Classes of Chemical Reactions
When chemical reactions occur, reactants (starting materials) are converted into new substances called
products. The new products often have properties that are entirely different from those of the starting
materials. For example, chlorine which is a deadly gas reacts with sodium, a metal that reacts violently with
any moisture, to form a new substance called sodium chloride, common table salt. Sodium chloride has
properties that are nothing like those of the reactants from which it is formed.
Four ways in which chemical reactions may be classified are combination, decomposition, single
replacement (substitution), and double replacement.
The reaction of two or more elements to form a compound is a combination reaction. This process may be
described by the general formula:
A + B → AB
The rusting of iron is a good example of this class of reaction.
4Fe(s) + 3O2(g) → 2Fe2O3(s)
f the milk does not cause a chemical reaction. With the proper equipment we can separate the coffee from the
milk using the different physical properties of the two solutions. This change in appearance is an example of a
physical change. On the other hand, the brown color on the surface of toasted bread does indicate that a
chemical change has occurred on that surface. We can scrape the toasted layer off of the bread, but we cannot
return those crumbs to their original untoasted condition.
Observations indicating a chemical change include:
1. Formation of a precipitate, or solid when two solutions are mixed
2. A color change, one that is not simply due to the dilution of either of the solutions
3. Evolution of a gas that may or may not have an odor
4. A change in temperature in the reaction mixture that is not caused by external heating or cooling.
We can detect these phenomena using sight, touch, and smell. One note of caution, smelling anything in a lab
should be done cautiously; some chemicals can cause serious harm if inhaled in sufficient quantities.
In many chemical reactions, a precipitate forms because one or more of the reaction products is insoluble.
The insoluble product or products usually settle to the bottom of the reaction container as a solid. The
insoluble product may also appear as a cloudy suspension that only slowly, if ever settles to the bottom of the
container.
When color changes are observed it is important to carefully distinguish between the terms clear and
colorless. Clear means that something is transparent, that is you can see through it. Colorless means that
something is without color. Water for example is clear and colorless whereas apple juice is also clear but it
has color. Most color changes are more easily observed if the sample is held against a white background for
contrast.
Every chemical reaction involves a transfer of energy. In many cases, this energy transfer involves easily
detectable heat changes. A heat change may cause an increase or decrease in the reaction mixture
temperature. If the reaction mixture temperature increases during the reaction we say that the reaction is
exothermic; this means that it gives off heat to the solution. If the reaction mixture temperature decreases
during the reaction we say the reaction is endothermic; this means that it absorbs heat from the solution.
is a they always do so in the same proportion by mass. For example, water is made up of 11% hydrogen
and 89% oxygen by mass. It does not matter where the sample of water is from; these two elements are
always found in this ratio. We also know that each molecule of water is made up of two hydrogen atoms and
one oxygen atom, this ratio is the same no matter where then sample of water is from.
In this experiment you will mix solutions of different substances and combine solids with solutions. From
your observations you will conclude whether or not a chemical change has occurred in each case.
Procedure
1. Obtain 12 clean and dry test tubes along with a test tube rack. Label the tubes 1-12
2. For all the following reactions you should take note of your initial observations and then
you should place the test tubes in the rack to go back to at the end of the experiment to see
if there have been any additional changes.
3. In test tube #1, place a small amount of CoCl 2•6H2O and add 2mL of ethanol. Mix well.
Record your observations.
4. Transfer 1mL of “Cobalt (II) chloride hexahydrate in 95% Ethanol” to test tube #2. Add
1mL of distilled water to the same tube. Mix well. Record your observations.
5. Place a small piece of zinc into test tube #3 then add 2mL of 1M HCl. Mix well. Record
your observations.
6. In test tube #4, place a small piece of zinc and add 2mL of 0.1M CuSO 4. Mix well.
Record your observations.
7. Transfer 1mL of 0.1M CuSO 4 to test tube #5. Add 1mL of 1M HCl to the test tube and
mix well. Record your observations.
8. Transfer 1mL of 0.1M CuSO 4 to the test tube labeled #6. Add 1mL of 0.5M NaHCO 3 to
the same tube and mix well. Record your observations.
9. Transfer 1mL of 0.1M CuSO 4 to test tube #7. Add 1mL of 1M NH 3 to the same tube and
mix well. Record your observations.
10. Place a small (approximately 1cm) piece of magnesium ribbon in test tube #8. Add 2mL
of 0.1M CuSO4 to the test tube and mix well. Record your observations.
11. Place a small (approximately 1cm) piece of magnesium in test tube #9. Add 2mL of 1M
HCL to the tube and mix well. Record your observations.
12. Transfer 1mL of 1M HCl to test tube #10. Add 1mL of 1M NaOH to the test tube and mix
well. Record your observations.
13. Transfer 1mL of 1M HCl to test tube #11. Add 1mL 0.5M NaHCO 3 to the tube and mix
well. Record your observations.
14. Place a small amount of NH 4Cl to test tube #12. Add 2mL of room temperature water to
the test tube and mix well. Record your observations.
15. Go back and look at all the test tubes in the rack, record any additional observations that
you might have.
16. Dispose of the cobalt chloride and ethanol solutions in the container provided. Dispose
any solids from the tubes in the container provided and rinse the liquids down the drain
diluting with a large amount of running water.
Name:______________________
Signs of Chemical Change
Test tube number
1
2
3
4
5
6
7
8
Observations
Was there a
chemical change?
9
10
11
12