Western Carolina University. Chem 132 Lab 04 Introduction to Physical Changes and Chemical Reactions Introduction

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1 Chem 132 Lab 04 Introduction to Physical Changes and Chemical Reactions Introduction This lab serves as an introduction to physical changes. Physical changes involve a change in the form of matter without a change in chemical structure. Example: melting ice to form water does not change the formula, but remains H 2 O: H 2 O (s) + heat H 2 O (l) Chemical reactions, on the other hand, involve the consumption of one or more chemical substances called reactants to form new chemical substances called products. Example: iron (Fe) rusting to forms iron oxide (Fe 2 O 3 ). Chemical reactions are often accompanied by visible changes such as formation of new colors, evolution of a gas, precipitation of a solid, or release of energy (heat or light). In this laboratory you will have the opportunity to observe a wide variety of physical and chemical changes. We will be working with a number of hazardous chemicals, so wear your safety goggles at all times while working on the lab. The instructor will inform you how to dispose of all chemical waste. Experimental Procedure 1. Heating Stuff. a. Place a small piece of ice in a small test tube. Heat it with a Bunsen burner until no further change occurs. Record your observations on your data sheet. (answer on data sheet) b. Place enough sugar to cover the bottom ½ inch of a small test tube. Heat it with a Bunsen burner until no further change occurs. Record your observations below. (Dispose of the test tube when finished, do not clean.) (answer on data sheet) c. Place enough salt to cover the bottom ½ inch of a small test tube. (Make sure the test tube is clean and dry!) Heat it with a Bunsen burner until no further change occurs. Record your observations below. 1

2 2. Canned Laughter. Answer all questions on your data sheet or in Chem21Labs a. Place about 10 ml of water in an empty soda can. Using a pair of tongs, heat the bottom of the can with a Bunsen burner to boiling, and continue to heat until you see the first evolution of steam. Continue boiling for another minute, then rapidly invert the can and immerse it into a dishpan containing cold tap water. b. Has a physical or chemical change occurred during the heating of the can (and the water)? Answers on Chem21Labs c. What is inside the hot can when you picked it up with the tongs, before putting it in cold water? d. What is inside the can after it was immersed in cold water? e. What is outside the can at all times? f. Using the answers to the previous questions, what do you think caused the changes observed? Answers on Chem21Labs 3. To Dissolve or Not to Dissolve, That is the Question. In this section we will investigate the ability of compounds to dissolve in different liquids, which is called solubility. If a substance dissolves in a liquid to form a uniform, homogeneous mixture, it is said to be soluble in that liquid. An example is table salt dissolving in water. A substance that does not dissolve in a liquid, but forms a heterogeneous mixture, is said to be insoluble in that liquid. An example would be oil and water, which do NOT mix to form a uniform solution. Liquids are composed of tiny particles called molecules, which represent the smallest unit that exists independently and retains its identity as the liquid. For example, a water molecule is composed of two hydrogen atoms and one oxygen atom, and has formula H 2 O. Cyclohexane, a common organic solvent, contains six carbon atoms and twelve hydrogen atoms and has formula C 6 H 12. Liquids may be classified as to whether or not the electrical charge in the molecule is uniformly arranged. A liquid whose molecules have a partial positive end and a partial negative end (non-uniform distribution of electric charge) are said to be polar. Water is an example of a polar liquid. A substance composed of molecules that have uniform distribution of charge is said to be nonpolar. Most organic liquids, like cyclohexane, are nonpolar. 2

3 The basic rule of solubility is summarized in three words: like dissolves like. Substances of similar polarities tend to dissolve in each other. In other words, nonpolar substances dissolve in nonpolar liquids, and polar substances dissolve in polar liquids. Some substances are intermediate in polarity, and may dissolve in both polar and nonpolar liquids. Here we will use two liquids: water and cyclohexane. We will then try to dissolve a number of compounds into these liquids. The compounds to be tested include sodium chloride (NaCl - table salt); naphthalene (C 10 H 8 - active ingredient in moth balls), copper sulfate (CuSO 4 ), urea (CH 4 N 2 O - excreted in urine), iodine (I 2 - which is used as an antiseptic), vegetable oil, vinegar, and isopropanol (C 3 H 8 O - rubbing alcohol). Once you determine solubility, you can then classify each compound as nonpolar, intermediate polarity, or polar. Obtain 8 small test tubes. Fill each ¼ full with water. Add a few grains of solid, or a few drops of liquid individually to one of the test tubes. Mix each test tube thoroughly with a stirring rod, making sure to clean the rod between test tubes. Make observations in the tables on your data sheet. Clean the test tubes thoroughly and repeat the procedure above for cyclohexane. Use acetone to ensure water is not present in the tubes as water will skew the results of cyclohexane solubility. Make sure to dispose the cyclohexane in the waste bottle marked cyclohexane waste, and rinse the test tubes with acetone into the cyclohexane waste bottle. Compound: Question Is it soluble in water? Is it soluble in cyclohexane? sodium chloride (NaCl) naphthalene (C 10 H 8 ) copper sulfate (CuSO 4 ) Record all results on Data Sheet. urea (CH 4 N 2 O) Is the compound nonpolar, intermediate polarity, or polar? Compound: Question iodine (I 2 ) vinegar oil Is it soluble in water? isopropanol (C 3 H 8 O) Is it soluble in cyclohexane? Is the compound nonpolar, intermediate polarity, or polar? 3

4 4. Do You Give Off Light? Light causes molecules to be excited from a lower energy level to a higher energy level. White light consists of all colors mixed together. The color of light that causes the excitation is absorbed by the molecules and all other colors are reflected. For example, a solution that absorbs green light appears red because green was removed from the white light and the red was reflected. Red is said to be the complimentary color of green. In addition to absorbing light, some molecules absorb and then re-emit light in a process called fluorescence. Fluorescence can occur in relatively large, flat molecules with many double bonds. In order to observe fluorescence, a light source that emits ultraviolet (UV) light, like a black light, is used. In most cases, the fluorescence can be observed by the eye as visible light. In this experiment we will investigate the fluorescence properties of a number of household products and chemicals. Obtain an ultraviolet lamp. Warning: ultraviolet light can damage your eyes. Do not look directly at the ultraviolet light. The following materials are available as samples: tonic water, sprite, a cigarette, fluorescein (a dye), Calgon, liquid laundry soap, dish soap, and sunscreen. Place the container containing each sample under the uv lamp. Record your observations on your data sheet. Note: for most samples do NOT open up the jar. Place the sealed container under the light. Shine the light on your hands. Does anything fluoresce? Now open up the jar of liquid laundry soap and apply a very thin layer to the back of one of your hands. View that area of your hand under the uv light and observe. Finally, rub a small amount of sunscreen on top of the laundry soap layer that is on your hand and observe. Sample Does it Fluoresce? Color of Fluorescence Brightness of Fluoresc. relative to H 2 O water light-colored clothing tonic water sprite cigarette fluorescein Calgon dish soap sunscreen your hands laundry soap on back of hand sunscreen on back of hand 4

5 Type answers to the questions below into the Chem21 lab program for this experiment. Laundry products frequently contain fluorescent compounds called optical brighteners. They serve to make clothes appear brighter. Most stains appear to yellow white clothes, while the brighteners emit blue light. How do you suppose the optical brighteners improve the appearance of clothes? (Hint: blue and yellow are complimentary colors.) Do optical brighteners really get your clothes any cleaner? Tonic water contains a compound called quinine which strongly fluoresces. It is also used as a treatment for malaria. Log into Chem21Labs to see the structure of quinine. Does it agree with the description of molecules in the introduction to Part 4 which are strongly fluorescent? How do you think sunscreen serves to prevent sunburn? Additional questions: 1. How can you distinguish physical changes from chemical reactions? 2. List two examples each of physical changes and physical properties that we did not investigate in this laboratory. 5

Experiment 2 - Using Physical Properties to Identify an Unknown Liquid

Experiment 2 - Using Physical Properties to Identify an Unknown Liquid We usually think of chemists as scientists who do things with chemicals. We can picture a chemist's laboratory with rows of bottles