Properties of Gases
Why does the average person often overlook the presence of gases? Probably because the properties of gases are so unobtrusive. All gases are transparent, and most are colorless. The major exceptions to the second half of this rule are fluorine, F2, and chlorine, Cl2, which are pale yellow-green; bromine, Br2, and nitrogen dioxide, NO2, which are reddish brown; and iodine, I2, which is violet.
Another important property of all gases is their mobility. Every gas will disperse to fill all space, unless prevented from doing so by a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. or liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container barrier or a force. (The force of earth’s gravity, for example, prevents air from escaping our planet.) Moreover, gases are capable of escaping through small holes (pores) in barriers such as plaster of paris or a balloon, even though the human eye sees such materials as continuous and impenetrable. The mobility of gases is also demonstrated by the minimal resistance they present to objects moving through them. You can wave your hand through air much more easily than you can through any liquid.
A third general characteristic of gases is their wide variation in densityThe ratio of the mass of a sample of a material to its volume. under various conditions. Densities of solids and liquids change by only a few percent when temperatureA physical property that indicates whether one object can transfer thermal energy to another object. or pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container. is doubled or halved. Similar changes in the conditions of a gas can alter its density by a factor of 2. This occurs because the volume of any gas increases greatly with an increase in temperature or with a reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. in pressure.