Chemical and Physical Change Demonstrations

Memorable Moments of

Change

Mike the Mouse Story

Paper chromatography achieves a physical separation of molecules. The separation is based upon different levels of “molecular love” (Zachmann’s term). A moving phase taunts the molecules that are being held back by the stationary molecules that are attached to the paper. Over time the group of molecules become smaller clusters. Molecules with the greatest level of love for the solvent travel the greatest distance. Molecules with the greatest love for the stationary phase trail behind. Since this is a physical process the molecules retain their original properties.

A Story of Survival

To illustrate the value of this tool Mr. Zachmann told the story of the miraculous survival Michael the mouse. In the story Michael was very ill. He could not eat with the other mice so he survived on food covered with a mysterious green growth. Over time Michael began to improve. Eventually, he became so strong he broke out of the cage, went on to Hollywood and landed a new gig as a magnificent mouse later to be called “Mighty Mouse.” A young graduate student suspected that there was something in the green scum that cured Michael so he ground up the green stuff and separated the material using paper chromatography. When the run was complete he collected the separated colors and then made pills of each. He tested the pills with other mice that suffered from the same illness as Michael. Using “double blind” study parameters he found that a combination of 2 parts blue and one part yellow provided the best results. The foreign drug company GSK bought the patent rights and now sells the drug as Tumor Necrosis Factor Five (TNF-5).

This exact story is not true. The story line however is very common. Pasture’s remark “chance favors the prepared mind” bears witness to this fact. Every day scientists test new possibilities for cures through the avenue of chemistry. As the sign on my door suggests we want to be masters of molecules.

Chemical Change

In general a chemical change involves an interaction of matter that results in the production of new materials. Since we can not see the molecules directly we relay of the appearance of new properties to suggest a new alignment of matter as been achieved. Three key visual indicators include the production of a precipitate, emission of a gas, or the formation of an unionized material from ionic sources.

Reagents for Change

The slide to the right presents various solutions and testing reagents. In the strange world of Mr. Zachmann these reagents are members of the CIA (Chemical Information Agency). CIA field agents are often called upon to deliver very specific information (AKA intelligence) about a chemical system. As students of chemistry you should become familiar with the intelligence these operatives can provide.

Reactions to Remember

Numbered from left to right the 3rd spot well shows the result of mixing sodium chloride with silver nitrate. Notice a white precipitate formed. The production of the precipitate suggests a chemical reaction has occurred and a new material has formed. At this point we do know if the precipitate is AgCl or NaNO3.

NaCl + AgNO3 AgCl + NaNO3

We questioned which of the expected products is the white precipitate. To help with that decision we tested the next cell with sodium acetate and silver nitrate.

Test Two

The 4th cell does not contain a precipitate. There is no evidence of a new material. What we learned from this test is that the previous white precipitate involved the chloride component.

Na(C2H2O2) + AgNO3 Ag (C2H2O2) + NaNO3

Test Three

The 5th cell shows the reaction of sodium hydroxide and silver nitrate. The color of the precipitate is valuable. We now believe silver hydroxide is a brown precipitate.

NaOH + AgNO3 AgOH + NaNO3

Since hydroxides are associated with bases, in the next step we treated the precipitate with sulfuric acid. This acid does not contain a chloride so perhaps it could reverse the reaction back to the original state for the reactants….can we handle this kind of power?

The Power of Chemistry

Compare the top picture to the bottom one. The addition of the acid did remove the precipitate. In the 6th well we reacted hydrochloric acid with the silver nitrate. Once again a white precipitate formed. The acid did not destroy the precipitate.

HCl + AgNO3 AgCl + HNO3

Litmus as a CIA Operative

The display to the right allows us to explore the investigative power of litmus. Litmus is an acid/base indicator. Litmus shows red in acid in an acid environment and blue in basic environment. The behavior is brain friendly because base and blue both begin with b.

Litmus in the FieldThe 1st cell shows the reaction of

ammonia (NH3) solution. The blue reaction indicates that ammonia behaves like a base. The 2nd cell tells us sodium acetate (NaC2H3O2) is a basic salt. The NaCl solution in the 3rd well did change either the red or blue litmus. NaCl is a neutral salt. The sulfuric acid solution (H2SO4) in the 4th well turned blue litmus paper red. This confirms the acid quality. The 5th well confirms the basic nature of sodium hydroxide solution (NaOH). The 6th well confirms the acid nature of hydrochloric acid (HCl).

Phenolphthalein

When litmus is not available phenolphthalein is a good replacement CIA operative. In a basic solution the agent shows pink. In acidic or neutral solution the response is clear. Compare the data on this slide to the previous slide. The three bases all responded appropriately with a pink reaction.

Lime Water Test

Early in the year Mr. Zachmann demonstrated the use of lime water as an indicator of carbon dioxide. In the picture to the right sodium bicarbonate is reacting with hydrochloric acid to liberate carbon dioxide gas. The test tub which contains lime water responded by turning cloudy. This reaction confirms the carbon dioxide.

NaHCO3 + HCl NaCl + H2CO3

H2CO3 H2O + CO2

Ca(OH)2 + CO2 CaCO3 + H2O

Fire Extinguisher

The system from the previous slide was used to put out a candle. The CO2 was applied to the bottom of the flask. As this dense gas filled the flask the candle flickered and then went out. After this demo Zachmann discussed the death pits of Africa.

Distillation

This system was used to distill a copper sulfate solution. The clear distillate was collected in the receiver. Distillation is a physical process.

Colloids

A characteristic test for colloidal suspensions is the Tyndall Effect. Colloidal particles are big enough to reflect light but not large enough to precipitate. The visible beam in the picture is an example of the Tyndall Effect. Although the colloidal particles are neutral the skin of the colloidal particles have the same charge. This charge is what tends to keep them apart. When colloidal mud in a river hits the salt of the ocean the charge on the skin is neutralized and the colloidal particles precipitate to the bottom.

Next Year

I failed to take a picture of what I called the Atomic Twin Towers (AKA the Hoffman Apparatus). I also failed to show the reactions of hydrogen peroxide (H2O2) and the value of splint tests. Next year I will repost this instructional powerpoint with these additions. I hope viewing this powerpoint helped you to remember this activity. Many colleges will use this kind of distance learning to present mind friendly reminders of past learning experiences. As a student you want to train your mind to capture visual data.