CHEMICAL CHANGES AND
EQUATIONS
There are many ways to detect or “observe” a chemical change. Some highly precise instruments enable us to
measure not only the degree of change in composition, but also the change in energy, which takes place when
substances react chemically. For many reactions, the changes that occur in energy and composition during these
reactions follow similar patterns.
A reaction that releases energy is exothermic. A reaction that absorbs energy is endothermic. After performing
many reactions, scientists are able to classify the reactions trends they have observed. From these trends, they can
develop generalizations about the nature of matter. Such generalization is the heart of chemistry.
A chemical equation represents a chemical reaction and indicates the substances that interact, the reactants,
and the new substances, or products that are formed. For the equation to be completely correct it must be
balanced. A balanced equation shows the same number of each kind of atom (represented by a symbol) on both
sides of the equation. For example, when iron rusts it combines with oxygen to form a new compound, which is
mostly Fe2O3. The chemical equation for this reaction would be:
4Fe + 3O2 -+ 2Fe2O3
Reactants yield Product
In this experiment, you will examine four classes of chemical reactions-synthesis, decomposition, single
displacement, and double displacement. You will be asked to identify some of the products formed in these
reactions and write balanced chemical equations for each.
Objectives
In this experiment, you will
• perform four types of chemical reactions
• identify some of the products of the reactions
• write and balance equations for the reactions observed
EQUIPMENT
goggles and apron
crucible
burner
ring stand and ring
clay triangle
spatula
small test tube
test tube holder
wood splint
forceps
evaporating dish
triangular file
flame spreader
glass tubing (15 cm)
2 large test tubes
one hole stopper
utility clamp
PROCEDURE
**Safety goggles and lab apron must be worn at all
times during this experiment**
A. Synthesis
1. Obtain a small sample of magnesium about 1-2g
or 10 cm of Mg ribbon. Place the Mg in a clean,
dry crucible and measure the mass of the crucible
and its contents carefully to the nearest 0.01 g.
Record the mass.
2. Place the crucible in a clay triangle, and place the
clay triangle on a ring. Begin heating, slowly at
first. Gradually increase the intensity of heat to the
hottest flame of your laboratory burner. CAUTION:
Mg is flammable and may begin to burn. If it does
burn, do not look directly into the flame and remove heat
3. When the Mg begins to glow brightly, stop heating. After the crucible has cooled slightly, remove
it from the clay triangle with forceps. Measure the
mass when completely cool, and then empty the
crucible’s contents into an evaporating dish.
4. Examine the contents for a change in composition.
Add a few drops of water to the residue and try to
detect the odor of ammonia gas, NH3).
5. Compare the new mass with the original mass.
6. Discard the solid contents into the waste container
designated by your teacher.
B. Decomposition
1. Obtain two small spatulas full of copper(II)
carbonate, CuCO3, and place them in a large, dry
test tube. CAUTION: CuCO3 is poisonous: avoid
skin contact.
gas by displacement of air by inverting another
test tube over the upturned gas delivery tube. (See
Figure 9-2.)
2. Insert the stopper and glass tube assembly into the
test tube containing the CuCO3.
3. Pour about 5 mL of limewater, Ca(OH)2(aq), into a
small test tube. CAUTION: Ca(OH)2 causes burns;
avoid skin contact. Place the open end of the right
angle glass tube in the limewater solution as shown
in Figure 9-1.
FIGURE 9-2. Air displacement method for collecting a
gaseous product.
3. Remove the test tube containing the gaseous products from the glass tubing. Keep it inverted and
bring a burning splint near its mouth. A “pop” or
“bark” indicates the presence of hydrogen gas.
FIGURE 9-1. Apparatus set-up for limewater test.
4. Heat the tube containing the CuCO3 while holding
the end of the glass tube in the limewater solution.
CAUTION: Do not point test tube at yourself or
any other person. Continue heating until bubbling
has nearly stopped. Remove the limewater tube
before removing the heat from the CuCO3. A
cloudy appearance in the Ca(OH)2 indicates the
presence of CO2.
5. Discard the solid leftover from heating the CuCO3
into the waste container designated by your teacher.
The limewater solution may be rinsed down the
drain using plenty of water.
C. Single Displacement (Replacement)
1. Clean and dry the apparatus used for Part B. Place
a small piece of zinc in a test tube and add 5 mL of
6M HCl. CAUTION: Keep away from open flame.
HCI causes bums; avoid skin contact. Rinse spills
with plenty of water. Insert the rubber stopper
containing the glass delivery tube and clamp test
tube in place as shown in Figure 9-2.
2. CAUTION: This set-up looks identical to the one
used for the decomposition of CuCO3, however,
DO NOT HEAT TEST TUBE WITH FLAME!
A reaction should occur and a gas should escape
from the tubing. With the glass tubing turned up,
collect some of the gas being liberated. Collect the
4. Remove the rubber stopper from the test tube.
Carefully add several mL of tap water to the contents of the test tube, then decant the liquid part
into the sink; rinse with plenty of water. The solid
may be discarded into a waste container or returned
to the reagent table as directed by your teacher.
D. Double Displacement (Replacement)
1. Add 5 drops of NaCI solution to an empty well of a
spot plate. To this, add 5 drops of AgNO3 solution.
Examine for the formation of a precipitate.
2. After observing the formation of a precipitate,
empty the liquid and solid contents into the waste
container designated by your teacher.