16 Physical And Chemical Changes Lab

Unit I: Introduction to Chemistry Physical and Chemical Changes Lab Learning Target(s): 6 Introduction As you have learned, chemistry is the study of matter and the changes that it undergoes. These changes can be classified as either physical or chemical. When a physical change occurs, the physical properties of a substance – such as its size, shape, density, or state – are altered, but its chemical composition remains the same. Examples of physical changes include melting ice, crushing gravel, tearing paper, grinding pepper, and boiling water. No new substances are formed as a result of these changes. Chemical changes, also known as chemical reactions, result in the formation of one or more new substances with different chemical properties and compositions from the original material. Examples of chemical changes include plants dying, leaves changing color, paper burning, bananas ripening, bread baking, or iron rusting. Some signs of chemical changes include a change in color, the formation of a precipitate (a new solid substance that settles out of solution), the production and release of gas, or a change in temperature. It is important that you distinguish between pure substances and mixtures as you observe physical and chemical changes in matter. Remember that pure substances – such as elements or compounds – are made up of one type of matter. Mixtures are two or more pure substances that are combined physically. Mixtures can be separated into their components by physical means, such as evaporation, filtration, or distillation. In this investigation, you will conduct tests on several substances and then use your data to determine whether the resulting changes are chemical or physical. As you observe each change, remember to ask yourself, “Has the change altered the identity of the substances?”

Pre-Lab Discussion Read the entire laboratory investigation and the relevant pages of your textbook. Then answer the questions that follow.

1. Identify the following as either a chemical or a physical change:

a. burning wood ______

b. dry ice (solid) changing to a gas ______

c. freezing water ______

d. ripening fruit ______

e. sugar dissolving in water ______2. What are some observable signs that indicate a chemical change is taking place?

3. Why are you instructed to feel the outside of the test tube after two chemicals are mixed, as in Steps 5 and 6 of this investigation?

4. What are the safety cautions that you need to observe during this investigation?

Problem How can you recognize and differentiate between physical and chemical changes in matter?

Materials chemical splash goggles sodium choride (NaCl) laboratory apron glass stirring rod latex gloves micropipette with silver nitrate (AgNO3), 0.1 M candle magnesium ribbon (Mg) 5 test tubes scissors test-tube tongs micropipette with hydrochloric acid (HCl), 6.0 M matches copper sulfate pentahydrate (CuSO4*5H2O) glass square mortar and pestle piece of paper, 2 cm X 2 cm laboratory balance watch glass iron filings (Fe) insulating pad powdered sulfur (S) tap water magnet graduated cylinder, 10-mL microspatula

Safety Wear your goggles and lab apron at all times during the investigation. Tie back loose hair and clothing to avoid any fire hazard. Always point the open end of a test tube away from yourself and others when heating a substance. Since glass retains heat without looking hot, heated glassware should be given ample time to cool before it is handled.

Use extreme caution when dealing with 6.0 M hydrochloric acid. It can cause severe burns if allowed to come into contact with the skin. Any spills should be reported to your instructor and cleaned up with cold water and sodium bicarbonate (NaHCO3). Silver nitrate solution will stain skin and clothing. Wear gloves and avoid spills and splashes. Heat iron and sulfur in a fume hood.

Procedure 1. Put on your goggles. Break off a small amount of wax from the bottom of a candle, and place it into a test tube. Holding the test tube with tongs, heat it gently over a burner flame until the wax melts completely. Place the test tube in the test-tube rack and let it cool for ten minutes. Record your observations in the data table. Caution: Keep loose hair tied back or covered. Hold the tube away from you and others while heating it. Use tongs to hold the hot test tube.

2. With the matches, light the candle. Secure the candle to the glass square by dripping wax onto the square and then holding the base of the candle in the molten wax until the wax hardens. Allow the candle to burn for several minutes. Record your observations in the Data Table.

3. Tear a small piece of paper into tiny pieces, and place the pieces in a watch glass. Place the watch glass on an insulating pad and ignite the paper with the matches. Allow the paper to burn completely. Caution: Keep all other flammable material away from the burning paper. Record your observations in the Data Table.

4. Measure 5 mL of water in the graduated cylinder. Pour the water into a test tube and add a microspatula of sodium chloride (NaCl). Stir the contents to mix. Put on latex gloves. Using the micropipet, add 10 drops of silver nitrate (0.1M AgNO3) to the NaCL-water mixture. Caution: Silver nitrate is toxic and can stain your skin and clothing, so avoid direct contact with it. Record your observations in the Data Table. Dispose of the solutions containing silver according to your teacher’s instructions.

5. Obtain a 5-cm piece of magnesium (Mg) ribbon and use scissors to cut it into 1-cm pieces. Place two of the pieces into a test tube an, using the micropipet, add a few drops of hydrochloric acid (6.0 M HCl). Caution: The 6.0 M hydrochloric acid is highly corrosive. Do not let any get on your skin or clothing. Wear latex gloves. Touch the outside of the test tube with your fingertip. Record your observations in the Data Table.

6. Grind several crystals of copper sulfate pentahydrate with the mortar and pestle. Place a microspatula of the powder into a test tube. Heat gently over a burner flame for 2 minutes. Allow to cool for 5 minutes; then add a few drops of water. Touch the bottom of the test tube with your fingertip. Record your observations in the Data Table.

7. One of the test tubes contains a mixture of powdered sulfur and iron filings. Run the magnet along the bottom and sides of the test tube. Record your observations in the Data Table. 8. Heat the iron-sulfur sample under a fume hood for several minutes until the mixture glows. Caution: Do not carry out this step unless a working fume hood is available. Noxious fumes are produced that must be evacuated by a fume hood. Allow the sample to cool for 10 minutes. Notice its appearance. Test with the magnet. Record your observations in the Data Table.

9. Clean up your work area and wash your hands before leaving the laboratory.

Data Table Step Observations 1

Critical Thinking: Analysis and Conclusions

1. Indicate whether the following changes are physical or chemical. Support your conclusions.

a. melting candle wax ______

b. burning candle wax ______

c. tearing paper ______d. burning paper ______

e. dissolving NaCL ______

f. mixing NaCl and AgNO3 ______

g. cutting Mg ribbon ______

h. adding HCl to Mg ______

i. grinding Copper Sulfate pentahydrate ______

j. heating Copper Sulfate pentahydrate ______

j. heating Fe and S ______

2. Name two possible indications that a chemical change has occurred, using examples from this investigation. ______

3. A change in color does not always indicate a chemical change. Explain why it could be the result of a physical change. ______

4. Answer the following questions using examples from this investigation to support your answers. a. How can substances in a mixture be separated? ______b. How can substances in a compound be separated? ______Critical Thinking: Applications

1. Sodium chloride dissolves in water, leaving a clear homogenous mixture with no physical evidence of the crystals with which you started. Design an experiment that you could perform to separate the sodium chloride from the water.

2. How would the experiment you designed in response to Question 1 differ if some sand was mixed in with the sodium chloride before it was added to the water?

Going Further

1. Based on what you have learned in this investigation, write a story in which you include as many physical and chemical changes as possible. When you finish the story underline the physical changes and circle the chemical changes.

2. Cooking involves both physical and chemical changes. Give examples that support this statement.