General Chemistry Principles, Patterns, and Applications, 2011

5. The molecular properties of a substance control its state of matter under a given set of conditions, not the
6.
other way around. The presence of strong intermolecular forces favors a condensed state of matter (liquid or
solid), while very weak intermolecular interaction favor the gaseous state. In addition, the shape of the
molecules dictates whether a condensed phase is a liquid or a solid.
7. Elements that exist as gases are mainly found in the upper right corner and on the right side of the periodic
table. The following elements exist as gases: H, He, N, O, F, Ne, Cl, Ar, Kr, Xe, and Rn. Thus, half of the
halogens, all of the noble gases, and the lightest chalcogens and picnogens are gases. Of these, all except the
noble gases exist as diatomic molecules. Only two elements exist as liquids at a normal room temperature of
20°C–25°C: mercury and bromine. The upper right portion of the periodic table also includes most of the
elements whose binary hydrides are gases. In addition, the binary hydrides of the elements of Groups 14–16
are gases.
[1] The distinction between a gas and a vapor is subtle: the term vapor refers to the gaseous
form of a substance that is a liquid or a solid under normal conditions (25°C, 1.0 atm). Nitrogen
(N 2) and oxygen (O 2) are thus referred to as gases, but gaseous water in the atmosphere is called
water vapor.
10.2 Gas Pressure
L E A R N I N G O B JE C T I V E
1. To describe and measure the pressure of a gas.
At the macroscopic level, a complete physical description of a sample of a gas requires four
quantities: temperature (expressed in kelvins), volume (expressed in liters),amount (expressed in moles),
and pressure (in atmospheres). As we explain in this section and Section 10.3 "Relationships among Pressure,
Temperature, Volume, and Amount", these variables are not independent. If we know the values of any three of these
quantities, we can calculate the fourth and thereby obtain a full physical description of the gas. Temperature, volume,
and amount have been discussed in previous chapters. We now discuss pressure and its units of measurement.
Units of Pressure
Any object, whether it is your computer, a person, or a sample of gas, exerts a force on any surface with
which it comes in contact. The air in a balloon, for example, exerts a force against the interior surface of
the balloon, and a liquid injected into a mold exerts a force against the interior surface of the mold, just as
a chair exerts a force against the floor because of its mass and the effects of gravity. If the air in a balloon
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