## Tuesday, October 23, 2012

Last week we made thermometers in fourth grade.  One of the standards is to understand that temperature is the measure of thermal or heat energy in the atmosphere.  The students are required to use a thermometer to compare air temperatures over a given time period.   So, to help them better understand thermometers and the idea of force, we have them build a thermometer.  There are lots of directions on the web, but I started with the Teach Engineering site.  The activity is directed toward fifth graders, and is mainly about making a scale and comparing their own scale to a celsius or fahrenheit thermometer.  However, we concentrate just on making the thermometer itself and getting the students to explain to me exactly how it works.  Here is a great photo of a working thermometer and an enthusiastic student, excited to see the apparatus working.

You start with a plastic soda or gatorade bottle.  If you use the gatorade bottle, you need to drill a hole in the cap to insert the straw.  For the soda bottle, just use the bottle as is without the cap.  CAUTION:  water bottles will not work!  We found out the hard way that water bottles will not work.  The manufacturers have taken so much plastic out of the water bottles that they are very thin walled.  The walls are not strong enough to resist the pressure changes and they will not work.

You will also need a mixture of rubbing alcohol (1 to 1 ratio), a straw, and clay.  I also have a bucket of ice on hand, containers for iced water and very warm water, say 110 degrees or so.

Here is copy of the sheet that I use:

EaSiEE as PiTM
Engineering and Science in Elementary Education

Thermometers

1. Place the straw in the bottle with the alcohol/water mixture but do not let it touch the bottom.

2. Use clay to seal the neck of the bottle by making a snake out of the clay, wrapping it around the straw and putting it on the opening of the bottle.  DO NOT shove the clay in the bottle, but just seal the top and around the straw.

You now have a homemade thermometer.  Test it and see if it works.

1. Cup your hands around the bottom of the bottle and see if the water/alcohol in the straw rises.  What is the approximate temperature of your body?  What is approximate room temperature?  What is the difference between the two?

If your thermometer doesn’t work, what could be wrong?  What are the three parts of the design.  What do you think you need to fix?

1. Put the bottom of the bottle in warm water, what happens?  What happens if you put it in ice water.  Keep observing the bottle, does something interesting happen in the alcohol/water mixture?

1. Why does the water/alcohol in the straw go up and down with temperature?

4.  Did you see bubbles in the water/alcohol mixture when you put it in the ice water?  Where did the bubbles come from?  Can you explain the phenomena?

Here are some students, getting the straw to the right height, and one of our wonderful volunteers looking on.

Here is a good photo of a thermometer working.  I color the water and alcohol mixture so that it is easier to see.  It is not easy to find clear straws.  Usually, you have to purchase them at a restaurant supply store.  But be careful, I did purchase some that were clear, the only ones they had, but they were brittle, so we had to go with the light colored straws.  The white ones that you can find are usually a bit opaque and not great for this application.  You can see the clay at the top of the bottle around the straw.  Also, this time, I could only find the straws with the flexible section.  You can cut them off, but it often makes them too short for the bottles.  Instead, have them place the flexible piece down inside the bottle. If you place it at the top, it often results in an air leak and a non-working thermometer.

The key to this device is the seal.  If the seal is not airtight, then the thermometer does not work.  You can see that the water/alcohol will rise just by heating with the hands.  If the water/alcohol does not rise in fairly short order, then there is a leak at the top.  Caution the students though, that if they squeeze the bottle, the mixture will shoot out the top!

What is happening?  The system is sealed, when working properly.  As you heat the alcohol/water mixture, the mixture expands, but it cannot expand into the air in the bottle because there is nowhere for the air to go, so the only place the water/alcohol mixture can go is up the straw!  With a little coaching and some leading questions, the students are able to figure this out on their own.

This is a fun activity, and all of them enjoy it.  There is always a bit of excitement when they work!