CHM 130 Physical and Chemical Changes

Transcription

1 Objectives CHM 130 and Changes 1. List physical and chemical properties of substances and explain how they are used in identification and separation of substances. 2. Become familiar with the concepts of solubility and miscibility and gain experience with both. 3. Outline the distinguishing characteristics of physical and chemical changes. Experimental A. Characterization of Substances by Observations of their and Properties 1. Look at the samples of the substances water, iodine, table salt, table sugar, ethanol, wax, copper (II) sulfate pentahydrate and mineral oil listed in Data Table A. Fill in the data table with color, form (liquid, crystal, powder, etc.) and any other properties you can observe. 2. Open the bottles and smell each substance carefully, as shown in the safety video). Which substances might be identified by their odors? Record your observation on the data table. 3. Behavior upon heating. It is important to note whether a substance melts, burns (combusts), or decomposes upon heating. Investigate this by using a small piece of aluminum foil (about 1 in by 1 in square) with a shallow depression in the center to hold a drop of liquid, or a few crystals of solid. Do not perform this experiment with iodine -- it reacts with the aluminum foil. You will observe its behavior upon heating in part C of the experiment. Place the foil on the lab bench and bring a lighted match close to the substance. Hold the match near the surface of the substance and observe what happens. Some changes occur slowly, and there may be more than one change per substance. Record all observations in your data table. B: Solubility and Miscibility. Solubility in water is another distinguishing characteristic of substances. Check the solubility of each solid substance (except I 2 ) by placing a few crystals in separate test tubes filing half full with DI water. Stir the mixture, and note the solubility at room temperature. Classify solubilities as low (or none), moderate, or high. Two liquids are said to be miscible if they mix in all proportions. If two liquids are immiscible, a distinct layer will form between them. In 3 separate test tubes, make mixtures of water/ethanol, water/mineral oil, and ethanol/mineral oil. Add about 1 to 2 ml of each liquid (note: 20 drops is about 1 ml) and mix the contents by agitating the test tube with your finger. Use different amounts of each liquid so you can tell which liquid settles to the bottom if the liquids are immiscible. Record your observations. Dispose of mixtures in bottles provided. 1

2 C. and Changes 1. Dissolve a small amount of baking soda (NaHCO 3 ) in 2 or 3 ml of H 2 O. Repeat using 1 M HCl (hydrochloric acid) instead of H 2 O. Compare the changes. 2. Prepare solutions of salt and sugar by adding one spatula full of the solid to about 5 ml of deionized H 2 O and stirring. Add a few drops of 0.1 M AgNO 3 to each. Be careful: silver nitrate stains hands. Also add a few drops of the AgNO 3 solution to tap H 2 O and to deionized H 2 O. Compare your observations. 3. Light a Bunsen burner and hold a copper wire with tongs in the hottest part of the flame for a few minutes until a change is observed. Remove and cool the wire. Scrape a section of the wire that was in the flame, and compare the interior appearance with that of the outer surface. Report your observations. 4. IN THE HOOD, your instructor will heat some solid I 2 (iodine) in a test tube placed in a boiling water bath. The change you should observe is called sublimation. Observe closely and record your observations when the test tube is removed from the boiling water. 5. Dispose of all chemicals in bottles provided. Matches should be soaked in water and then disposed of in the paper trash. 2

3 Name _Answer Key Section Lab Partner CHM 130LL Experiment #1 - and Changes A. Characterization of substances by observations of their physical and chemical properties Substance (Formula) Color Form Odor Heating Water (H 2 O) Colorless Liquid None Puts out match (may see some steam) Table Salt (NaCl) White Crystalli ne solid Ocean Not much may see some discoloration Table Sugar (C 12 H 22 O 11 ) White Crystalli ne solid None Turns brown or black smells like roasting marshmallows Ethanol (C 2 H 5 OH) Colorless Liquid Alcohol Burns vigorously flame can be orange or blue Wax (CH 2 CH 2 CH 2 CH 2 ) White Flakes of solid Candle Melts might burn Copper (II) sulfate (CuSO 4 5H 2 O) Blue Crystalli ne solid Dusty Green flame crystals turn brown or white Mineral oil (a hydrocarbon mixture) Colorless Thick liquid None Either puts out match or burns using the match as a wick A1. Which of the above might be easily confused if you tried to distinguish them by appearance alone? Record your answers as "water and sugar," or "salt, water, and iodine. (Note these are not correct answers.) Water ethanol mineral oil (all colorless liquids) Table salt table sugar (both white crystalline solids) A2. If you were given an "unknown" colorless liquid which was one of the materials you tested, what specific steps would you take to identify it using the techniques above? Odor only ethanol has an odor, Thickness only the mineral oil is a thick liquid, Heating the ethanol burns quickly with an orange or blue flame and the mineral oil burns slowly using the match as a wick A3. How would you identify a white solid unknown which was one of the materials you tested? Heating salt will do nothing, sugar will turn brown and smell, wax will melt 3

4 B. Solubility and Miscibility B1. Solubility (if students use too large a sample, not everything will dissolve and it can show moderate solubility) Observation Solubility (high, moderate, or none) Table Salt in H 2 O: With enough stirring all solid dissolves High Table Sugar in H 2 O: Wax in H 2 O: CuSO 4 5H 2 O in H 2 O: With enough stirring all solid dissolves Wax floats on surface no matter what With enough stirring all solid dissolves High Insoluble high Question B1a: In your 10 ml graduated cylinder, add about 2 ml of table salt and then add deionized water until the liquid level reaches about 4 ml. Shake vigorously. Does all the table salt dissolve? Based on your result here and your result in Table B1, can you see that solubility depend on the relative amount of solvent (water in our experiments) and solute (what we are adding to water)? Less soluble if there is less solvent, more soluble if there is more solvent B2. Miscibility (should see ethanol on top of oil, but if it not mixed enough the oil will stay on the top if poured into the tube 2 nd ) Miscible or Immiscible More dense liquid (if immiscible) Water and Ethanol Miscible Can t tell Water and Mineral Oil Ethanol and Mineral Oil Immiscible Immiscible Water is more dense Mineral Oil is more dense Question B2a: After you have mixed water and ethanol and shaken the test tube, is it possible to tell which liquid is more dense? Why or why not? No it is not possible to tell which is more dense. Water and ethanol are miscible so they form a single new solution Question B2b: If you have done your experiment carefully, you should be able to rank the 3 liquids in order of increasing density. Show your ranking and explain your reasoning. Ethanol < Mineral Oil < Water, based on the observations, water is more dense than mineral oil and mineral oil is more dense than ethanol 4

5 C. and Changes C1. Dissolving NaHCO 3 : NaHCO 3 in H 2 O: NaHCO 3 in HCl: Observation Solid dissovles in the water Formation of bubbles Type of Change C2. Testing with AgNO 3 : Salt solution: Sugar solution: Tap H 2 O: Deionized H 2 O: Observation cloudy solution forms with AgNO 3 formation of a solid Solution might look slightly white due to dissolving AgNO 3 cloudy solution forms with AgNO 3 formation of a solid No change clear Type of Change C3. Heating Cu wire: Inner Appearance: Outer Appearance: Observations Still looks like shiny copper Black coating Type of Change No Change Observation on heating I 2 : Observation on cooling I 2 : C4. Heating and Cooling I 2: Observations Type of Change Silvery solid sublimates to purple gas Purple gas deposits as a silvery solid Use the observed behavior of Cu and I 2 to comment on the reversibility of chemical vs. physical changes. Changes are reversible they can undone, Changes are not reversible permanent change 5

6 D. Extra Questions 1. List 3 chemical properties and 3 physical properties of substances you observed in this experiment. properties: Reaction upon heating, with acid (HCl), with other compounds (AgNO 3 ) properties: Color, odor, solubility, miscibility give specific examples 2. Classify the following as physical or chemical changes: Freezing a saltwater solution - physical Dissolving sugar in your coffee - physical The tarnishing of silverware - chemical 3. Carefully define and distinguish between melting and burning (combustion). Give an example of each from this experiment. Melting a physical change from the solid phase to the liquid phase, the compound does not change, melting wax from Part A Burning a chemical change that results in the creation of a new compound, heating sugar, ethanol or mineral oil from Part A 4. Is the behavior of Cu in the flame most closely related to melting or burning? Justify your answer. Cu in the flame is closer to burning a new compound, the black coating on the wire, was created rather than a phase change to liquid copper 6