Physical and Chemical Changes

Transcription

1 Exercise 3 Page 1 Illinois Central College CHEMISTRY 130 Laboratory Section Name Physical and Chemical Changes Objectives The purpose of this experiment is to observe carefully several changes in matter and to determine if they are physical changes or chemical reactions. Background A physical change is one in which the appearance of a substance changes but its composition or identity is unaffected. No new compounds are formed. Physical changes are usually reversible. Examples of physical changes include the boiling of water to produce steam, the filing of a metal to produce dust or filings, and the dissolving of sugar in water to form sugar syrup. In most cases one or two simple processes are all that is needed to reverse the physical change. For example, sugar may be isolated from sugar syrup merely by evaporating the water. A chemical change is one that results in a change in the composition and identity of a substance. New compounds are formed. Chemical changes are usually irreversible. Some chemical reactions are the burning of wood to form carbon dioxide and water, the rusting of iron to form iron oxide, and the heating of limestone to produce lime and carbon dioxide. Usually the reversing of a chemical reaction requires an involved process of many steps. For example, wood is made from carbon dioxide and water by green plants using photosynthesis--a complex series of reactions that science has not yet duplicated. In a chemical reaction, one or more substances called reactants are necessary. The substance or substances formed are called the product(s) and these product(s) always differ from the reactants in their physical and chemical properties. Physical properties are properties that do not involve the reaction of a substance with other substances. They include such things as color, state (solid, liquid, or gas), density, and melting and boiling points. Chemical properties are displayed when the substance reacts to produce products or undergoes a chemical change. A chemical reaction is indicated by any of the following observations. 1. Change of color. 2. Production of heat, light, or sound. 3. Evolution of gas. 4. Formation of a solid where none was present before.

2 Exercise 3 Page 2 These observations are not infallible indications that a chemical reaction has occurred. For example, when ice forms from liquid water, a solid is present where none was before, but a physical change rather than a chemical reaction has occurred. In this case, one would classify the change as physical based on the ease of reversing it. Special Note: Quantities need not be measured precisely in this experiment. If reagents are dispensed from dropper bottles, one squirt from the dropper should deliver about l ml, or l cc, of the reagent. (Or, if you prefer, 1 ml is approximately 20 drops) For reagents dispensed from bottles with stoppers or screw caps, simply estimate the amount of liquid as you pour it into a test tube. An ordinary test tube holds about ml. Therefore, liquid quantities can be estimated as follows: Quantity of Liquid Approximate Height in Test Tube 10 ml about 1/3 full 5 ml about l inch 2 ml about l/2 inch 1 ml about 20 drops Whenever laboratory experiments are qualitative rather than quantitative in nature, volumes of liquid can be estimated in this manner.

3 Exercise 3 Page 3 Procedure Observe what changes occur in each of the following procedures and classify the change as a physical change or a chemical reaction. Observe means: 1. Look carefully at the reactants, the products, and the change. 2. Listen to the change as it occurs. 3. Cautiously smell the reactants and the products. (Smell by holding the reaction vessel near your nose after the change is complete and fanning the vapors toward your nose with your hand. Never breathe the vapors directly above the vessel.) Never taste any chemical! A. Changes Caused by Heat Place a little of the substance indicated in a test tube and heat it until a change occurs. 1. NH 4 Cl 2. CuSO 4. 5H iodine (demonstrated by the instructor) 4. urea 5. CoS0 4. 7H (NH 4 Cr (demonstrated by the instructor) Hold a small piece of the substance indicated with a pair of tongs, and hold it directly in the flame of a Bunsen burner. 7. zinc (demonstrated by the instructor) 8. magnesium B. Changes Caused by Liquids and Solutions 1. Add a small amount of solid CuSO 4. 5H 2 O to a few ml of water. Shake the tube to get complete mixing. 2. To the solution from part 1 add 1 ml of dilute NH 4 0H. 3. Add a small crystal of NiCl 2. 6H 2 0 to a few ml of water. 4. To 1 ml of calcium chloride solution add 1 ml of NaHCO 3 solution. 5. To 1 ml of lead nitrate solution add 1 ml of K 2 CrO To 1 ml of NaHCO 3 solution add 1 ml of dilute hydrochloric acid (HC1). 7. To 1 ml of NaOH solution add 1 ml of dilute HC1. (Be sure to feel the outside of the tube.) 8. To 1 ml of Hg(NO 3 solution add 1 ml of KI solution.

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5 Exercise 3 Page 5 Illinois Central College CHEMISTRY 130 Laboratory Section REPORT SHEET Physical and Chemical Changes Name For each indicated part in this experiment, describe what you observed. Then identify what happened as a physical change or a chemical reaction. A. Changes Caused by Heat 1. NH 4 Cl Observations and description phys/chem 2. CuSO 4. 5H 2 O 3. iodine 4. urea 5. CoS0 4. 7H 2 O 6. (NH 4 Cr 2 O 7 7. zinc 8.magnesium

6 Exercise 3 Page 6 B. Changes caused by Liquids and Solutions 1. CuSO 4. 5H 2 O + H 2 O observations and descriptions phys/chem 2. CuSO 4 +H 2 O+NH 4 OH 3. NiCl 2. 6H 2 O + H 2 O 4. CaCl 2 + NaHCO 3 5. Pb(NO 3 + K 2 CrO 4 6. NaHCO 3 + HCl 7. NaOH + HCl 8. Hg(NO 3 + KI

7 Exercise 3 Page 7 Illinois Central College CHEMISTRY 130 Laboratory Section Name PRELAB: Exp.3 Physical and Chemical Changes 1. What is a physical change? Give several examples. 2. What kinds of observations indicate that a chemical reaction has occurred? 3. What is the safe procedure to use when smelling a substance? 4. Refer to the examples in your text and write the correct formula for the following pairs of ions. a) Cr +3 and S -2 b) Al +3 and Nitrate ion c) Mg +2 and Phosphide ion d) Fe +2 and Nitride ion e) Ca +2 and Phosphate ion

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