Teaching Tips

Teaching Tips: The Mystery Powder Experiment

Synopsis:
In this module students use a simulation to compare an unknown solid power to standards using a series of tests. Students use a robotic controller to measure the pH, reaction with common substances, and appearance tests to identify their unknown substance. A group of common materials are used as standards for comparisons with their unknown.

Big Ideas:
a. Substances have unique chemical and physical properties that can be used for identification.
b. Comparing and contrasting properties of materials can be useful in categorizing substances.
c. Chemical properties describe how one substance reacts with other substances.
d. Physical properties describe characteristics of a substance, such as color, density, and solubility.

Misconceptions Addressed:
a. Chemical changes are additive, not interactive.
b. Elements, compounds, and mixtures all describe the same quantity.

NSES Grade 5-8 Standards
a. Physical Science
i. Properties and changes of properties in matter
b. Science in Personal and Social Perspectives
i. Personal health
ii. Science and technology in society

The Experiment

The Mystery Powder PSI Sim™ is a virtual representation of an experiment that is often done in the classroom. If you do this experiment, it may be a good idea to do the PSI Sim™ first, since there are aspects to the module that will help students do the real experiment more efficiently. Although the experiment uses a robotic arm and a magnified display, the experiment is very much like the one that has been done for years in the classroom. The simulation is built to remind students to always use a clean sample, since the system won't work correctly unless a clean sample is used. Today's students are familiar with the work of crime scene forensic teams and the idea of examining powder to look for clues should seem commonplace to them. The methods section of the sim is quite long, but students should be encouraged to move carefully through the field notes and the chemical analysis section. The collecting data section checks their familiarity with the background information presented in the methodology section as well as the ideas presented in the simulation section.

Physical and Chemical Properties

Students often have a difficult time understanding the difference between physical and chemical change. It may be helpful to remind them that in a physical change no new substance is produced. Some might find it helpful to think about physical changes as reversible, and chemical changes as irreversible. When we chop wood, the result is still wood. When ice melts, it is still water, etc. In the methodology section we discuss physical changes in sugar from cubes to granulated to rock candy.

In a chemical change one or more new substances is produced. Burning wood, for example, produces new substances. Ask the students to try to imagine rust changing back into iron, or un-burning a log. Students may have misconceptions about rust, thinking that every impurity on a metals surface is rust. Rust is specifically iron oxide, but other types of corrosion (oxides) are also the result of oxidation reactions. Oxidation reactions occur when a chemical substance loses electrons to another substance. Combustion is an extreme example of oxidation. For example, in gasoline explosions carbon and hydrogen in the gasoline oxidize to form water and carbon dioxide.

Demonstrations and Other Experiments

You may wish to do a demonstration with real powders or mixtures of powders. Powders can be tested with vinegar, heat, and iodine. Powders such as baking soda and other powders may fizz when vinegar is added. That is, just because a powder fizzes with vinegar does not mean it is baking soda. However, if a powder does NOT fizz with vinegar, the students can be fairly certain that baking soda is NOT present. The idea here is that qualitative analysis involves deduction and the process of elimination. This is why it is so important for experimenters to keep accurate notes. Similar tests with iodine will turn any mixture that includes even a small amount of starch to a blue-black color.

Answers to Number 3e. on "Collecting Data and Reporting" Sheet:

Baking soda fizzes with vinegar (chemical reaction).
Cornstarch turns black with iodine (chemical reaction).
Plaster of Paris turns hard and warm with water (warm: chemical reaction; hard: physical change).
Sugar turns brown, then black with heat (chemical reaction).
Salt tastes salty; sugar, sweet (physical change).
Sugar and salt dissolve in water (physical change).
Iodine changes powders to its own color, but not a new one (physical change).