CHEMICAL COMPOSITION
Life exists on Earth because of the abundant presence of liquid water. While
other planets have water, it may be primarily found as either a gas, as on Venus,
or as a solid, such as on Mars. However, recent explorations of the surface of Mars have revealed the
presence of water that existed millions of years ago. Thus the chemical nature of water is an
important chemical to us, all living organisms, and to the survival of our planet.
THE CHEMICAL COMPOSITION OF WATER
Compounds can be separated into their elements by chemical means, such as heating them (to very
high temperatures) or passing electricity through them. In any chemical reaction, atoms are
conserved. This means that if a given number of atoms react in a chemical change, the same kind and
number of atoms will be present in the products. In other words, chemical reactions produce new
arrangements of atoms, not new atoms. For the decomposition of water, we can see that atoms are
conserved by comparing the number of atoms (not molecules) present in the reactants and products.
PROCEDURE
In your supplies you will find magnetic spheres of different colors and sizes. The larger spheres
represent oxygen atoms and the smaller spheres represent hydrogen atoms. Construct 6 models of
water molecules.
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How many hydrogen ATOMS in total do the models contain? __________

How many oxygen ATOMS? __________

What is the ratio of the hydrogen atoms to oxygen atoms? __________
Decompose the water molecule models to form molecules of hydrogen gas (H2) and oxygen gas (O2).
Hydrogen and oxygen gas are examples of diatomic molecules, that is, they only exist free in nature
as a combination of two atoms. Nitrogen gas (N2) and chlorine gas (Cl2) are also diatomic gases.

How many oxygen MOLECULES are formed by the DECOMPOSITION? __________

How many hydrogen MOLECULES are formed? __________

What is the ratio of hydrogen to oxygen molecules? __________

Write a balanced chemical equation for the DECOMPOSITION of water.
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PART I: ELECTROLYSIS OF WATER
The decomposition of a substance using electricity is called ELECTROLYSIS.
PROCEDURE
Sulfuric acid is hazardous: Safety glasses
are REQUIRED during this experiment.
Set up the apparatus as shown in the diagram to the right. Make sure that
all connections are tight. Notice that the end of the electrode should not
go all the way up into the test tubes.
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Also, note the color of wires that are connected to the terminals of the power source:
Positive Terminal (red)
Negative Terminal (white)
Plug in the power source.
Since this reaction is very slow with pure water it is necessary to add a substance to aid in the
conductance of electricity. Add approximately 20 mL of sulfuric acid to the water and stir. Make
certain that the electrodes are under the collecting cylinders and that all the gas produced is
collected into the cylinders.
When one of the test tubes is full of gas, unplug the power source. Estimate the volume of gas in
the other test tube (one quarter, one half, two-thirds, etc.)
Positive Terminal (red)
Gas Volume: ______________
Negative Terminal (white)
Gas Volume: ______________
Remove the electrode from the test tube full of gas, but keep the electrode in the beaker. Place
your finger over the end of the tube and remove the cylinder from the beaker. Keep the cylinder
upside-down once it has been removed.
This cylinder contains hydrogen gas. To test for this gas, ignite a wood splint and place the
BLAZING splint quickly inside of the cylinder. What are your observations?
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Reconnect the power supply and continue collecting oxygen gas in the other test tube until it is
full. To test for oxygen, ignite a wood splint and blow it out so that an orange GLOWING ember is
present. Place the glowing splint into the test tube and record your observations.
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PART II: COLLECTING OXYGEN & HYDROGEN IN THE SAME TUBE
Repeat the experiment, this time collecting BOTH gases together in one test tube. Be sure that the
two electrodes do not touch while in the test tube. Set up the apparatus as before and unplug the
electrodes when the test tube is completely full of gas. Ignite the mixture of gases with a BLAZING
SPLINT. What are your observations? Why is this reaction more vigorous than with the previous gases
you collected?
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In the space shuttle’s main engines, liquefied hydrogen and oxygen
(stored in the central external fuel tank) react to form water. The three
main components of the external tank are an oxygen compartment
containing approximately 550,000 liters of liquefied oxygen at -300 oF and
a hydrogen compartment that holds 1.5 million liters at a temperature of
-423 oF. The hydrogen tank is 2.5 times larger than the oxygen tank but
weighs only one-third as much when filled to capacity. The reason for the
difference in weight is that liquid oxygen is 16 times heavier than liquid
hydrogen.
Disassemble the apparatus and rinse all the components with water. Wipe up any spills which
occurred during the experiment.
PART III: COMBINING OXYGEN & HYDROGEN
Before Ignition
After Ignition
gas
water
water
In the last reaction where a mixture of gases was ignited, hydrogen and
oxygen gas reacted to form water vapor. As in the decomposition process,
2 parts of hydrogen will combine with one part of oxygen. If more hydrogen
or oxygen is used than in a 2:1 ratio, some of the remaining gas will remain
unreacted.
The test tube to the right contains a mixture of 10.0 mL of hydrogen and
10.0 mL of oxygen gas which is suspended in a beaker of water. To react
the gases, an electrical spark can be used, the reaction occurs with a
bang (as you saw in the last experiment) and the hydrogen and oxygen
gas react at a 2:1 ratio to form water. Since 10.0 mL of hydrogen gas will
react with 5.0 mL of oxygen gas, there was 5.0 mL excess of oxygen gas
which remains in the test tube after ignition. The water in the
10 mL Hydrogen
10 mL Oxygen
beaker then rises to fill the volume where the reacted gases once
occupied.
5 mL Oxygen
For the following sets of reactions of hydrogen and oxygen gas determine the identity and quantity
of unreacted gas (if any). Sketch a line indicating the volume of unreacted gas that remains (if any) at
the top of the cylinder and water below it.
Before Ignition
After Ignition
5 mL Hydrogen
15 mL Oxygen
____________
Before Ignition
After Ignition
20 mL Hydrogen
20 mL Oxygen
____________
Before Ignition
After Ignition
20 mL Hydrogen
10 mL Oxygen
____________
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PART IV: CHEMICAL COMPOSITION OF HYDROGEN PEROXIDE
O
H
O
While hydrogen peroxide (H2O2) resembles the chemical composition of water, it has H
significantly different chemical properties. The bond between the two oxygen atoms is
weak, so that H2O2 readily decomposes. H2O2, when pure, is extremely unstable, therefore it is
commonly sold as a 3% solution in water, making it much safer to handle, but it is still quite reactive.
The decomposition of H2O2 is also accelerated by the presence of light, so H2O2 is normally sold and
stored in brown bottles.
In this experiment yeast is added to act as a catalyst. A catalyst is a substance that increases the rate
of a chemical reaction without being consumed or permanently changed. In the previous experiment
sulfuric acid was also used as a catalyst. This does not mean that it is not involved in the reaction. It is
"attached" or bonded to the reactants temporarily in the reaction, but the catalyst then returns to its
former composition by the time the reaction is complete.
Hydrogen peroxide decomposes when applied to a cut. The catalyst that speeds up the
decomposition in this case, is an enzyme that is found in the blood. Enzymes are complex proteins
that catalyze many reactions in the body. When the hydrogen peroxide decomposes, oxygen is
released which kills anaerobic bacteria that often cause infections.
Procedure
Using the magnetic spheres, construct 2 models of hydrogen peroxide molecules.

How many hydrogen ATOMS in total do the models contain? __________

How many oxygen ATOMS? __________

What is the ratio of the hydrogen atoms to oxygen atoms? __________
Decompose the hydrogen peroxide molecule models to form molecules of oxygen gas (O2) and
water (H2O).
 How many oxygen MOLECULES are formed by the decomposition? __________
 How many water MOLECULES are formed? __________
 Construct a balanced chemical equation for the decomposition of hydrogen peroxide.
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DECOMPOSITION OF HYDROGEN PEROXIDE
Materials: 50 mL beaker, spatula, wood splints, matches, 3% hydrogen peroxide solution, yeast.
Pour 20 mL of hydrogen peroxide into the beaker.
Add a small sample of yeast to the hydrogen peroxide and observe the chemical reaction. After
30 seconds or so, you will notice that the reaction becomes more vigorous.
Light a wooden splint and gently blow it out to form a glowing splint. Place the glowing splint into
the beaker directly above the solution. What evidence did you observe indicating the production
of oxygen gas?
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Dispose the mixture in the sink and rinse out the beaker.
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PART V: BAGGIE CHEMISTRY
There are many types of chemical reactions. When chemicals are mixed several
possible observations indicate that a chemical change has taken place:
In some chemical reactions heat is generated, thus we can feel the reaction to
see if the products of the reaction are getting hotter. Other chemical reactions
absorb heat, thus the products become cooler.
In some chemical reactions, one of the products is a gas. Evidence of this type
of reaction is that gas bubbles form when the reaction takes place.
In some chemical reactions one of the reactants may change color.
In some chemical reactions, one of the products may not be soluble in water,
and a precipitate will form. Evidence of this type of reaction is the formation
of an insoluble solid.
In this experiment, two chemicals that can be found around the house will be mixed, and several
chemical changes will be observed. CALCIUM CHLORIDE is the ingredient in some types of products that
are sold in the winter to melt ice on sidewalks. BAKING SODA or SODIUM BICARBONATE is found in most
kitchens and is used in cooking. The third chemical that we will use is UNIVERSAL INDICATOR. Universal
indicator changes color when the reaction involves an acid or base.
The reaction that occurs in the experiment is:
CaCl2 (aq) + 2 NaHCO3 (aq)
2 NaCl (aq) + 2 CO2
(g)
+ Ca(OH)2 (aq)
PROCEDURE
1. Pinch the zip-loc bag with your thumb and forefinger at the bottom to form two partitioned
areas.
2. Place one spoon of calcium chloride into one corner of a zip-loc plastic baggie.
3. Place one spoonful of baking soda into the other corner of the bag. Do not mix the two
chemicals at this point.
4. Place approximately 30 mL of water and 10 drops of universal indicator into the bag WITH THE
CALCIUM CHLORIDE ONLY. Make sure that the water does not come in contact with the baking
soda. Feel the temperature as the calcium chloride reacts with water.
5. Record your observations that indicate a chemical reaction is taking place.
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6. Seal the bag. Un-pinch the baggie and mix the calcium chloride solution with the baking soda.
Again, feel the temperature of the bag. Record your observations that indicate a chemical
reaction is taking place.
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7. Ignite a wooden splint. Carefully open the baggie and place the BLAZING SPLINT into the gas that
was generated during the chemical reaction.
Observations:_________________________________________________________________
What gas was produced during the chemical reaction? __________________________
8. If you were to measure the mass of the baggie and chemicals prior sealing the baggie, and
again measured the mass after the chemical reaction was completed, would the mass of the
baggie and its contents changed in mass? (Increased, decreased, or remained constant?)
Defend your answer.
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9. Rinse the baggie in the sink with plenty of water and dispose of the baggie in the trash.
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