Colligative properties are BACK!

Snowy Day in New England

Winter in New England is the perfect time to teach colligative properties. The snow plows are out nearly every week prepping the roads for another winter storm, coating everything with rock salt. Ah Ha! Freezing point depression in action.

I brought back colligative properties this year because it is in the UConn ECE curriculum. Even though the topic was removed from the AP Chemistry curriculum in the recent overhaul, the undergraduates at UConn are required to learn it so we are too. I was secretly dreading it. However, the addition of a freezing point depression lab into our schedule made it all worth it. And, thanks to Flinn, I found a few fun demonstrations for class discussion that helped to bring this mini-unit to life.

Freezing Point Depression Lab

I spent a lot of time preparing for the freezing point depression lab. I poured over the pages from the UConn lab manual, trying to adapt the procedure to the equipment and chemicals I had available at my school. This lab presented the perfect opportunity to learn how to use the Bluetooth Vernier temperature probes that my students can run from their iPads. With only minor adjustments to the procedure, an excellent pre-lab video I found online (prelab video link), and some email exchange with the lab technician at UConn, we conducted this experiment with great success. Our cooling curves were absolutely gorgeous! Never mind that the melting point of our pure solvent was not exactly accurate, more to discuss in the error analysis. We didn’t have any of the unknowns that the technician recommended for the lab, so I bought a box of (smelly) moth balls from Lowes to use for our “unknown”. The lab turned into an analysis of the technique rather than an identification lab. With our beautiful graphs, the analysis was straightforward. About half the students calculated the molar mass of the moth balls to within 10% accuracy. Not a rousing endorsement for this technique, but still a valuable lab experience that I plan to do again next year.

These students are watching their solutions freeze.

One thing I loved about the lab was the “meta-lab” experience. To record the freezing point of the solution, the students had to make an ice/salt mixture for the cold bath. They had to use the freezing point depression of water to study the freezing point depression of cyclohexane. Boom! That’s my students’ minds blowing in lab.

Ice Cube Challenge!

In addition to the freezing point lab, we did several fun demos in class related to colligative properties and solubility. The first one was the classic “lift the ice cube with a string” challenge (Flinn Scientific Publication No. 91359). Using only salt and a string, I challenged my students to  lift an ice cube off the table without touching it. It’s harder than it looks! The salt melts the ice cube just enough to freeze the string to the ice cube. It’s delicate work.

I also used the classic demonstration of the conductivity of electrolytes with the light bulb conductivity tester. This quick and easy demo shows student how to identify an ionic compound from a molecular compound, and the difference between a strong and weak electrolyte. But, this year I added another wrinkle to the demonstration. Using a solution of lime water (saturated calcium hydroxide), I attempted to “blow out” the light bulb by bubbling carbon dioxide through the solution (Flinn Publication No. 91353). This reaction was a great review of precipitation reactions, and a way to give another tangible observation to go along with the cloudy product formed. The students could see the ion concentration decrease as the light grew dimmer, while simultaneously watching the solution get more cloudy from the insoluble calcium carbonate formed.

Look at the solubility of gases at three different temperatures.

Solubility of gases is a tricky topic to throw into the mix when learning about solutions. Just when my students are getting the hand of solubility trends, I added gases which are completely opposite from solid solutes. I used a simple, yet effective, demonstration of the solubility of carbon dioxide at three different temperatures to help them understand the trend. Using a can of soda and three water baths, I showed the kids how a higher temperature causes a gas to come out of solution faster (Flinn Scientific Publication No. 91457).

Solubility of gases is a tricky topic to throw into the mix when learning about solutions. Just when my students are getting the hand of solubility trends, I added gases which are completely opposite from solid solutes. I used a simple, yet effective, demonstration of the solubility of carbon dioxide at three different temperatures to help them understand the trend. Using a can of soda and three water baths, I showed the kids how a higher temperature causes a gas to come out of solution faster (Flinn Scientific Publication No. 91457).

Super Cool Demo!

I added another fun demo to the list, even though it was a little bit of a side step from the topic. All the kids have seen videos on YouTube of water bottles freezing while being poured. The trick is to make the water super-cooled in the freezer; a very delicate state that takes the liquid below the freezing point but still in the liquid state. This can only happen with liquids that are packed under pressure. I decided to try this supercool demo with my students, (Flinn Scientific Publication No. 91605). I used small bottles of club soda that I chilled in the refrigerator overnight. To make the super-cooled liquid, I made an ice/salt bath (freezing point depression again!) to cool the soda to -8 degrees for 10 minutes. I tried this demo three times, and I only got one good freezing event to happen. The super-cooled soda has a mass freezing upon opening, that spreads from the top all the way down to the bottom of the bottle. It’s very exciting to watch the ice crystals spread throughout the soda. I was nervous about the bottles exploding if the soda actually froze, so I didn’t cool them thoroughly enough to get the freezing effect until the last try.

Success! 

The colligative properties unit was a lot of fun to teach this winter. Adding molality to our repertoire seemed worth the effort to explore the freezing point depression of water and other interesting properties of solutions.