Today is 22 July 2009 -- We continued exploration of Boyle's law

We used mini marshmallows and a 60 ml syringe to study pressure/volume relationships.  When we put our thumbs over the nozzle and pressed on the plunger, the marshmallow shrank.  This had to be because the marshmallow was full of air.  When we pulled a vacuum on the syringe, the marshmallow grew larger because the air in it expanded.  This really demonstrated the inverse relation on a fixed amount of gas by volume and pressure.  If the pressure goes up, the volume does down, and vice versa. 

Students were asked to write up a paragraph reflecting their understanding of this principle. 

Then we explored pressure itself.  We made a water barometer.  Students had to understand that mercury, used in barometers since the time of Torricelli, are prohibited in schools.  Mercury, being 13.543 times as dense as water can be used in a glass column 760 mm long to measure the pressure of the atmosphere.  In other words, a column of mercury 760 mm long will balance the pressure of the atmosphere.  You learned that the weight of a column of air one inch square, from sea level to the stratosphere weighs 14.7 lbs.  We call that "air pressure" and it is stated as lbs per sq. inch.  Since we can't use mercury, we used water.  Students found that the column of water required to balance the atmosphere was about 33 feet.  (760 mm x 13.534 g/cc)/(25.4 mm per inch x 12 inches per foot).  We took a length of plastic tubing 35 feet long, filled it with water and hung it from the 3rd floor landing.  The top was sealed and the bottom was in a bucket of water.  The water column in the tube fell to 33 feet, creating a vacuum in the top of the tube.  The students were told to follow a specified report format.

We then started talking about Charles' law which relates temperature and volume.  The students saw a demonstration of what happens when a balloon is fitted to a flask, and the flask is heated.  The air inside the flask expands, forcing the balloon to inflate.  When water is put in the flask, the balloon expands even more as the water is vaporized. 

Students had a homework assignment to express the temperatures 212, 32 and 800 degrees F in Kelvins.  They were shown the formulas:  K = degrees C + 273.15;  degrees C = (degrees F - 32) x (5/9).

Tomorrow we will continue with Charles' law, solving problems.  We would also like to do a lab on finding absolute zero.