## Fermi Questions: Back of the Envelope Calculations

The Art of Estimation
When I was in high school, in the days before electronic calculators were available, we learned how to use mechanical slide rules (see ChemMatters, April 2004, p.4) for our calculations. While they were great for getting a good answer to math problems, they didn’t keep track of the decimal place. That had to be done by keeping track of an order of magnitude estimate of the answer. Continue reading “Fermi Questions: Back of the Envelope Calculations”

## Mixtures

All of us realize we are surrounded by “stuff” or more precisely, matter. But getting a handle on the various types of matter around us is a different story. It is a little like walking down a shopping mall with stores that have no store signs labeling what kind of goods they sell. All you can do is look through the windows to get an idea of what they offer for sale.

To sort out the kinds of stuff that surround us, chemists look at the physical and chemical properties of matter and classify it in to various groups.

One of the largest distinctions is between pure substances and mixtures. Pure substances (or just substances) are homogeneous (that is, the same throughout) and have a definite composition, which means they have simple whole number ratios (by mass) between elements that make them up. These are the elements (like gold or carbon) and compounds,(like salt or calcium carbonate) that we tend to spend a lot of time studying in chemistry, because they are the building blocks for all the other types of matter we encounter.

Mixtures on the other hand are a bit messier. They are combinations of two or more materials. This means mixtures can usually be separated back into their original materials by physical processes. A good non-chemical example of a mixture is a jar of mixed nuts, or a drawer with miscellaneous kinds of screws, bolts and washers.

Solutions, suspensions and colloids are similar in that they are combinations of smaller amounts of solute in larger amounts of solvent. The main reason for the varying characteristics among them is the particle size of the solutes.

Emulsions are a special case. An emulsion can be made with two liquids that would normally not mix, such as oil and water. In the case of mayonnaise an egg yolk, which contains the emulsifier lecithin, is used to suspend the oil in tiny droplets. The lecithin coats the oil droplets so its “fat loving” side is on the inside with the fat, while the “water loving” side faces the aqueous solvent side.

Although solutions are often introduced with examples such as sugar dissolving in a pitcher of water for Kool-Aid, solute-solvent relationships can involve various phases beyond solids in liquids.

Air is a common example of a gas-gas solution and soda is carbon dioxide gas dissolved in liquid water. Hydrogen gas can dissolve in palladium metal as an example of gas-solid solutions. You might be wearing a solid-solid solution right now in the form of alloy metals used in jewelry. Similar phase combinations occur in suspensions and colloids. A nice challenge would be to come up with other everyday examples of various phases in solutions, suspensions, colloids or other mixtures.

Finding other examples could help sort out the messy mix of mixtures that surround us!

This graphic is a winning entry in the 2015-2016 ACS ChemClubs/ChemMatters Infographic Contest. Students, teachers, and other chem enthusiasts were challenged to take a chemistry topic and turn it into an original informational graphic. Entries were judged on originality, and the ability to convey accurate science details clearly and creatively. This infographic was conceived by Aaron Herrera and Emerald Rawls from Mapleton Expeditionary School of the Arts in Thornton, CO.

## AACT: A Great Resource for Teachers of Chemistry

The American Association of Chemistry Teachers (AACT) officially launched in September. As a ChemClub advisor, you can further supplement your students’ chemistry experiences by joining for the annual fee of only \$50, and preservice teachers can join at the discounted price of \$25.

Some member benefits include being part of a community of K–12 teachers of chemistry, access to high-quality resources, and subscriptions to the ACS publication ChemMatters and the new AACT online periodical Chemistry Solutions, which is written by and for teachers of chemistry.

AACT’s website has collections of original multimedia that may be of interest to ChemClubs. AACT partnered with New York Times bestseller Sam Kean to bring his book “The Disappearing Spoon” to life—each short video in the series features Sam narrating a story inspired by his book about various elements.

There is also a series of videos about the founders of chemistry: Learn about Mendeleev’s journey to assemble the periodic table, the ancient chemists who paved the road for what chemistry is today, the first female Nobel Laureate Marie Curie’s life story, and others.

One mission of ChemClubs is to provide students with fun, authentic, and hands-on opportunities. The library of classroom resources on AACT’s site includes lessons from teachers across the country and around the world. Peruse the collection and try some of the demonstrations with your club members. Safety precautions in each lesson are outlined in detail, so if you have an outreach program, you can gauge right away whether the activity is appropriate for your students to run, or whether it should be facilitated by an instructor. AACT has a number of lessons that are designed for student-to-student interactions, including this cabbage activity. And if you have a great activity you’ve done with a club, you can submit it to the AACT library, and AACT will credit your ChemClub with its contribution.

In each issue of Chemistry Solutions there is a column called Chemistry Fun! In the September issue, the column featured pictograms of phrases that were puns of chemistry concepts. For example, what is this? Your club members may get a kick out of activities included in this column.

You can subscribe to AACT email updates by completing the “Stay in Touch” field in the footer of teachchemistry.org. Or follow us on Facebook, Twitter, or Pinterest for other news and information from AACT. We are excited to be part of the K–12 chemistry community, so share with us ideas you have by emailing AACT@acs.org.