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December 2016

Honors Chemistry 16 – Molarity & Solutions

img_6793Today we did the Molarity Calculation Lab from Ian Guch’s 24 Lessons That Rocked the World.  Students were given a beaker of 2.0 M (2.0 molar ) solution of NaCl (salt) and asked to produce 1.0 grams of NaCl.  They had to calculate how much of the solution to put in a small beaker, that would produce 1.0 gram of salt after boiling away the water.  They also needed to estimate how much they might lose in the process of heating (salt sputtering out of container during heating) and adjust their initial amount of solution.  One group asked if they could use an evaporating dish with lid to keep from loosing product and another group choose a bigger beaker after watching the first group lose some salt to sputtering.    img_6798The groups got pretty good results and this was a nice short lab for the last class before break.

I also set up a hot ice demonstration with a supersaturated solution of sodium acetate.  I prepared the supersaturated solution the night before, heating 160g of sodium acetate with 30 ml of water in a beaker on a hot plate.  A supersaturated solution contains more solute then it really wants to, so any little perturbation can cause it to crystallize and unfortunately my solution did this right before class.  I reheated it but it kept crystalizing in the beaker or was still too warm to crystallize instantly when poured.  But we still had fun watching it crystallize and got some cool crystals.img_6812

Here are some videos showing how the demo is supposed to work.

And here are the videos I asked the students to watch before class:

Intro Chem 14 -Dissolving stuff in Water

The labs we did this week all came from Chapter 5 of the American Chemical Society’s free Middle School Chemistry curriculum.  The first activity involved cutting out sodium (Na) and chloride (Cl) ions and water molecules. The curriculum has a page of them to print, but before I printed them out for the students I went a head and colored them on the computer to save some time.  Students cut them out and arranged the Na and Cl ions to form a salt crystal.  I had used this activity a few years ago and still had a set that I had taped to little magnets so I could arrange the ions/molecules on my white board, so the kids could follow along with them.   Once we had the NaCl crystal we ‘dissolved’ it in water by bringing the water molecules up to the crystal and the surrounding the different ions.  The positive side (H+) of the water molecules are attracted to the Cl- ion and the negative (O-) side of the water molecules are attracted to the Na+ ions.  I challenged the students to take the paper models home and make stop motion movies of the water molecules dissolving the NaCl.  Some of the students taped the  ‘dissolved’ Na and Cl ions surrounded by the polar water molecules into their notebooks.img_6778

The next activity in the curriculum asks the students to find out whether isopropyl alcohol or water would be better for dissolving salt.  So we talked about what experiment we could do to figure that out.  Should we use a lot of salt or a little?  Should we use a liter of water or a smaller amount?   Should we use the same amount of alcohol and water?  Should we stir for the same amount of time?  I wrote down what we came up with on a marker board and then the students did the experiment.  They got two cups, labeled one water and one iso, then measured 5 grams of salt for each cup.  They started a timer and poured 50 ml of water into one of the cups and started stirring.  When all the salt was dissolved they wrote down the time.  Then they did the same with the alcohol, but found salt would dissolve in the alcohol.   I asked them what they thought about that and with some prodding realized the alcohol molecules are not as polar as water so they aren’t strong enough to pull apart the Na and Cl ions.  I had them write down their procedures and results in their books.

We still had some time (guess I should have let them color the molecules themselves) so we did the next activity which was dissolving baking soda in water – does it dissolve as well as salt?  They basically repeated the above experiment, 5 g of baking soda in 50 ml of water and found it didn’t dissolve as easily as the salt.

We then watched a couple of  movies. I’m pretty sure I had already showed the Dogs teaching chemistry but since we’ve been talking about covalent and ionic bonds a lot I decided to show it again.

Then I showed Tyler Dewitt’s video on “What Happens When Stuff Dissolves?”

 

Honors Chemistry 15 – Charles’ Law

Last week we looked at Boyle’s law  – holding a sample of gas at constant temperature while varying the pressure and watching the volume change.  Today we investigated Charles’ Law by keeping the pressure constant and changing the temperature so we could see how the volume would vary.  We used the same 10ml syringes that we used last week but tried to do a better job lubricating the plungers with vegetable oil.  They still seem to get stuck so the students gave them a bit of push/pull/twist to free them and then measured where the plunger came to rest.

img_6574The lab is from the Home Scientist Chemistry kit manual, CK01A, Session VIII-2: Observe the Volume-Temperature Relationship of Gases (Charles’ Law).  To get a range of temperatures we put the syringes and thermometers in my kitchen freezer which got us to -12C,  an ice/water bath, boiling water and cups of water heated in the microwave to get a few points in between.  The students made sure to reset the syringe before each data point and the results of this experiment came out much better than last weeks.  The data analysis was a lot easier in this lab since the only math was converting their measured temperatures in Celsius to Kelvin.   You can see from one set of results below that the data came out very linear, as you increase the temperature of a gas the volume goes up as expected.  Since this was a relatively short lab I talked about percent error a bit before we started and showed them how to put error bars on their data points.  I also had the students graph their data as they went so they could see what temperatures they might want to do to fill in gaps in their graph.  This also allowed them to see when something might have gone wrong (plunger stuck) and they needed to retake a data point.

IMG_6581.jpg

If I only had time for one of these gas law experiments I would definitely recommend this one over the Boyle’s law lab.

Intro Chem 13 – Water

img_6563We started class by watching some videos on youtube, mainly slow motion videos of water balloons popping and the Amoeba Sisters video explaining how the special properties of water, like surface tension are due to the polarity of water molecules.

After the movies the kids floated paper clips in a glass of water. They had to be careful putting them on the surface or they would sink.  Then they used disposable pipettes to see how many drops of water they could put on a penny before it over flowed.  They repeated the same experiment with isopropyl alcohol and saw they got fewer drops to stay on the penny.  And finally they saw that detergent could break the surface tension on the pennies and make the paperclips sink in the cup.  Here are some slow motion videos we took today.

Slow motion movie from class of isopropyl alcohol drops on penny already covered with isopropyl alcohol.

Sticking a toothpick in a big water drop on a penny.

Breaking the surface tension with a toothpick dipped in dishsoap.

The activies we did today are described in Chapter 5 of the Middle School Chemistry curriculum by the American Chemical Society and you can also see them in Physics Girl’s video below.

I highly recommend doing the milk one at the beginning. Its a lot of fun.

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