PHYSICAL AND CHEMICAL CHANGES
INTRODUCTION: Matter undergoes many changes. In some cases only the temperature, physical state, or size of a particle changed. Ice melts and water evaporates, Such changes are physical.
In other cases different substances with new characteristic properties are formed. Wood burns and metals tarnish. Such changes are chemical. A chemical reaction is indicated by any of the following observations:
· Change of color
· Production of light
· Change in temperature
· Evolution of a gas
· Formation of a solid precipitate from a liquid or solution
MATERIALS: Alka-seltzer tablets; baking powder; copper foil; magnesium ribbon; platinum wire test rod; sandpaper, fine grit or steel wool; zinc mossy; filter paper; hydrochloric acid, dilute (1:4); solution of silver nitrate (0.2 M); wooden splints.
Lab Notebook: For each procedure put a heading in your notebook for that procedure and record your observations, and answer any questions IN COMPLETE SENTENCES in that section. You should have 7 sections for this experiment.
SAFETY: WEAR SAFETY GOGGLES
PROCEDURE:
I. Platinum wire in Bunsen burner:
Examine the platinum wire test rod. Observe the color and luster of the metal. Record your observations. Hold the wire in the flame of your burner for about two minutes.
Describe the appearance of the wire while held in the hottest part of the flame. Allow the wire to cool and re-examine it.
1. Did a chemical change occur?
II. Magnesium in Bunsen burner:
Sandpaper or steel wool, a piece of magnesium ribbon about 3 cm in length to remove the tarnish. Note the color, luster: and flexibility of the metal. Record your observations. Holding one end with the forceps, ignite the other end in the burner flame. CAUTION: Do not look directly at the magnesium while it is burning. Compare the ash, which may be collected on the ceramic pad, with the original metal.
1. Is a new substance formed as magnesium burns?
2. Does this represent a chemical or physical change?
3. What do you think reacted or combined with the magnesium?
Disposal: Throw ash and any remaining magnesium in the trash.
III. Copper foil in Bunsen burner:
Clean a piece of copper foil and heat it in the outer cone of the burner flame for 1-2 minutes. Avoid melting. Let it cool and re-examine it. See if you can scrape off some of the black scale from the surface of the copper. Compare the properties of this scale with those of metallic copper. Heat the foil a second time.
1. How does the scale that forms when copper is heated differ from the pure copper?
2. Is this a chemical or physical change?
3. What do you think would be the ultimate result of successive heating and scrapings of the copper?
Disposal: Return copper to the waste container in the front of the room.
IV. Copper (II) carbonate with heat:
Place a small amount of copper (II) carbonate in the bottom of a large test tube. Holding the test tube with the test tube clamp heat the tube in a Bunsen burner flame for about one minute. Place a burning splint in the test tube and record all of your observations.
1. Did a chemical or physical change take place?
Disposal: Place the remaining material from the test tube in a waste beaker at the front of the room.
V. Zinc in hydrochloric acid:
Add a small piece of mossy zinc to a test tube containing a small amount of hydrochloric acid. Record your observations. Let the action continue for about 5 minutes. When the reaction is proceeding vigorously, bring the flame of a burning splint to the mouth of the test tube. The dilute hydrochloric acid is a water solution of HCl.
1. Are there any chemical changes?
2. Based on your observations what gas do you think is being evolved during the reaction?
Disposal: Pour any acid down the sink. Flush with water. DO NOT ALLOW THE ZINC TO GO DOWN THE DRAIN! THROW ANY ZINC IN THE TRASH.
VI. Silver nitrate and hydrochloric acid:
Put a small amount of silver nitrate solution in a small test tube then add a few drops of dilute hydrochloric acid. Record your observations.
1. Is there a chemical change or physical change?
2. What forms when the two substances are mixed?
3. Was there any evidence of a gaseous product of this reaction?
Disposal: Pour the contents down the sink and rinse with water. Make sure the test tube is clean – use a test tube brush if necessary.
VII. Alka-seltzer in water:
Place a small amount of water in a small test tube and then add ¼ of an Alka-Seltzer tablet. After the reaction proceeds for 10 seconds, thrust a burning splint into the upper portion of the test tube. Record your observations.
1. Was there a chemical or physical change?
2. What gas do you think was evolved when the alka-seltzer was mixed with water? Why?
Disposal: Pour the contents down the sink and rinse with water.