UNIT I
INVESTIGATING MATTER
Physical changes
Chemical changes
Water freezes.
A candle burns.
Ice melts.
The metal on a car starts to rust.
Sugar dissolves in water.
A piece of wood burns in a fireplace.
Frost forms on windows.
A firecracker explodes.
Detergent removes grease from
a dirty pot.
Concrete becomes hard after it
is poured.
The burner on an electric stove
glows red.
An egg becomes hard when it is
cooked.
Coffee changes color when
cream is added.
TABLE 2.1 Examples of Changes in Matter
FIGURE 2.3
What happens as a log burns?
Can you “unburn” a log?
ACTIVITY 2B / OBSERVING CHANGES IN MATTER
I
n this activity, you may observe
physical changes, chemical
changes, or situations where nothing appears to change.
MATERIALS
Part I
safety goggles
apron
gloves
small piece of steel wool
two pieces of copper wire
(2 cm long)
tongs
Bunsen burner
dilute hydrochloric acid
four test tubes in rack
magnesium ribbon (2 cm strip)
sodium carbonate solution
calcium chloride solution
copper sulfate crystals
Pyrex test tube
test tube holder
water in dropper bottle
PART 1 Student Experiments
PROCEDURE
1. In your notebook, draw two data tables like Tables 2.2 and 2.3.
Use them to record the properties of the starting materials and any
changes that occur.
2. Put on your safety goggles, apron, and gloves.
3. With the steel wool, polish a piece of copper wire. Examine it carefully and record the properties of the wire in your table. Pick up the
wire with tongs and hold it in the hottest part of a Bunsen burner flame
for 5 seconds. Remove the wire, let it cool, and examine it again.
Record your observations in your data table.
Procedure step
Starting materials
3.
Copper wire
4.
Dilute hydrochloric acid
5.
Magnesium ribbon
6.
Sodium carbonate solution
Properties
Calcium chloride solution
8.
Copper sulfate crystals
TABLE 2.2 Properties of Starting Materials in Activity 2B
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CHANGES IN MATTER
Procedure step
Description
of change
Type of change
(physical
or chemical)
CHAPTER 2
Reasons for
classification as
physical or chemical
3.
4.
etc.
FIGURE 2.4
Heating copper sulfate
TABLE 2.3 Sample Data Table for Activity 2B
C
C A U T I O N !
■ Dilute hydrochloric acid is very
corrosive and can cause damage to
eyes and skin. If you get any in your
eyes or on your skin, immediately
rinse with cold water for 15 or 20
minutes and inform your teacher.
■ Copper sulfate is poisonous. When
heating the test tube, hold it pointing
away from yourself and everyone
else.
4. Carefully pour dilute hydrochloric acid into a test tube to a depth of
about 4 cm. Polish the second piece of copper wire as you did in step
3 and carefully place it in the dilute hydrochloric acid. Leave it in the
acid as you continue with Steps 5 to 7. Then observe the wire carefully and describe any changes in your data table.
5. With the steel wool, polish a piece of magnesium ribbon. Examine it
carefully and record the properties of the magnesium ribbon in your
data table. Pour dilute hydrochloric acid into a test tube to a depth of
about 4 cm. Add the magnesium ribbon to the test tube; describe any
changes.
6. Carefully pour sodium carbonate solution into one test tube and calcium chloride solution into another, both to a depth of about 2 cm.
Examine each solution and record the properties of each in your
data table.
7. Slowly and carefully pour one solution into the other. Describe any
changes that you observe in your data table.
8. Place approximately 2 g of copper sulfate crystals in a Pyrex test
tube. Hold the test tube as shown in Figure 2.4 and heat the crystals
for 10 s. Allow the tube to cool, then heat it again. Replace the test
tube in the rack and allow it to cool completely. Record your observations in your data table.
9. Add a few drops of water to the crystals. Record your observations in
your data table.
MATERIALS
Part II
ammonium chloride in Pyrex
test tube
test tube holder
glass wool
Bunsen burner
10. Dispose of the materials according to your teacher’s instructions.
11. Wash your hands thoroughly after you have completed the activity.
DISCUSSION
1. Why did you have to wear safety goggles, an apron, and gloves to
carry out these experiments?
➥
25
UNIT I
INVESTIGATING MATTER
C
C A U T I O N !
■ Your teacher will carry out Part II
of the investigation if the classroom
has good ventilation.
2. Using the steel wool in Steps 3, 4, and 5 removed the evidence of a
change. Why was it better to polish the copper wire and magnesium
ribbon before studying them?.
3. (a) List all the solid substances used or produced in this experiment
(If you do not know the name of the substance, include only a
description in your list.)
(b) Repeat part (a) for all liquids and all gases.
4. In Step 6, solutions of sodium carbonate and calcium chloride are
used. Suggest a reason why these solutions are used rather than solid
sodium carbonate and solid calcium chloride.
ammonium
chloride
glass
wool
PART II Teacher Demonstration
PROCEDURE
1. Your teacher will place one scoop of ammonium chloride in a Pyrex
test tube. The test tube will be plugged with glass wool to slow the
escape of any gas (Figure 2.5). The bottom end of the test tube will
be heated gently over a flame for 3 seconds only and then
allowed to cool.
2. Record your observations in your notebook.
DISCUSSION
FIGURE 2.5
Is there evidence of a change as
the test tube cools?
1. What are some characteristics of physical changes? Give an example
of a physical change with each characteristic.
2. What are some characteristics of chemical changes? ❖
DISTINGUISHING BETWEEN A PHYSICAL CHANGE
AND A CHEMICAL CHANGE
It is not always easy to tell the difference between a physical
change and a chemical change. Some chemical changes may produce flames, sparks, and noise (Figure 2.6), but many do not. The
chemical changes that take place when you bake a cake, polish
brass, or clean the oven are much less spectacular. However, certain clues may tell you that a chemical change has occurred:
FIGURE 2.6
This welder is using a chemical
change to create a physical
change. How does his oxyacetylene welding torch work? What
are the two changes?
26
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•
•
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A new color may appear.
Heat or light may be given off.
Bubbles of gas may be formed.
Solid material may form in a liquid.
The change may be difficult to reverse.
Any of these clues could also be part of a physical change. You
need to consider several clues in order to determine which type
of change has taken place.