Monday, Dec 13, 2010

Collected Wood Labs and checked off names as I collected them.

Explained how to do the Heating and Cooling of an Unknown substance lab.
Described set-up with both a picture and using the sample set-up.

Wrote procedure on board:
1. Prepare data table to take time and temperature measurements every 30 seconds for about 40 minutes.
2. Put 150 ml of water in the 250 ml beaker and place on the hot plate
3. Attach a test tube clamp to the ring stand and clamp on test tube with unknown substance and thermometer.
4. Place the test tube in the water so that it is about 1 cm above the bottom of the beaker.
5. Take time and temperature. Then turn on hot plate to high and continue to take time and temperature every 30 seconds (every half a minute)
6. When the temperature of the substance is 75 deg C, turn off hot plate, lift test tube out of water and swing to side, and continue to take time and temperature measurements every half a minute until the substance cools to 40 deg C.

Students are to make a graph of their data and annotate it showing what is happening in each stage, drawing horizontal lines to show melting and freezing points, and writing those melting and freezing temperatures on the graph.

On the reverse side of the paper, students make the following observations:
1. Describe the substance in the solid form before starting to heat it.
2. At what temperature does the substance begin to melt?
3. Does the substance begin to melt from the top down or bottom up?
4. Is the substance more dense in the liquid or solid phase and how can you tell?
5. Describe the substance in its liquid form.
6. At what temperature does the substance begin to freeze?
7. Does the substance begin to freeze from the bottom up or from the top down?

I should have also asked the students to record the temperature at which the substance appeared to be completely melted.

Normally I do this lab with a test tube in the water as well but I decided to omit this complication.

When students swung out their test tubes, I gathered up their hot water and unplugged their hot plates.

After students had a long plateau, I put their test tubes in a beaker of cold tap water to speed up the cooling process and get the "tail" on the curve.

I showed students how to set up their axes, landscape, 20 deg C to 90 deg C by 10 deg C on the y-axis and each division is 1 minute on the x-axis.

Many students did not complete their graph so I asked them to make sure they finished it for the start of tomorrow's class.