0.13 Physical properties of gases

Introduction

The individual molecules of different compounds have characteristic properties, such as mass, structure, geometry, bond energies, bond lengths, bond angles, polarity, diamagnetism, or paramagnetism. We have not yet considered the properties of mass quantities of matter, such as density, phase (solid, liquid or gas) at room temperature, boiling and melting points, reactivity, and so forth. These are properties which are not exhibited by individual molecules. It makes no sense to ask what the boiling point of one molecule is, nor does an individual molecule exist as a gas, solid, or liquid. However, we do expect that these material or bulk properties are related to the properties of the individual molecules. Our ultimate goal is to relate the properties of the atoms and molecules to the properties of the materials which they comprise. This is where our work on the properties of individual atoms and molecules begins to pay big dividends in our understanding of matter.

This goal is, in general, very difficult to achieve both conceptually and quantitatively, and for many substances, relating the molecular properties to bulk properties is an active area of research. But there are many substances and properties for which we can make conceptual progress, and in some cases, we can even develop quantitative relationships. In the next Concept Development Study, we will study the relationship between the properties of gases and the properties of the individual particles which make up gases. In this Concept Development Study, we lay the groundwork for that study by analyzing the physical properties of gases, including pressure, density, and temperature.

Foundation

We assume as our starting point the Atomic Molecular Theory, which states that all matter is composed of discrete particles. The elements consist of identical atoms, and compounds consist of identical molecules, which are particles containing small whole number ratios of atoms. We also assume that we have determined a complete set of relative atomic weights, allowing us to determine the molecular formula for any compound.

Of particular use will be Avogadro’s Law, which states that equal volumes of gases contain equal numbers of gas particles, regardless of the type of gas particles, provided that the volumes are taken at equal pressure and temperature. You may recall that Avogadro based this statement on the Law of Combining Volumes.

Observation 1: pressure-volume measurements on gases

It is an everyday observation that air has a "spring" to it: if you squeeze a balloon, the balloon rebounds to its original shape. As you pump air into a bicycle tire, the air pushes back against the piston of the pump. Furthermore, this resistance of the air against the piston clearly increases as the piston is pushed farther in. The “spring” of the air is measured as a pressure , where the pressure P is defined:

P = F/A

F is the force exerted by the air on the surface of the piston head, and A is the surface area of the piston head.