Experiment 2. Physical and Chemical Changes

Objectives

Learning Objective 1: To observe physical and chemical changes.

Learning Objective 2. To identify physical and chemical changes.

Introduction

A physical change is one in which the appearance of a substance changes but its composition or identity is unaffected. No new substances 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 table salt in water. In most cases, one or two simple processes are all that is needed to reverse the physical change. For example, salt that has been dissolved in water may be recovered water by evaporating the water from the solution.

A chemical change (Chemical reaction) is one that results in a change in the composition and identity of a substance. New substances are formed. Chemical changes are usually irreversible. Examples of chemical reactions include 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, reversing a chemical reaction requires one or more chemical reactions.

In a chemical reaction, one or more substances called reactants are necessary. The newly formed substance(s) is(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 precipitate.

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.

SAFETY INFORMATION:

1. NO EATING or DRINKING IS ALLOWED IN CHEMISTRY LABORATORIES.

2. OBTAIN and WEAR EYE GOGGLES AT ALL TIMES.

3. NOTIFY YOUR INSTRUCTOR IF YOU HAVE ANY QUESTIONS.

4. WASH YOUR HANDS BEFORE LEAVING THE LABORATORY.

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 1 mL (or approximately 20 drops), or 1 cc, of the reagent. 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 large test tube holds about 20 mL. Therefore, liquid quantities can be estimated as follows:

Quantity of Liquid (Approximate height in a large test, 20-mL, tube): 10 mL is about 1/2 full

5 mL is about 1/4 full

2 mL is about ½ inch

1 mL is about 20 drops

Whenever laboratory experiments are qualitative rather than quantitative in nature, volumes of liquid can be estimated in this manner.

For all demonstrations and tests, you must record in your data sheet the appearance of the substances before the change and what occurred during and after the change(s).

Procedure C. Three Pennies Experiment NOTE:

Very clean pennies are required.

Procedure D. Sublimation of Iodine crystals NOTE:

Hot plate gives satisfactory results.

STUDENT EXPRIMENTS:

NOTE: Tests 1, 6, and 10 can be run effectively in a spot plate.

MATERIALS: Steel wool, well plates, small test tubes, calcium carbonate, baking soda, wooden splints, magnesium ribbon, potassium chlorate, 0.1 M NaCl, 0.1 M AgNO3, CaCl2 NH4NO3, universal indicator, 0.5 M CuCl2, Dry ice, 6 M NaOH, zinc powder.

PRELAB QUESTIONSNAME: Matthew Ramirez

1. Identify the three Physical States of matter.

2. Identify four changes which are indicative of a chemical change.

3. Provide the two examples of chemical changes discussed in the experimental handout.

4. Fill in the blanks.

a. 1 mL is about

b. 1cc is about

c. ½ full test tube is about

5. Circle the piece of equipment which can be used to more precisely determine the volume.

DropperTest Tube