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Physical and Chemical Properties Objectives The main objective of this experiment is to experimentally determine several chemical and physical properties of two unknown compounds and based on the set of properties identify each substance as either ionic or molecular. Another objective is to compare chemical and physical properties of unknown substances with those of known substances in order to identify the unknown substances. Introduction All substances can be characterized by their unique sets of physical and chemical properties. Properties that can be determined without changing the composition of a substance are referred to as physical properties. Characteristics such as melting point, boiling point, density, solubility, color, odor, etc. are physical properties. Properties that describe how a substance changes identity to produce a new substance are chemical properties. Reactions of a substance, either with itself or with another substance are chemical properties. Physical and chemical properties can be used to classify a substance as ionic or molecular. Ionic compounds are those consisting of cations and anions which are bonded to each other by electrostatic attractions. Ionic compounds usually have high melting points, in excess of 300oC, are often soluble in water, conduct electricity when in aqueous solution or molten, and do not burn in a flame. Molecular compounds consist of neutral molecules that are held together in the solid (or liquid) state by attractions between these molecules called intermolecular forces. Intermolecular forces are also electrostatic in nature, but are much weaker than the ionic bonds that hold ionic compounds together. Consequently, molecular compounds usually have low melting points, often less than 300oC, are usually less dense than ionic compounds, and are often less soluble in water than ionic compounds. When molecular compounds do dissolve in water, they do not conduct electricity. There are exceptions to everyone of the “usual” properties associated with ionic and molecular compounds. Consequently, to characterize a substance as ionic or molecular several properties must be determined and the set of properties is used to characterize the substance as ionic or molecular. Physical and chemical properties can sometimes be used to identify a substance. In order for you to do this you will be given a list of substances from which your substance was chosen. You will need to look up some of the properties of these substances in the literature. Resources that you can use include The Handbook of Chemistry and Physics, The Merck Index, or you may do a Google search. You can then compare the properties that you observed in lab with those that you found in the literature to identify your unknown substances. Special Safety Precautions Be careful not to burn yourself with either the Bunsen burner or the melting point apparatus. Do not breathe the vapors of the solids as you attempt to burn them; burn them near or under a snorkel. Protocol Obtain 2 unknowns from your instructor and record the code numbers of your unknowns in your record book. 1. Solubility in Water For each unknown determine the solubility in water as follows: Place 0.25 g of the solid in a medium size test tube. Add 1 mL of water from a 10 mL graduated cylinder and stir the mixture with a stirring rod. If after 2 minutes the solid has not all dissolved, add another milliliter of water and repeat the stirring process. Continue adding water in 1 mL increments until all the solid has dissolved and then record the total volume of water required to dissolve each solid. If the solid does not dissolve after the addition of 20-mL of water, discontinue the addition of water and record the fact that the solid did not dissolve after the addition of 20 mL of water in your record book. Based on this data alone, classify your unknowns as either ionic or molecular. 2. Conductivity Add 1 g of each solid in a 50 mL beaker along with about 20 mL of water. Stir the mixture thoroughly for 1 to 2 minutes and test the conductivity of the solution even though the sample may not completely dissolve. Your instructor will show you how to measure the conductivity. Record whether the solution resulted in the light shining brightly, dimly, or not at all. A bright light indicates that the solid is an electrolyte; no light at all indicates that the substance is a nonelectrolyte. Based on this data alone, indicate whether your substances are ionic or molecular. 3. Melting Point Pour a small amount of each of your unknowns on a piece of weighing paper or onto a watch glass. Tap the open end of a melting point tube into each of your unknowns so that about 2 to 3 mm height of your unknown is in the melting point tube. Make sure that the temperature of the melting point apparatus is below 100oC. Place the tube in the melting point apparatus and turn on the heating dial to about 5. Look through the magnifying glass and record the temperature at which the solid just begins to melt and the temperature at which the sample has completely melted. This is known as the melting point range. If the unknown does not melt by 300oC, discontinue heating the sample and record the melting point as simply being greater than 300oC. Make sure that you turn off the melting point apparatus. Some samples, especially molecular compounds, decompose before they melt. This is observed by the sample becoming charred. If the sample begins to char, record the temperature at which charring begins and discontinue heating. Based on this data alone, indicate whether your substances are ionic or molecular. 4. Color and Odor Record the color and odor of each of your unknowns. This is one more piece of data that along with several others may help you in identifying the unknown. Color or odor alone will not indicate whether a substance is ionic or molecular. 5. Oxidation with Oxygen (Combustion) Place a small amount, about the size of half a pea, of each unknown on the end of a spatula and place this in the flame of a Bunsen burner that is near or under a snorkel. Be careful not to melt the snorkel hood. Record whether or not the unknown burns. Based on this data alone, indicate whether your substances are ionic or molecular. Physical and Chemical Properties Name _________________________________________ Section __________ Date ________ A. Data and Results 1. Solubility Unknown Number _______ Minimum Amount of Water Needed to dissolve 0.25 g _____________ Prediction: Ionic or Molecular: _____________ Unknown Number _______ Minimum Amount of Water Needed to dissolve 0.25 g _____________ Prediction: Ionic or Molecular: _____________ 2. Conductivity Unknown Number _______ Conductivity(Bright, dim, no light) _____________ Prediction: Ionic or Molecular: _____________ Unknown Number _______ Conductivity(Bright, dim, no light) _____________ Prediction: Ionic or Molecular: _____________ 3. Melting Point Unknown Number _______ Melting Point Range ________________ Prediction: Ionic or Molecular: ________________ Unknown Number _______ Melting Point Range ________________ Prediction: Ionic or Molecular: ________________ 4. Color and Odor Unknown Number ______ Color ________________Odor ________________ Unknown Number ______ Color ________________Odor ________________ Physical and Chemical Properties Name _________________________________________ Section __________ Date ________ 5. Combustion Unknown Number ______ Burn with oxygen? (yes or no) ______ Prediction: Ionic or Molecular: _____________ Unknown Number ______ Burn with oxygen? (yes or no) ______ Prediction: Ionic or Molecular: _____________ 6. List of Possible Unknowns m.p. Solubility Color Reference* Ionic Compounds Potassium chloride _____ ________ _____ ________________________ ________________________ Calcium chloride, anhydrous _____ ________ _____ ________________________ ________________________ Copper(II) chloride dihydrate _____ ________ _____ ________________________ ________________________ Sodium sulfate _____ ________ _____ ________________________ ________________________ Molecular Compounds Adipic acid _____ ________ _____ ________________________ ________________________ Sucrose _____ ________ _____ ________________________ ________________________ Acetylsalicylic acid (aspirin) _____ ________ _____ ________________________ ________________________ Vanillin _____ ________ _____ ________________________ ________________________ * References must have the title, author or editor, publisher, page(s), year of publication. If the reference is an internet site, the complete URL to the specific webpage that has the information you provided must be given. Physical and Chemical Properties Name _________________________________________ Section __________ Date ________ Post-laboratory Questions 1. Discuss in detail how you would classify each unknown based on the overall set of data for each unknown. a) Unknown Number ______ b) Unknown Number______ 2. Based on the literature values for some physical and chemical properties of the unknown possibilities, discuss how you determined the identity each unknown. a) Unknown Number_______ Compound Name (from list) ________________ b) Unknown Number_______ Compound Name (from list) ________________ |
Determine whether each property is a physical property or a chemical property.
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