December 2012 - January 2013

# Snow exploratory

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[This collection of snow activities was published in the Colorado Chemistry Teacher’s Association (CCTA) newsletter in the winter of 2010. Using six stations, students investigate a variety of snow-like substances and have some fun with chemistry. The teacher’s notes at the end describe the setup for each station along with some classroom advice. This activity gives a general description of how one could do stations with various snow-like substances. Use the appropriate lab safety precautions, such as goggles, tying ones’ hair back, etc. The author is not known and Chem 13 News would be interested in finding out who put this collection together and credit the advice given in the teacher’s notes. If you made up this activity, or know who did it, let us know. Email Jean Hein.]

## Station 1: Silver snow (class demo/precipitation)

Observe what happens to the “tree” and the “sky” over the course of a period. Write a balanced equation for the reaction of silver nitrate with copper metal to form cupric nitrate and silver metal. Classify this equation.

## Station 2: Flaming snow (gel)

Place about 15 mL of ethanol in an evaporating dish. Add about 5 mL of the calcium acetate solution and stir. What happens? You have formed a gel: a liquid dispersed in a solid. Sprinkle a bit of strontium chloride or cupric chloride on your gel. WARNING: Be sure any open bottle of alcohol is capped before the next step. Light a wood splint and use that to ignite your “snow”. Safety precautions for an open flame must be followed: Keep all combustible materials and other chemicals away from the flame and attend to the flame at all times.

What color are the flames? Name an advantage of a solid fuel such as this Sterno, over liquid fuels such as alcohol.

## Station 3: Falling snow (solubility)

Fill a 100 mL graduated cylinder about half full with the potassium sulfate solution. This is a saturated solution. Now fill most of the rest of the graduate with 70% isopropyl alcohol. Observe. Potassium sulfate is soluble in water but insoluble in alcohol. Alcohol is soluble in water. From these facts, hypothesize why the potassium sulfate precipitates.

## Station 4: Bouncing snow (polymers I)

Pour about 20 mL of sodium silicate solution into a paper cup. Add about 5 mL of ethanol and stir with a wood splint. When the mixture gels, place it in your hands and gently press until you get a round ball that does not crumble. Be patient! You will probably need to moisten the ball by holding it in a stream of water. Investigate and write down the bouncing properties of your “snowball”. You have made a silicone polymer. Use a dictionary or your textbook to find out what a polymer is, and write down a definition in your own words.

## Station 5: Slimey snow (polymers II)

Stir together 12.5 g white glue and 20 mL water for 30 seconds in a beaker. In a paper cup dissolve 0.6 g sodium tetraborate with 15 mL water. Add 25 mL PVA to the glue mixture and stir for 1 minute. Then add the sodium tetraborate solution and stir another minute. Take the product out of the beaker and knead for 2 minutes. Test and describe 2-3 properties of your “snow”.

## Station 6: Snow in a bag (ice cream)

Make sure you have washed your hands before starting this station!

1. Fill a large zip-lock bag half full of ice.
2. Add about 6 tablespoons of rock salt.
3. In the small zip-lock bag put 1/2 cup of milk and 1 tablespoon of sugar.
4. If you are using plain milk (not chocolate milk or eggnog) add 1/4 teaspoon of vanilla.
5. Seal the small bag and place it inside the large bag, then seal the large bag.
6. Shake about 5 minutes. (Mittens are useful here!)
7. Be sure to wipe off the mouth of the small bag before you open it, or you may get salt in your ice cream.
8. Enjoy!

Why is salt added to the ice?

## Teacher’s notes

### Silver snow

First, make one “tree” out of copper wire for each of your classes. I use a piece of fairly thick copper wire, about 6” long, and bend it into a coil around a pencil. Then I gradually widen the bottom of the coil so I have a spiral cone shape. The tree must stand upright on its own.

You will need a jar (I use a battery jar or a small fishbowl) large enough for all your trees to stand in. Make enough silver nitrate (0.1 M works well) to fill the jar to a depth slightly deeper than your trees. [I do this station as a demo because it can get prohibitively expensive to make enough silver nitrate for students to do it in small groups.] Place it where you will not have to move it.

For each class, let them pick out a tree and use forceps to place the tree in the solution. Over several hours, the silver in solution will precipitate out as silver “snow” and fill in the “branches” on the tree, and the solution will turn blue as the copper reacts.

This classic single replacement is:

2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s).

Over several days the silver will darken as it tarnishes to silver oxide. The trees are very delicate and will not withstand being taken out of the solution. The trees may collapse if the wire is too thin or the solution is too concentrated, because too much of the copper infrastructure will react.

### Sterno

This gel works best with 100% ethanol, but other alcohols also work. The calcium acetate is a saturated solution, made by dissolving 35 g of calcium acetate per 100 mL of warm water. The strontium chloride and cupric chloride provide red and green coloring to the flames, which can be seen most effectively if this station is in a darkened part of the room. The chlorides can easily be left out and still make an effective station.

If the gel does not form immediately, you need more calcium acetate. Keep watch to be sure alcohol does not get splashed around, or you may find your whole lab bench aflame! This station will encourage experimentation by those who like fire, so I keep it under my closest eye during the class. You may want to keep the bottle of ethanol with you, away from the Sterno station to ensure the bottle is capped after students take the required amount.

### Solution precipitation

Available as rubbing alcohol, 70% isopropyl alcohol is available in drug stores. The saturated potassium sulfate solution is slow to make. Dissolve 15 g of K2SO4 in 100 mL deionized water for 30 minutes. Not all the solute may dissolve: decant off the saturated solution.

This station is also effective as a class demo with a giant 2000 mL graduated cylinder or hydrometer cylinder. As the isopropyl alcohol mixes with the water in the top layer of the solution, the polarity of the top layer changes and the solubility of potassium sulfate decreases. A cloud of K2SO4 precipitates, and as the K2SO4 particles aggregate they get heavy enough to fall. After all the K2SO4 has been precipitated from the top layer the “snowstorm” stops. The mixture can be stirred to mix the isopropyl alcohol with lower layers of solution: this will precipitate more K2SO4.

Some students may find themselves dreamily staring at the falling snow and losing track of time!

### Silicone polymer

Sodium silicate is available premade as a solution from chemical supply catalogs, or in stores as “water glass”. You may want to have students wear gloves as water glass has a pH of 12.

Students need to be patient: forming the balls takes time and gentleness. This station will attract your craftsmen. Once made, the balls do last well. Students can take their snowballs home in zip lock bags: they will have to wet them occasionally to keep the polymer from drying out. The silicone polymer formed is of the form O-SiR2-O-SiR2-O-, where “R” represents an ethyl group.

[Editor’s note: Consider having students return, if they want, at the end of the day to limit the possibility of a “snow-ball” fight in the hallways.]

### Slimey snow (polymers II)

The formula for Elmer's glue is a secret but it likely contains polyvinyl alcohol, PVA and polyvinyl acetate (PVAc). Here a polymer is not formed but the polymers present, crosslink as a result of the sodium tetraborate. The properties of the slimey snow are a result of the hydrogen bonds in the crosslinked polymer. These bonds are easily broken and reformed allowing the polymers to slide past each other while still being kept together.

### Ice cream

This is my students’ favorite part! You will need gallon-sized and pint-sized zip-lock bags. The large bags can be reused but the small bags should be thrown out after each use. Remember to provide spoons as well as kitchen-type measuring devices for this station. Eating and experimenting in a lab setting is controversial, and you may wish to save this activity for another day. I prefer to do it and use it as an opportunity to remind them of safety. Provide a clean space well away from the chemical hazards of the other stations. And remind students to wash hands before starting this station and to clean up after themselves. I keep my “second eye” (the one not watching the Sterno) on this station.

I have experimented with chocolate milk, white milk, and eggnog: all work well. Other flavors of milk, as well as fillers such as chocolate chips, are also possible.

I also encourage students to take this home as an activity they can share with their family over the holidays. To that end, provide extra handouts of just this “recipe”.

The ice-filled bags get quite cold. Students can bring gloves to wear or use lab towels or lab hot gloves to protect their hands.

## General notes

To keep some order in the class, I do require students to do some writing. We have both formal lab write-ups and informal “exploratories” in our chem classes: this one is of the latter types.

I require them to attach the procedures, write down all observations, and answer the questions. I grade them on a (ü+), (ü) or (ü-) system. I also find that playing soft Christmas music during the lab tends to focus students on inquiry instead of tomfoolery.

In an 80-minute period I tell students they have to get 3 stations done before they make ice cream, and have to get 4 stations (including ice cream) completed. For 40 minute periods I usually have them do the silver tree, one other station, and ice cream.

I tell the students ahead of time we are “squeezing one more lab” into the before-vacation week, but don’t tell them ahead of time about the lab or give a “prelab” assignment. For more mystery, I indicate mittens or gloves are required, but do not tell them why!

This does take a lot of set-up, and can be very difficult logistically if you’re sharing space with another teacher who’s not doing the same lab. But the smiles of the students as they head out the door (as well as the genuine investigation that happens) is worth it.

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