it.
SAFETY INFORMATION:
1. NO EATING or DRINKING IS ALLOWED IN CHEMISTRY LABORATORIES.
2. OBTAIN and WEAR EYE GOGGLES AT ALL TIMES.
3. NOTIFY YOUR INSTRUCTOR IF YOU HAVE ANY QUESTIONS.
4. WASH YOUR HANDS BEFORE LEAVING THE LABORATORY.
Special Note: Quantities need not be measured precisely in this experiment. If reagents are dispensed from dropper bottles, one squirt from the dropper should deliver about 1 mL (or approximately 20 drops), or 1 cc, of the reagent. For reagents dispensed from bottles with stoppers or screw caps, simply estimate the amount of liquid as you pour it into a test tube. An ordinary large test tube holds about 20 mL. Therefore, liquid quantities can be estimated as follows:
Quantity of Liquid (Approximate height in a large test, 20-mL, tube): 10 mL is about 1/2 full
5 mL is about 1/4 full
2 mL is about ½ inch
1 mL is about 20 drops
Whenever laboratory experiments are qualitative rather than quantitative in nature, volumes of liquid can be estimated in this manner.
For all demonstrations and tests, you must record in your data sheet the appearance of the substances before the change and what occurred during and after the change(s).
Procedure C. Three Pennies Experiment NOTE:
Very clean pennies are required.
Procedure D. Sublimation of Iodine crystals NOTE:
Hot plate gives satisfactory results.
STUDENT EXPRIMENTS:
NOTE: Tests 1, 6, and 10 can be run effectively in a spot plate.
MATERIALS: Steel wool, well plates, small test tubes, calcium carbonate, baking soda, wooden splints, magnesium ribbon, potassium chlorate, 0.1 M NaCl, 0.1 M AgNO3, CaCl2 NH4NO3, universal indicator, 0.5 M CuCl2, Dry ice, 6 M NaOH, zinc powder.
PRELAB QUESTIONS NAME: Matthew Ramirez
1. Identify the three Physical States of matter.
2. Identify four changes which are indicative of a chemical change.
3. Provide the two examples of chemical changes discussed in the experimental handout.
4. Fill in the blanks.
a. 1 mL is about
b. 1cc is about
c. ½ full test tube is about
5. Circle the piece of equipment which can be used to more precisely determine the volume.
Dropper Test Tube
A. Baking Soda and Vinegar
What will happen if I mix vinegar and baking soda? Is it a chemical change or a physical change?
Procedure:
1. Weigh about 5 grams of baking soda using a spatula. Place the baking soda in into a container.
2. Pour about 2 mL of vinegar into the container.
3. Note observations.
B. Clock Reaction
Color changes as a function of time, when with mixing solutions - Is it a chemical change or a physical change?
Procedure:
NOTE: The following solutions are prepared, in advance, by the laboratory coordinator. Can be substituted by any other iodine reaction procedures.
Solution A: 250 mL of 3% Hydrogen peroxide (H2O2)
Solution B: 7.25 g Potassium Iodate (KIO3) in 100 mL of water. Add 2.15 mL of 6.0 M sulfuric acid (H2SO4). Warm and stir until KIO3 dissolves. Dilute to 125 mL with water.
Solution C: 2.6 g malonic acid and 0.55 g Manganese (II) sulfate (MnSO4.H2O) in 100.0 mL water. Heat 50 mL of water to a boil in a 100-mL beaker. In a 50-mL beaker, make a slurry of 0.2 g starch in
5.0 mL deionized water. Pour the slurry into the boiling water. Stir to dissolve. Now pour 15 mL of this
into the malonic acid/manganese(II) sulfate solution. Dilute to 125 mL with water.
Combine solution A and B. Add a magnetic stirring bar and stir to create a large vortex in the mixture. Now pour in solution C. Use cell phones timers, note seconds for reaction.
C. Three Pennies Experiment (Optional)
Has the Alchemists' Dream Come True? Is it a chemical change or a physical change?
The Alchemists' dream was to change base metals such as lead into gold. In this experiment, you start with three clean shiny copper pennies (minted on or before 1982) and end with one copper penny (base metal left unchanged), one silver penny (coated with zinc), and one golden penny (bronzed penny). Your problem is to classify the process which changes the copper penny to the silver penny and the silver penny to the gold penny as a physical or chemical process.
Leave one of the three pennies on the bench top (this penny remains unchanged during the experiment). Place a pea size scoop of granular zinc metal in a 50-mL beaker. Put about 20 mL of 2.0 M ZnCl2 in the beaker, drop two pennies in the solution and heat on a hot plate. After the solution boils for about 5 minutes and the top side of the pennies look silvery turn the pennies over using your spatula or crucible tongs. Continue heating for another 5 minutes or until both sides of the pennies are silver in color. Turn off the hot plate and take the beaker off the hot plate. Let the solution cool and carefully pour the solution into the waste container. Remove the pennies and dry them. Place the solid zinc in the appropriate waste container. What color are the pennies?
Put 25 mL of cold water in the 50-mL beaker. Light a Bunsen burner and adjust the flame so that it is not too hot (no inner light blue cone but not a yellow flame either). Pick up one of the pennies with your crucible tongs and carefully heat it in the top of the flame until it suddenly turns golden. Immediately remove the penny from the flame, and drop it in the beaker of cold water. (If you heat the penny too long or with too hot a flame, the penny will melt.) What color is the penny? D. Sublimation of Dry Ice (if available) or iodine.
Procedures: Obtain dry ice from the prep room and place it on a workbench. Note your observations. DO NOT TOUCH the dry ice. It is -80oC. It will burn your hand. Use CAUTION.
Heat solid iodine in a beaker and collect the vapors using an ice cooled evaporating dish.
STUDENTS EXPERIMENTS
Test 1: Copper(II) sulfate (CuSO4) solution and steel wool
a. Place about 10 mL of the blue copper(II) sulfate solution in a small beaker.
b. Dip a piece of steel wool, which is composed mainly of elemental iron, into the blue copper(II) sulfate solution by holding it in the solution with tongs for about 30 seconds and then take it out.
c. Examine the coating on the steel wool. Record your observations. Now put the steel wool back into the solution and let it remain for at least 15 minutes while you perform the other procedures. d. Record your observations.
Test 2: Decomposition of the calcium carbonate (CaCO3).
a. Working in the fume-hood, add dilute hydrochloric acid (HCl) dropwise to a test tube containing a pea size portion of calcium carbonate.
b. Perform a splint test on any gases that are produced: light a wooden splint and bring close to the mouth of the test tube.
c. Record your observations.
Test 3: Burning of the magnesium ribbon.
a. Working in the fume-hood, ignite a small strip of magnesium ribbon by holding it with tongs in the flame of the burner. Do not look directly at the burning magnesium flame.
b. Record your observations.
Test 4: Decomposition of potassium chlorate (KClO3).
a. Mix a small quantity of solid potassium chlorate with solid manganese(IV) oxide, MnO2, in a test tube and attach the test tube to a stand with a clamp at a 45º angle.
b. Working in the fume-hood, heat the contents of the test tube over the Bunsen burner flame.
c. Perform a splint test on any gases that are produced: light a wooden splint and bring it close to the mouth of the test tube.
d. Record your observations. (Note: MnO2 is a catalyst in this reaction and so does not need to be part of the chemical equation. A catalyst is a reagent that speeds up the chemical reaction without being consumed).
Test 5: Thermal decomposition of sodium bicarbonate (NaHCO3). a.Place a small quantity of solid sodium bicarbonate in a test tube. b.Attach the test tube to a stand with a clamp at a 45º angle.
c.Working in the fume-hood, heat the contents of the test tube using a Bunsen burner flame. d.Perform a splint test on any gases that are produced.
e.Record your observations.
Test 6: Precipitating sodium chloride solution (NaCl) with a silver nitrate (AgNO3) solution
a. Place 10 drops of sodium chloride solution in a test tube.
b. Add 2 drops of silver nitrate solution to the test tube.
c. Record your observations.
Test 7: Formation of the calcium chloride (CaCl2) solution.
a. Put about a teaspoon of calcium chloride (CaCl2, a solid) in a test tube.
b. Touch the bottom of the test tube to the inside of your arm to get a sense of its temperature.
c. Add deionized water to the test tube (about 1/3 full) and stir with a glass stirring rod.
d. Touch the bottom of the test tube to the inside of your arm again. If you don’t feel a difference, addmore solid calcium chloride. Record your observations.
e. When finished, place the contents of the test tube in the waste container. Rinse the test tube with water. You do not need to dry the test tube.
Test 8: Formation of an ammonium nitrate (NH4NO3) solution.
a. Put about a teaspoon of ammonium nitrate (NH4NO3, a solid) in a test tube.
b. Touch the bottom of the test tube to the inside of your arm to get a sense of its temperature.
c. Add deionized water to the test tube (about 1/3 full) and stir with a glass stirring rod.
d. Touch the bottom of the test tube to the inside of your arm again. If you don’t feel a difference, addmore potassium nitrate. Record your observations.
e. When finished, place the contents of the test tube in the waste container. Rinse the test tube withwater. You do not need to dry the test tube.
Test 9: A natural indicator (red cabbage juice is "universal indicator").
a. Obtain 3 test tubes and fill each one with 5-10 drops of universal indicator.
b. To the first test tube, add an equal amount of deionized water.
c. To the second test tube, add an equal amount of 0.10 M HCl (hydrochloric acid).
d. To the third test tube, add an equal amount of 0.10 M NaHCO3 (baking soda).
e. Note any color changes and record them on your data sheet.
f. Pour the liquids into a waste container. Thoroughly rinse the test tubes with water.
Test 10: Magnesium (Mg) with copper(II) chloride (CuCl2) solution.
a. Add about 2-3 mL of CuCl2 (copper(II) chloride) solution to a test tube.
b. Record your observations. (Include color, temperature, states of matter, etc.)
c. To this test tube, add a 1 inch piece of magnesium ribbon.
d. Touch the test tube to the inside of your arm. Wait for a minute and record your observations. (Include color, temperature, states of matter, etc.)
e. When finished, place the magnesium and copper chloride solution in a labeled waste container. Rinse the test tube with water (it does not need to be dry).
DATA SHEETS
For each reaction, write down your observation(s) before and after the reaction that you believe provide evidence that a chemical change took place. Be clear and concise; use only enough detail to communicate your findings.
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EXPERIMENT |
OBSERVATIONS
BEFORE CHANGE AFTER CHANGE |
TYPE OF CHANGE (Physical or Chemical) |
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A |
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B |
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C |
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D |
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Test #1 |
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Test #2 |
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Test #3 |
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Test #4 |
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Test #5 |
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Test #6 |
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Test #7 |
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Test # 8 |
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Test #9 |
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Test #10 |
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POST LABORATORY ASSIGNMENT NAME:
Label each process as a physical (P) or chemical (C) change:
a. reflection of light on a mirror ( )
b. bending a copper wire ( )
c. grinding spices ( )
d. burning paper ( )
e. slicing potatoes for fries ( )
f. mixing sugar with water ( )
g. strongly heating a mixture of steel wool and sulfur ( )
h. a nail rusting ( )
i. paper ripping ( )
j. wood burning ( )
k. mixing water and food coloring ( )
l. food molding (rotting) ( )
m. tarnishing copper ( )
n. dyeing fabric ( )