Although it is important for students to learn to solve problems involving "the gas laws", the reason for including the study of gases in the curriculum goes far beyond this; the subject provides an ideal opportunity to illustrate, by example, how the creative use of some simple principles of mechanics can provide a surprisingly good description of the simplest state of matter. A major objective of these lessons is to help students to get a feeling for the kind of reasoning that goes into the development of the scientific model that we call the "ideal gas". Beginning students in Chemistry are notoriously hesitant to undertake the development of conceptual models of their own, preferring instead the relative comfort of learning the usual standard set problems. It is hoped that bringing the students into more intimate contact with the thinking behind the laws will build both interest and confidence.

The eleven lessons of this unit are mostly quite short, so that students can concentrate on just a few concepts at a time, and they can go through them in any sequence. Each ends with a summary screen and the display of a "score" which students are encouraged to use for self- assessment. The emphasis is very definitely on concepts rather than on learning "formulas" or the proper mathematical development of the kinetic molecular theory. The level is aimed at first-year college/university, which means that the lessons should for the most part be suitable for high school use as well. The fact that a number of the topics in the last two sections are normally considered "beyond the scope" of many introductory courses should not prejudice instructors against recommending them to their students; these topics are interesting and they introduce concepts that can be built upon later. At the other end of the scale, the lesson on atmospheric pressure and the barometer was developed in response to our experience that few of our incoming first-year university students are able to explain how a barometer works.

Throughout the lessons there is continual emphasis on predicting relationships
and on in- terpreting graphical information. This is one area in which interactive
CAI can probably do considerably more than can traditional methods of instruction.
A good example is the discussion of the three-dimensional *PVT* surface
in the fourth lesson, which is difficult to convey in lecture (students
have enough difficulty taking notes in two dimensions!), and for some reason
is considered too trivial (or intimidating?) for textbooks. The goal of
all this goes considerably beyond teaching "facts" about the properties
of gases; our hope is that these lessons can help students develop habits
of questioning and analysis that will serve as useful tools in any quantitative
subject.

This three-dimensional P-V-T surface is explored in Lesson 5.

The Properties of Gases lessons are organized into eleven sections as detailed below:

**Introduction.**Pressure as the fundamental observable property of a gas; how pressure is measured; definition and kinetic-molecular origin of gas pressure.**Effect of temperature on P and V**. How to keep the pressure constant as the temper- ature is increased. Charles'law in graphical and algebraic forms, and expressed as 1/273 change in volume per degree; extrapolation to absolute zero.**Mole-volume relations in gas-phase reactions**. Avogadro's law; comparing the volumes of equal masses of different gases. Gas-volume stoichiometry.**Pressure-volume relations**. Exploring the relation between pressure and volume; Boyle's experiment and Boyle's Law.**The ideal gas law**. the PV-product and its dependence on the temperature; combining Boyle's, Charles' and Avogadro's laws. The gas constant and its units. Exploring the PVT-surface.**Diffusion**. Kinetic energy, mass, and molecular velocities; Graham's Law. Relative diffusion rates of NH3 and HCl.**Mixtures of gases**. Partial pressure; Dalton's Law. Application to scuba diving.**Gas density, molar volume and molar mass**. Effects of P and T on the density.**Atmospheric pressure and the barometer.**The relation between pressure and altitude. Operation of mercury and water barometers.**Molecules in motion.**How velocities are averaged. The kinetic energy of a gas and the effect of collisions in redistributing molecular velocities. Understanding velocity distribution curves. Boltzmann distribution and the temperature. Average molar kinetic energy.**Real gases and intermolecular interactions**. Assumptions of the kinetic-molecular theory not applicable to real gases. Excluded volume and intermolecular attractions; the conditions under which they become significant. Understanding PV vs P plots; effects of temperature and molar mass. The van der Waals equation.