Chemistry Reference
In-Depth Information
hydroxides and their solutions. The label indeed reveals the ingredients sodium
hydroxide and aluminum. In the actual substance white sodium hydroxide in grainy
form can be observed, and additionally little silver-colored aluminum splinters.
When water is added to the mixture, a strong exothermic and alkaline reaction sets
in: hot concentrated sodium hydroxide solution is produced. Adding paper to the
reaction mixture demonstrates the decomposition reaction of organic substances.
With added aluminum and sodium nitrate gaseous ammonia is produced which
improves the properties of those cleaners. The formation of gases is being analyzed
and their functions are being discussed (see also Chap. 8).
P9. Controlling the Learning Process.
E5.19: Reactions of Alkaline-Earth Metals with Water
The learning process has been successful, if students are able to transfer familiar
reactions of alkaline metals with water to similar reactions of magnesium or
calcium with water. The formation of alkaline hydroxide solutions and hydrogen
gas is familiar, but additionally solid hydroxides form suspensions. Also composi-
tion and formulae M(OH)
2
(M
Mg or Ca) of these hydroxides are different
leading to different chemical equations for the metal-water reactions.
ΒΌ
P10. Practicing Experimental Skills.
E5.20: Student Experiments on the Lithium-Water Reaction
Student experiments with hazardous alkaline metals and caustic bases are possible
with lithium, as it can be handled much more safely than sodium or potassium. The
reaction with water is not only possible in an open beaker, but also quantitatively in
a closed apparatus. A piece of lithium can be added to a glass cylinder, which has
been filled with water and opened under water in a glass bowl pneumatically (it is
not allowed to let sodium and potassium react in this way!).
The student work sheet (see E5.20 at the end of this chapter) exemplifies how
work sheets should look in general. It also shows which experiments may be carried
out safely and can be interpreted by the students themselves.
Selection criteria for experiments:
After the different roles of experiments in
chemical education have been experienced, further criteria for the selection of
experiments in chemistry classes shall be offered and discussed: The planned
experiment should
- Be suited for the age group
- Be based on the student's knowledge
- Be productive scientifically and educationally
- Match the school's chemical collection
- Have a high probability of success
- Not have any safety hazards
- End during an adequate period of time
- End with a clearly recognizable effect
- Be usable as a student experiment
Selection criteria can also be found for specific classroom topics, e.g. for an
initial experiment toward the introduction of chemical reactions of different
elements (see Table
5.3
):
