Charles' Law of Gases A Simple Experimental Demonstration John T. Petty Alabama School of Mathematics and Science, 1255 Dauphin Way, Mobile, AL 36604 Of all the simple gas laws, Charles'law, which relates the volume of a gas to its temperature, is one that lends itself to a simple demonstration ( I ) . The trend is clear; all that is needed is some method of measuring the amount of expansion or contraction in order to obtain a quantiative result. Experimental Charles' law states that for a sample of gas a t constant pressure, a change i n temperature results in a proportional change in the volume. In this experiment, air is used to test Charles'law. Assemble the apparatus shown i n the figure. Clamp the graduated cylinder so i t is held vertically upside down and half submerged i n the beaker full of water. After the system i s assembled raise the graduated cylinder about 25 mm and let i t stand for a minute. If the system is air tight, i t will support a column of water. If the water level falls, check the system for leaks before continuing. Once the system is air tight, remove the pipet bulb, squeeze i t and reconnect it to the system. Slowly release the bulb. The final water level should read between 30 and 40 mL on the graduated cylinder. Remember when reading the scale, that the graduate cylinder is upside down. If the column rises above the 30-mL mark, let some air into the system until the level is within the indicated range. To start the experiment, place the flask in a n ice bath for 5 min. The level should not be allowed to rise above either the calibration marks on the cylinder or the top of the glass tubing. Record the reading on the graduated cylinder (R1) and the temperature of the ice bath (TI). Pairs of data points are obtained by heating the water bath to some etc.) and then recording the other temperature (Tz, T3, Tg, new water level shown on the graduated cylinder (Rz, R3, Rpetc.) a t that temperature. After sufficient data has been recorded, weigh the flask empty and then full of water and take the difference. Convert the grams of water needed to fill the flask to its volume by dividing by water's density 1.00 gImL. This i s used a s the starting volume (V1) of the gas a t TI. To obtain the volumes of gas a t the other temperatures, add the change in volume (the difference in the readings on the graduated cylinder) for that temperature to the original volume of air in the flask. Vz= V, + (Rz-Rl) V3=V1+(R3-R,) etc.
As a conclusion, plot gas volume (VI, VZ,V3 etc.) versus temperature on a graph with the scales set a t 0-300 mL and -350 to +I00 "C. Extrapolate a "best fit" straight line through the data points to a volume of "0" mL to yield a value for absolute zero. Discussion This experiment is designed to be easy to do and requires no special equipment, although i t is a rather inelegant use
1W mL gmduated
Clamp
----800 mL beaker
10W mL
0
beaker
Apparatus for Charles' law experiment. of a graduated cylinder. This experiment also can be done using 25-mL or 50-mL graduated cylinders with a 125-mL Erlenmeyer flask a s the gas container without seriously effecting its accuracy. If a beaker of sufficient depth to immerse the graduated cylinder is available, the overall accuracy of the data can be improved by raising or lowering the graduated cylinder to match the water levels inside and out before taking the volume readings. If this is not possible then the difference in water levels will introduce an error of about 1% into the results. Once the apparatus is assembled any number of data points can be taken, although four should be the minimum Hnd they should he at least'l0 "C apart. Because only the gas actually i n the flask is being expanded each time, t h e plot of the d a t a points will be sliehtlv curved. A more accurate. but more involved. method for handling the data would he to solve the equa: s average the retion for each successive air of ~ o i n t and sulting values for X.
Here X represents the temperature below 0 "C that a scale must start in order to give the correct expansion for each set of points, in other words absolute zero. I have used this experiment for several years both in teaching general chemistry a t the college level and AP and general chemistry a t the high school level. Results in the area of -270 10 "C are normal and even the poorer students get answers between -250 and -300 "C. This experiment serves several useful purposes. I t reinforces the student's intuitive feel for Charles' law with quantitative numbers they can see. I t introduces the idea of extrapolating experimental data to obtain a theoretical value and gives a physical quantitative meaning to the concept of absolute zero. Literature Cited 1. Haworth. D. T. J Ckem Educ 1967.44.353
Volume 72 Number 3 March 1995
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