Teaching Tips

Teaching Tips:The Chemical Mixer Experiment

Synopsis:
In this module students use a simulation that incorporate color and sound to study how elements combine in definite proportions to form compounds. Students use a “mixer’ to determine the chemical makeup of a compound. The compound name is provided and students must determine the chemical formula of the compound.

Big Ideas:
a. Compounds consist of two or more elements chemically
combined.
b. Compounds consist of elements combined in specific
proportions.
c. The periodic table arranges elements based on their electron configuration.
Misconceptions Addressed:
a. A given compound has a variable combining ratio of elements.
b. Elements and compounds described the same quantity.

NSES Grades 5 – 8 Content Standards Addressed:
a. Physical Science
i. Properties and changes of properties in matter

A Virtual Experiment

The PSI Sim™ Chemical Mixer Experiment is unusual in the sense that it has no real counterpart in reality. There is not such thing as a chemical mixer, it is simply construct to help students understand The Law of Definite Proportions through independent inquiry. Students often have problems understanding how chemical compounds are formed Simply put (and the law represents a fairly simple concept), The Law of Definite Proportions states that a pure sample of a compound will always have the same proportion by mass of the elements from which it is formed. The law was discovered by French chemist J. L. Proust in quantitative experiments and observations in 1799.

While the law of definite proportions seems trivial today, it did not seem so trivial in Proust's time. In fact, when Proust first proposed it, it was very controversiall. Many chemists, including Claude Louis Berthollet, another French chemist argued that elements could combine in any proportion. The debate between chemists of that day shows that they, like many modern students, did not really understand the difference between chemical compounds and mixtures

Note: Students fond of researching may discover that the Law of Definite Proportions is not universally true. Nonstoichiometric compound, whose elemental composition can vary from sample to sample, thus making the idea of a "pure" sample questionable, are exceptions. Unfortunately, Prousts instruments were not accurate enough to catch the exceptions.

Classroom Management

The Chemical Mixer Experiment makes significant user of sound. This can make a computer lab quite noisy. If possible, procure headphones for the students before the lab. You may find that many of your students carry headphones with their portable music players. Those headphones should work fine when plugged into the computer headphone jack. Note that it is possible to complete The Chemical Mixer Experiment without sound. The simulation also has a color indicator that indicates the color of the attempted compound and the target compound. When these colors are the same, the attempt is correct. Also, note that the red lights on the left side of the mixer indicate the number of atoms in the compound being assembled (the attempt) and the red lights on the right indicate the correct number of atoms in the target compound. See the diagram below:

A Better Periodic Table

Students sometimes move quickly through the Methods section to get to the simulation. This is not wise in The Chemical Mixer Experiment (if it is ever wise!) since there is a great deal of content in the Methods section. The section "A Better Periodic Model" is a modern, animated interpretation of the "Oatmeal Barrel" trick used by chemistry teachers in days past. The table was given away by a chemical society and teachers used to wrap it around a large cardboard oatmeal container to illustrate the concepts in our video.