EXPERIMENT 6 Physical and Chemical Changes Part 2 INTRODUCTION Evidence of chemical change can be the evolution of heat or light, the formation of a gas (seen in Experiment 5), the appearance of a material with a new color (seen in Experiment 5), or the forming of a solid when two solutions are mixed. A solid formed from the mixing of two solutions is a precipitate. PROCEDURE 1. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. PART A ALKA-SELTZER WITH WATER 2. Obtain one-half of an Alka-Seltzer tablet from the cart, and a wood splint from the back counter of the lab room, or from drawer 035. Place the Alka-Seltzer tablet in a 100-mL beaker and record any observations of the tablet in your Data Table. Measure out 5 ml of water in a 10-mL graduated cylinder. Ignite your burner and light the end of the wood splint. Pour the 5 ml of water into the beaker containing the Alka-Seltzer tablet. After 5-10 seconds, thrust the burning splint into the upper portion of the beaker (but not into the liquid!). Observe carefully for any signs of chemical action, and any observations concerning the gaseous product and its interaction with the burning splint. Record any observations in your Data Table. Alka-Seltzer contains a solid substance, which acts as an acid, and sodium hydrogen carbonate. The remaining solution can be washed down the sink. PART B SLIVER NITRATE SOLUTION WITH HYDROCHLORIC ACID 3. Place 3 ml of silver nitrate solution in a medium test tube. Observe the silver nitrate solution and record any observations. Add 10 drops of dilute hydrochloric acid to the test tube containing the silver nitrate solution. Record any observations in your Data Table. 4. Prepare a filter set-up as shown on the right. Transfer the entire contents of the test tube to the filter set-up, and collect all of the precipitate in the filter paper. If necessary, use a deionized water wash bottle to flush all of the precipitate out of the test tube. Observe the precipitate and record any observations in your Data Table. 45
5. Discard the filtrate in the bottle for Liquid Waste in Fume Hood A. Unfold the filter paper and place it on a piece of paper toweling. Take it outside and expose it to direct sunlight for several minutes, or place it in a UV box for several minutes. Observe the precipitate again and record any observations in your Data Table. Dispose of the filter paper in the bag for Solid Waste in Fume Hood A. PART C HEATING IODINE CRYSTALS 6. Place 4 small crystals of iodine from Fume Hood B in a clean, dry 250-mL beaker. Cover the beaker with an evaporating dish, and put ice in the dish. CAUTION: Iodine burns and stains. Do not allow it to come in contact with your skin or clothes. Observe the iodine crystals and record any observations in your Data Table. 7. Position the snorkel hood so that it is about 18 inches above the lab bench. Do so by loosening the three knobs and then adjusting the snorkel hood until it is in the position you want. Open the flow valve by turning it until it is parallel with the tubing, indicating that the vacuum is on. 8. Attach an iron ring to a ring stand. Place a ceramiccentered wire gauze on the iron ring and support the beaker with the evaporating dish on a ceramic-centered wire gauze. Place a burner below the beaker so that when the burner is lit, the tip of the inner cone just reaches the bottom of the ceramic-centered wire gauze. Slide the apparatus so that the beaker and evaporating dish are under the snorkel hood, as shown below. 9. Light the burner and gently heat the beaker until a substance collects on the bottom of the evaporating dish. Observe the substance on the bottom of the evaporating dish and record any observations in your Data Table. Record any observations of the substance on the bottom of the evaporating dish. 10. When you are done, use a clean 100-mL beaker to obtain one pump of saturated sodium thiosulfate solution from the pump bottle on the lab bench in front of the instructor s desk. Hold the beaker under the spout of the pump bottle, pull the top of the pump up as high as it will go, and gently push it down as far as it will go. Pour the saturated sodium thiosulfate solution into the iodine beaker and swirl it around to neutralize any remaining iodine. Then, while carefully holding the evaporating over the sink, pour the sodium thiosulfate solution from the beaker onto the evaporating dish to neutralize the crystals adhering to the bottom of the evaporating dish. When all solids have been neutralized, rinse the beaker and evaporating dish in the sink with water. 11. Clean and wipe dry your laboratory work area and all apparatus. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. 46
EXPERIMENT 6 LAB REPORT Name: Student Lab Score: Date: Lab Station Number: DATA TABLE PART A Observations Alka-Seltzer Dry Alka-Seltzer Added to Water Gaseous Product Appearance Gaseous Product Action with Burning Splint PART B Observations Silver Nitrate Before Adding Hydrochloric Acid Silver Nitrate After Adding Hydrochloric Acid Precipitate Before Sunlight Precipitate After Sunlight 47
PART C Observations Iodine Crystals In Beaker Crystals On Evaporating Dish 48
POSTLAB QUESTIONS 1. What change occurs when an Alka-Seltzer tablet was placed in water, chemical or physical? 2. What is the evidence for your answer to question 1? 3. What is the name of the gas that is evolved in the reaction between the acid and the sodium hydrogen carbonate in the Alka-Seltzer tablet? 4. What change occurs when silver nitrate was mixed with hydrochloric acid, chemical or physical? 5. What is the evidence for your answer to question 4? 6. What change occurs when your precipitate was exposed to sunlight, chemical or physical? 7. What is the evidence for your answer to question 6? 8. By referring the Solubility Table on page 4, determine the name of the insoluble product (the precipitate) produced when solutions of silver nitrate and hydrogen chloride (hydrochloric acid) are mixed. 9. What was an important use for this light-sensitive precipitate in the 20 th century? 10. What change occurs when you heated iodine crystals, chemical or physical? 11. What is the evidence for your answer to question 10? 49
12. Classify each of the following as an example of a compound (C), mixture (M), or element (E). (a) water, H 2 O (b) air, N 2 & O 2 & Ar & CO 2 (c) sulfur, S (d) salt, NaCl 13. State whether the following properties are typical of a metal (M) or nonmetal (N). (a) good heat conductor (b) low melting point (c) dull/brittle solid (d) ductile 14. Indicate whether these observations are most likely evidence for physical change (P) or chemical change (C). (a) iron and sulfur powder mix together (b) iron and sulfur powder change color and texture upon heating together (c) mixiung lead (II) nitrate and hydrochloric acid solutions form a precipitate (d) silver metal deposits onto a copper wire in a silver nitrate solution (e) mothballs gradually disappear at room temperature 50