Chemistry/Chem 11 Demos

FIRE WRITING

TOPIC

• Introductory demonstration 1
  

EQUIPMENT

• Large sheet of porous (non–coated) paper to make a “WELCOME” sign
• Cotton swab
• Tape (to suspend paper from blackboard)
• Matches
• Wooden splint

CHEMICALS

• 100 mL of saturated potassium nitrate (KNO₃)

PROCEDURE Wet a cotton swab with potassium nitrate and use the swab to write ”WELCOME” on the paper. The letters must be continuous and at least 3/8” wide. Allow the paper to dry thoroughly. On the day of the demonstration, suspend the paper from a blackboard with tape. Touch a glowing hot wooden splint to the start of the message. The message will be traced out by the glowing paper.

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INVISIBLE INK MESSAGE

TOPIC

• Introductory demonstration 2
  

EQUIPMENT

• long sheet of absorbent paper
• large piece of cardboard (larger than the sheet of paper)
• adhesive tape
• 2 – atomizer spray bottles
• paint brush (1/2” to 1” wide)
• 3 – large beakers

CHEMICALS

• 10 g of ferric chloride
• 2 g of ammonium thiocyanate
• 5 g of potassium ferrocyanide (CARE: not ferricyanide)

PROCEDURE Dissolve 10 g of ferric chloride in 200 mL of water. Use this solution to paint a welcoming message such as “ CHEMISTRY RULES!” on the paper, in such a way that “CHEMISTRY” is separated from “RULES!” by at least 8–10 inches, and allow to dry overnight. Using adhesive tape, attach the paper to the large sheet of cardboard to act as a stiff backing. Dissolve 2 g of ammonium thiocyanate in 200 mL of water and put into one atomizer. Dissolve 5 g of potassium ferrocyanide in 200 mL of water and put into the second atomizer. Spray the left side of the message with potassium ferrocyanide to reveal the word “CHEMISTRY” in dark blue. Spray the right side of the message with ammonium thiocyanate to reveal the word “RULES!” in blood–red colour.

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POP BOTTLE CANNON

TOPIC

• Introductory demonstration 3

EQUIPMENT

• 750 mL plastic pop bottle (no bigger!)
• 10 cm x 10 cm square of aluminum foil
• rubber stopper to fit bottle
• tesla coil

CHEMICALS

• methanol

PROCEDURE Roll up the aluminum foil into a tube and insert into the pop bottle. Add about 5 mL of methanol to the bottle, stopper and shake a few times. Turn on the tesla coil and let a spark jump from the tip of the coil to the piece of aluminum foil inside the bottle. A satisfying explosion will bounce the rubber stopper off the ceiling. Use this to get the attention of the class on the first day. Three such bottles make a “three gun salute” to welcome students to Chemistry 11. The methanol vapour and oxygen form an explosive mixture which is ignited by the spark. Before you decide to make it “bigger and better” you should know that adding pure oxygen to the bottle gives a blast which is almost deafening and is strongly advised against because it shreds the bottle into fragments which can cut students standing 20 feet away.

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A SIMULATED “ACID IN YOUR EYE” ACCIDENT

TOPIC

• The need for safety goggles

EQUIPMENT

• Large petri plate
• Overhead projector
• Permanent marker (black preferred)
• Dropping pipet

CHEMICALS

• 6 M HCl (hydrochloric acid)
• Raw egg (white only)
• 50 mL of saturated NaHCO₃ (sodium bicarbonate)

PROCEDURE Draw a large eye on the bottom of the petri plate, place the petri plate on the overhead projector and place the egg white in the petri plate. [Both egg white and the eye’s pupil are protein gels.] Place several drops of acid on the egg white; it instantly becomes opaque. Adding some baking soda neutralizes the acid but does not undo the damage to the egg white. NaOH solutions work the same way but the opaque area continues to expand for several hours!

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WHY WORRY ABOUT SAFETY?

IRA REMSEN’S FIRST EXPERIMENT

TOPIC

• Working with dangerous chemicals

EQUIPMENT

• 1 L flask, with a stand and clamp to hold the flask
• one hole stopper to fit flask, fitted with a glass tube and rubber tubing
• 1 L graduated cylinder and 40 cm of 6-8 mm OD glass tubing
• 100 mL graduated cylinder (for nitric acid)
• safety goggles

CHEMICALS

• penny
• 60 mL concentrated nitric acid

PROCEDURE Almost fill the 1 L graduated cylinder with water and place the 40 cm glass tube into the cylinder. The 40 cm glass tube should go to the bottom of the 1 L graduated cylinder when connected to the rubber tubing leading to the flask. Set up the stand and clamp so the flask can be supported vertically while allowing the rubber tubing to connect to the glass tube in the graduated cylinder. [Read the following to the class. Tell them that they must record all their observations.] Ira Remsen was a 19th century chemist who recorded his observations on the first experiment he ever did, before he had learned anything about safety in the Chemistry laboratory. He wrote While reading a textbook of chemistry, I came upon the statement “nitric acid acts upon copper.” I was getting tired of reading such absurd stuff, and I was determined to see what it meant. Copper was more or less familiar to me, since copper cents were then in use. [Hold up a copper penny] I had seen a bottle marked nitric acid on a table in the doctor’s office where I was then “doing time.” [Put on goggles and hold up a stock bottle of concentrated nitric acid. Read some of the warnings on the side.] I did not know its peculiarities, but the spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words “act upon” meant. The statement “nitric acid acts upon copper” would be something more than mere words. In the interest of knowledge, I was even willing to sacrifice one of the few copper cents then in my possession. I put one of them on the table, opened the bottle marked nitric acid, poured some of the liquid on the copper and prepared to make an observation. [Explain that for safety reasons you are doing the experiment a bit differently than Remsen did. Remove the stopper and pour about 60 mL of acid into the flask. Tilt the flask and slide in the penny. Replace the stopper and clamp the flask in place. Allow 4-5 students at a time to get a closer look. After a couple of minutes, continue reading.] But what was this wonderful thing which I beheld? The cent had already changed, and it was no small change either! A green–blue liquid foamed over the cent and over the table. The air in the neighbourhood of the performance became coloured dark red. A great coloured cloud arose. This was disagreeable and suffocating. How should I stop this? I tried to get rid of the objectionable mess by picking it up and throwing it out of the window. I learned another fact. Nitric acid not only acts upon copper, but it acts upon fingers. The pain lead to another unpremeditated experiment. I drew my fingers across my trousers and another fact was discovered. Nitric acid acts upon trousers. Taking everything into consideration, that was the most impressive experiment and, relatively, probably the most costly experiment I have ever performed. It was a revelation to me. It resulted in a desire on my part to learn more about that remarkable kind of action. Plainly, the only way to learn about it was to see its results, to experiment, to work in a laboratory. [When the reaction has stopped, ask one student to describe the penny – its gone – and have students hypothesize where it went. After 5–10 minutes another observation is made: the water travels up the glass tube and into the flask. The resulting solution in the flask is turquoise blue and the brown gas in the flask fades.]

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INDICATOR SPONGE

TOPIC

• The need for clean work benches

EQUIPMENT

• Cellulose sponge
• Rubber gloves
• Large container to hold sponge and solution

CHEMICALS

• 1 g of congo red indicator
• 100 mL distilled water
• 100 mL of 1 M acetic acid
• 100 mL of saturated sodium hydrogen carbonate (about 10 g / 100 mL)

PROCEDURE Dissolve about 1 g of congo red in about 100 mL of distilled water. Soak the sponge with the congo red solution and ring out the sponge using rubber gloves. Let the sponge sit in the liquid overnight, squeeze out the liquid and allow to dry completely. Rinse with fresh water a few times and the sponge is ready for use. Students are often careless about cleaning up their benches, especially at the start of the year. Clean up a little 1 M acetic acid and saturated sodium hydrogen carbonate from the bench to show students what happens when the sponge is used to clean up acidic or basic solutions.

U–TUBE WITH UNEQUAL ARMS

TOPIC

• Density of water vs alcohol

EQUIPMENT

• Big U-tube (14–18 mm glass tubing or 1/2” tygon tubing, about 18 in high)
• stand and 2 clamps

CHEMICALS

• ethanol or methanol
• water

PROCEDURE Clamp the U–tube with the bend downward. Fill the tube about 1/2 full of water and then carefully, so as to minimize mixing, pour alcohol into one side arm until the tube is almost full. The result will be a U–tube having liquid at a higher level on one side than on the other. Demonstrate that there is no blockage by gently blowing into one side arm and showing that the liquid can move freely. What is Happening: Normally, a U-tube has liquid at the same height in each arm because the mass of the liquid in each arm pushes down equally (otherwise the liquid would move) and therefore requires equal heights of liquids. The alcohol has a lower density than the water and therefore a greater volume of alcohol is required to have the same mass as a lesser amount of water. Hence, more alcohol must “pile up” to equal the downward push of the smaller amount of water.

THE MYSTERIOUS SUNKEN ICE CUBE

TOPIC

• Density of ice vs water or alcohol

EQUIPMENT

• 2 – 250 mL beakers

CHEMICALS

• two ice cubes
• 500 mL of ethanol or methanol
• 500 mL of distilled water

PROCEDURE Half–fill one beaker with alcohol and half–fill the other with distilled water before presenting the beakers to the class. Have a student drop one ice cube into each beaker. The ice in water floats (density of ice = 0.9 g/mL, density of water = 1.0 g/mL) whereas the ice in alcohol sinks (density of alcohol = 0.79 g/mL). Ask students to explain what must be happening.