EXPERIMENT 3

[change.lab]

OBJECTIVES:

1. To learn to identify a change as being chemical or physical.

2. To see how some common chemicals behave when they are combined with each other.

3. To become familiar with the chemical names and formulas of some common substances.

DISCUSSION:

We will make several combinations of substances. Some will result in physical changes and the others in chemical changes. If a chemical change occurs, we call it a CHEMICAL REACTION. Physical changes are not reactions, since the substances remain the same as before.

>>> Chemical reactions are identified by ONE or MORE of the following changes:

1. heat change (hotter = exothermic; colder = endothermic)

[Endothermic changes are usually physical changes.]

2. color change

3. gas products (bubbles and/or odor)

4. precipitates form (insoluble substances)

In the procedures following, we will NOT measure the substances. The directions say to use "some" or "a small amount". In the case of water or solutions, about 5 mL is enough. Use your judgement. After each combination, decide whether it is a Physical Change or a Chemical Change.

NOTE: You do not need to know the chemical formulas at this time, but you should start to learn them. They are listed for showing you how chemists will refer to these substances. Get used to using these formulas. Some of the bottles have only the formulas on them.

EXPERIMENTAL PROCEDURE:

1. Magnesium [Mg] and vinegar [acetic acid, HC₂H₃O₂]

Place a small amount of vinegar in a test tube. Feel the temperature of the tube. Drop in a few small pieces of Mg. Feel the temperature again. {Do this temperature comparison for every test.}

2. Magnesium and dilute sulfuric acid [ H₂SO₄ ]

Place a small amount of sulfuric acid in a test tube. This is a stronger acid than the vinegar. Drop in a few small pieces of Mg.

3. Baking soda [sodium bicarbonate, NaHCO₃] and vinegar.

Put a small amount of baking soda in a test tube. Add a few milliliters of vinegar in another test tube. Pour the vinegar from the second test tube into the first tube.

4. Copper sulfate solution [CuSO₄] and water [H₂O or HOH]

Place some copper sulfate solution in a test tube. Add an equal amount of water.

5. Ammonium nitrate [NH₄NO₃] and water.

Place some solid ammonium nitrate in a test tube and add some water. Shake well.

6. Calcium hydroxide solution [Ca(OH)₂] and magnesium chloride solution [MgCl₂]

Place some calcium hydroxide solution in one test tube and an equal amount of magnesium chloride in another test tube. Mix them.

7. Calcium hydroxide solution and phenolphthalein solution [PP].

Put a SMALL amount of calcium hydroxide solution in a test tube. Add 2 or 3 drops of PP. Record the result. SAVE for #8.

8. Vinegar and #7

Pour some vinegar in a test tube. Use about twice as much as you used for the calcium hydroxide in Part 7. Add this to the PP and calcium hydroxide saved from Part 7.

9. Iron (III) nitrate [Fe(NO₃)₃] and potassium thiocyanate [KSCN].

10. Barium chloride [BaCl₂] and sodium sulfate [Na₂SO₄].

Put some sodium sulfate solution in a test tube. Add a little barium chloride solution. Mix and let stand 5 minutes.

11. Iron (III) nitrate and potassium ferrocyanide [K₄Fe(CN)₆).

Pour some potassium ferrocyanide solution in a test tube. Add 1 or 2 drops of iron nitrate solution. If the mixture is too dark to see the color, add water to dilute it.

(Tests 9, 10 and 11 are our salute to the 200th anniversary of the United States Constitution.)

EXP #3 - LAB REPORT Name ____________________ Sec ____

A: Feel the test tube as you add the second reagent. Heat change may be EXOTHERMIC or ENDOTHERMIC. Write EXO, ENDO or NONE in this column. Endothermic changes are NOT usually chemical changes.

B: If there is a color change, name the final colors.

For example: green.

C: Gas products may be very small bubbles or a foaming action. Don't confuse gas bubbles with a solid. At this time, we will not identify the gases. Common gases formed are H₂ (hydrogen) and CO₂ (carbon dioxide). Answer YES or NO.

D: Solid products are called PRECIPITATES (ppts). If you start with a solid reactant and some of it remains afterwards, it is NOT a product. A precipitate must be something NEW. Give the color of any precipitate.

E: If a change occurs in any of columns A-D, then the change was a CHEMICAL CHANGE. An exception is the endothermic heat change, which we will take as a physical change. Look (and feel) carefully. Some changes are very minor and difficult to detect. Enter P or C.