GCC CHM 130LL: Chemical and Physical Changes page 1 of 7 CHM 130LL: Chemical and Physical Changes Objective Introduction In this experiment you will - make and record observations of properties of substances. - cause changes to occur and classify these changes as physical or chemical based on evidence provided by your observations. Every pure substance can be described by a unique set of chemical and physical properties. For example, sugar looks, tastes and behaves the same, whether it is from your cupboard or your neighbor’s. You count on this fact whenever you use sugar and would be surprised if its behavior were different. If you needed to decide whether an unlabeled canister contained sugar or something else, you would take a look at it, feel it, taste it—this is OK in the kitchen, but not OK in chemistry lab! (Of course, to know for sure that its sugar, you should determine several physical properties. Some sugar-substitutes look and taste remarkably like the real thing but do not behave the same way in the body.) While you may not realize it, you were using a basic principle of chemistry—that every pure chemical is unique, but every sample of one pure chemical behaves the same way. Practicing chemists in industry are interested in converting chemicals to better serve the marketplace, and research chemists are interested in making new chemicals. Thus, it is of vital importance for chemists to recognize what changes have occurred, to predict what kinds of changes may occur, and to understand how to control these changes. Two general types of changes are involved: Physical changes: These involve only a change in physical state but no change in the composition of the individual atoms or molecules present. For example, dissolving sugar in water still results in a sweet taste because the sugar molecules are still present. Chemical changes: These do change the composition of the substance. Burning the sugar in a saucepan results in a dark caramel, a substance that is different from the original sugar! Some of the sugar molecules have been converted to caramel molecules! To determine if a change was physical or chemical, a chemist must observe what happens to the the individual atoms or molecules. If they are converted to other kinds of molecules, the change was chemical; if not, the change was physical. Because such observations are impossible, chemists rely on evidence that can be observed. In general, if the properties of the substance change, one can assume that the substance has undergone a change in composition (chemical change). If the properties are the same, then no change in composition has occurred (physical change). A change in just one property of a substance does not always mean that a chemical change has occurred. For example, the particle size and appearance of granulated sugar can be changed by grinding into powdered sugar, but tasting it will convince you that it is still sugar. Changes in appearance can be deceiving. Tests of reactivity are better indicators of chemical change.GCC CHM 130LL: Chemical and Physical Changes page 2 of 7 Experimental Procedure In this experiment, you will cause changes in several pure substances then test the substances before and after the changes occur. Record detailed observations, especially with respect to any change in appearance (color, texture) and/or change in physical state indicating a new substance was produced—e.g. fizzing which indicates a gas is given off or cloudiness which indicates a solid precipitate has formed. Based on your tests and observations, you will classify the changes as physical or chemical. If the test results are the same for the original and the changed sample, then the change was physical. If the test results are different for the original and the changed sample, then the change was chemical. An example is shown below: Heating Copper(II) Sulfate Pentahydrate Original Sample Heated Sample Appearance of Sample Blue crystals Gray chunky solid sample + H2O Blue solution, solid at bottom Blue solution, solid at bottom The evidence above indicates that heating the original sample results in a _physical change. Note: You will need 2 dry test tubes for part A only. The rest of the tests can be done in clean wet test tubes. Waste disposal: A waste jar will be provided for all waste generated in this lab. A. Copper Carbonate and Heat 1. Exchange two wet clean test tubes for two dry test tubes. Use the tip of the spatula to place a very small pea-size amount of copper (II) carbonate, CuCO3, into each of the two dry test tubes. There is a sample test tube at the instructor station that shows you how much copper (II) carbonate you should use. Record the color and the appearance of the sample. 2. Gently tap the tubes on the bench top so any sample clinging to the sides falls to the bottom. 3. Heat one sample directly over a Bunsen burner flame for about 5 minutes. Record the color and the appearance of the sample after heating. 4. Allow the test tube to cool for about 10 minutes in a beaker. (Do not put the hot test tube in a plastic test tube rack, or it will melt the plastic!) 5. Add 5-6 drops of 2 M HCl to both samples. Compare the results and record your observations for both samples. B. Silver Nitrate and Copper Metal CAUTION: Silver nitrate stains skin and clothing. Rinse spills immediately. Note: You may use clean wet test tubes for this test. 1. Clean a small piece of copper wire with sandpaper, coil it, and drop it into a small test tube. Add enough 0.1M AgNO3 to completely cover it. Also add anGCC CHM 130LL: Chemical and Physical Changes page 3 of 7 approximately equal volume of the 0.1M AgNO3 to a second test tube. Wait at least 5 minutes and observe the contents of the first test tube carefully. 2. Pour the AgNO3 solution off the piece of copper into a third test tube. Add 3-4 drops of 3M NH4OH to this solution, and also to the solution that was not in contact with the copper metal. Compare the results. C. Sodium Bicarbonate and Hydrochloric Acid. CAUTION: Hydrochloric acid is corrosive and can burn skin and damage clothing. Note: You may use clean wet test tubes for this test. 1. Place about 10 drops of saturated NaHCO3 (sodium bicarbonate) in each of two test tubes. To one, add 2M HCl (hydrochloric acid) dropwise until you no longer see any bubbles