The Physical Behavior of Gases


Some of the earliest work leading to formulation of the fundamental laws of chemical combination (Law of Multiple Proportions, Law of Definite Composition, Law of Combining Volumes, etc.) was done with gases. In part this happened because the physical behavior of gases was already well understood (at least in an empirical way) by the very early 1800s. In fact some of the earliest experiments with gases were quite simple and yet powerful because they inevitably led to an understanding of matter as particulate and linked in its behavior to energy (generally as heat).

The work of Robert Boyle (Boyle's Law, 1660) and Jacques Charles (Charles' Law, ca. 1800) established the predictable behavior of gases under conditions of (respectively) changing pressure and temperature. Their experiments eventually led to an understanding of the nature of all gases and the development of the concept of "ideal gases".

In this activity you will have a chance to see for yourself how gases behave. Boyle's original experiment was done with a "J" shaped tube in which air was trapped by a column of mercury . To avoid the problems inherent in the use of toxic mercury, you should use an apparatus similiar to that pictured below:

The syringe is connected to a pressure gauge and about 30 mL of air has been trapped in the syringe at atmospheric pressure (~ 1 atmosphere). The gauge reads additional pressure and the inside scale is in atmospheres. By changing the volume of the trapped air sample you can determine how the pressure is affected.

The experiment by which Charles deduced the relationship between temperature and volume was also originally done with mercury and a trapped air sample. In this investigation you might consider using the apparatus pictured below:

The colored liquid is oil used in vacuum pumps with a dye added to make it more visible. A sample of air is trapped in the long side of the "J" shaped tube. The temperature of the apparatus is controlled by adding ice to the water bath. The entire assembly can be placed on a magnetic stirrer to help maintain an even temperature throughout the beaker. The ruled scale by the side of the J tube is used to measure the relative volume of the air sample (proportional to length since the cross sectional area of the tube is constant).

Design an experiment to determine the relationship between pressure and volume for a constant amount of gas and constant temperature (hint: take syringe volume readings for each increment on the pressure gauge scale; DO NOT EXCEED 2 ATMOSPHERES. That means 1.0 on the inside scale!!!!!).

Design an experiment to determine the relationship between temperature and volume for a constant amount of gas and constant pressure (hint: begin at room temperature and add ice gradually, recording the length measurement for every degree, down to about 1 oC OR recording the temperature for each mm change in length--whichever seems more practical).

Analysis

1. Plot a graph of pressure vs. volume for your first experiment (remember that the gauge readings are in addition to1 atmosphere).

a. Is the relationship linear?

b. Describe in words the apparent relationship between pressure and volume for a gas

2. Plot a graph of pressure vs. the reciprocal of the volume (i.e., calculate 1/V for each reading and plot these on a new graph--using the decimal values for 1/V--as the x-axis.

a. Is the relationship linear?

b. Describe in words the relationship between pressure and the reciprocal of volume for a gas

3. Plot a graph of volume vs. temperature for your second experiment.

a. Is the relationship linear?

b. Describe in words the apparent relationship between temperature and volume for a gas

4. Extrapolate your volume vs. temperature graph to volume = 0. What temperature do you obtain? [if you have done your graph on a computer or calculator, you can use the equation of the line to solve for this answer].

One of the results of Charles' experiment was the determination of what we today call Absolute Zero, or the lowest temperature that can be attained. This establishes the lower limit of the Kelvin temperature scale. In degrees Celsius the temperature is -273 oC. How well does this agree with your result? Suggest some reasons for differences.


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Created by James R. Fromm (jfromm@3rd1000.com)