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Physical and Chemical Changes

 

Introduction

           

            As you read this, it is fall.  Summer flowers are fading and dying.  Leaves are changing from green to red, yellow, and orange.  All these changes involve chemistry.

            Changes can be classified as either physical or chemical changes.  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.

                        1) Give 2 more examples of physical 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 matter.  Example of chemical changes include: plants dying, leaves changing colors, paper burning, bananas ripening, bread baking, or iron rusting.  Some signs a chemical changes include a change in color, the formation of a precipitate (new solid substance that settles out of solution), the production and release of gas, or a change in temperature.

            2) Give 1 more example of a chemical change

            3) What is a precipitate?

You must 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.

 

Objectives of this Lab

            You will conduct tests on several substances

            You will collect data to determine whether the resulting changes are chemical or physical

            You will make clear and concise observations about the reactions or changes

            You will conduct a pre-lab discussion

            You will conduct a post-lab discussion pertaining to the lab

 

Pre-Lab Discussion

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

            a.  Burning wood                                                                      __________________

            b.  Dry ice (solid) changing into gas                                           __________________

            c.  Freezing water                                                                     __________________

            d.  Ripening fruit                                                                       __________________

            e.  Sugar dissolving in water                                                      __________________

2.  What are some observable changes 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? ____________________________

 

      ___________________________________________________________________

 

SAFETY!!!!                 

 

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

            A.

            B.

            C.

            D.

            E.

            F.

            G.

 

YOU WILL NOT BE PERMITTED TO CONTINUE THIS LAB INVESTIGATION IF THE SAFETY REQUIREMENTS ARE NOT FOLLOWED!!!!

 

IF YOU DO NOT FOLLOW THE SAFETY REQUIREMENTS YOU WILL GET A “0” FOR THIS LAB ASSIGNMENT!

 

GOGLES AND LAB APRONS MUST BE WORN AT ALL TIMES, NO EXCEPTIONS!    

 

Focus Question:

 

            How can you recognize the differentiate between physical and chemical changes?  

 

 

Materials:

Chemical splash goggles                                    sodium chloride (NaCl)

Lab apron                                                        glass stirring rod

Latex glove                                                       micropipet with silver nitrate 0.1 M

Birthday candle                                                 magnesium ribbon

5 test tubes                                                       scissors

Test-tube holder                                               micropipet with HCl 6.0 M

Laboratory burner                                            copper sulfate pentahydrate

Test-tube rack, wood or metal                          mortar and pestle

Matches                                                           Tap water

Graduated cylinder                                           Insulating Pad

Microspatula                                                    Watch glass

Glass square                                                     Piece of paper, 2 cm  X   2 cm                                    

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 yourselves 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 handles.

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.  Note the caution alert symbols here and with certain steps of the procedure.  Refer to you guide to Laboratory equipment handout.

1) What happens if you don’t follow the safety required in this lab?

2) What can happen if hydrochloric acid comes in contact with you?

3) What do you do to a test tube when you are heating it?

 

Procedure:

1.  Put on your goggles and lab apron.  Break off a small amount of wax from the bottom of a birthday candle, and place it into a test tube.  Holding the test tube with the test tube holder, 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 your data table.

 

            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 it goes out.  Record your observations in your 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.  Record your observations.

 

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 to the NaCl-water mixture.  Record your observations.  Dispose of solutions

in the BEAKER UP FRONT!

 

            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, and using the micropipet,

add a few drops of hydrochloric acid (6.0 M HCl).  Wear latex gloves.  Touch outside of the test tube with your fingertip.  Record your observations.  Dispose of the solution in the beaker up front!

 

6.  Grind several crystals of copper sulfate pentahydrate with the mortar and pestle.  Place a microspatula of the powder into a test tube.  Heat gentily 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.

 

7.  Clean up your work area and wash your hands and lab bench before you leave!  RECORD YOUR OBSERVATIONS!

 

Data Table

Steps

Observations

1

 

2

 

3

 

4

 

5

 

6

 

7

 

 

Classify the following changes: physical or chemical, Use your experiments to verify your answers.

a.  Melting Candle wax

b.  Burning Candle wax

c.  Tearing paper

d.  Burning paper

e.  Dissolving NaCl

f.  Mixing NaCl and AgNO3

g.  Cutting Magnesium ribbon

h.  adding HCl to Mg

i.  Grinding CuSO4 * 5 H2O

j.  Heating CuSO4 * 5 H2O

 

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

 

2)  Using your lab experiments:

            a.  How can substances in a mixture be separated?

            b.  How can substances in a compound be separated?

 

Critical Thinking

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