Physical and Chemical Changes
Introduction:
As you read this, it is probably fall. Summer flowers are fading and drying. Leaves are changing from green to
red, yellow, and orange. All these changes involve chemistry.
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 alter, but its chemical composition remains the same. Examples of physical
changes include melting ice, crushing gravel, 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, 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 ad 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 Questions:
1. Identify the following as either a chemical or a physical change, explain your reasoning.
a. Burning wood
d. Ripening fruit
b. Dry ice (solid) changing into a gas
e. Sugar dissolving in water
c. Freezing water
2. What are some observable signs that indicate a chemical change is taking place?
3. What are the safety precautions you need to observe during this investigation?
Materials:
Chemical splash goggles
Pieces of paper, 2cm by 2cm
Magnesium (Mg) ribbon
Laboratory apron
Watch glass
Scissors
5 test tubes
Tap water
Micropipette with
Test tube holder
Graduated cylinder, 10mL
hydrochloric acid (HCl), 6.0M
Bunsen burner
Spatula
Copper sulfate pentahydrate
Test tube rack
Sodium chloride
Mortar and pestle
Matches
0.1M silver nitrate (AgNO3)
Balance
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 in
contact with the skin. Any spills should be reported to your instructor and cleaned up with cold water and sodium
bicarbonate. Silver nitrate solution will stain skin and clothing.
Procedure:
1. Put on your goggles and lab apron. 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/counter and ignite the paper with the matches. CAUTION:
Keep all other flammable material away from the burning paper. Allow the paper to burn completely. Record
your observations in the Data Table.
2. Measure 5mL 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. Using the micropipette, add 10 drops of silver nitrate (0.1M
AgNO3) to the NaCl-water mixtures. 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.
3. Obtain a 5cm 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 a micropipette, add ten drops of hydrochloric acid (6.0M HCl). CAUTION: the
6.0M hydrochloric acid is highly corrosive. Do not let any get on your skin or clothing. Touch the outside of the
test tube with your finger tip. Record your observations in the Data Table. Dispose of the solution according to
your teacher’s instructions.
4. Grind several crystals of copper sulfate pentahydrate (CuSO4*5H2O) 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 finger tips. Record your
observations in the Data Table.
5. Clean up your work area and wash your hands before leaving the laboratory.
Data Table
Task
Observations
Tearing Paper
Burning Paper
NaCl and Water
Adding Silver Nitrate to NaCl and water
Cutting Magnesium Ribbon
Adding Hydrochloric acid to Magnesium
Grinding of Copper Sulfate Pentahydrate
Heating of Copper Sulfate Pentahydrate
Adding water after heating Copper Sulfate Pentahydrate
Critical Thinking:
1. Indicate whether the following changes are physical or chemical. Support your conclusions.
a. Tearing paper
e. Cutting Mg ribbon
b. Burning paper
f. Adding HCl to Mg
c. Dissolving NaCl
g. Grinding Copper sulfate pentahydrate
d. Mixing NaCl and AgNO3
h. Heating Copper sulfate pentahydrate
2. A change in color does not always indicate chemical change. Explain why it could be the result of a physical
change.
3. Sodium chloride dissolves in water, leaving a clear homogeneous mixture with no physical evidence of the
crystals with which you started. Design an experiment that you could perform to separate sodium chloride from
the water.
4. Cooking involves both physical and chemical changes. Give examples that support this statement.
5.
Using the drawing above, determine if you think a physical change is occurring or a chemical change. Explain
your reasoning.
Conclusion:
Discuss the differences between physical and chemical properties and how this applies to the cooking of scrambled
eggs.