10 – 1
Properties of Pure Substances
Observe, and in some cases quantify, a number of physical properties of pure substances:
boiling, freezing, sublimation, and triple-point behavior.
The properties of a substance are those characteristics used to describe or identify it.
Properties can be classified as either physical or chemical. Physical properties such as color,
odor, amount, temperature, viscosity, and boiling point can be determined without altering the
chemical composition of the sample: boiling water results in the conversion of liquid water to
water vapor, a change in the physical state of the substance, yet at the molecular level both the
liquid and the gas are composed of H
O molecules. Chemical properties, conversely, do change
the chemical composition of a sample: the combustibility of molecular hydrogen (H
) results in
its reaction with oxygen (O
) to form a new substance, water.
This experiment is concerned with physical properties of substances. More specifically,
changes in the physical properties of substances will be observed in this experiment: the freezing
-butanol and the sublimation of carbon dioxide are two of the physical changes that will be
observed. The conditions at which each change occurs can be used to characterize the substance
being observed. For example, the temperature-pressure conditions under which carbon dioxide
sublimes are characteristic of carbon dioxide. Understanding the relationship between these
external conditions and the physical properties of pure substances is the goal of this experiment.
Part A: Phase Behavior of Water, Carbon Dioxide, and t-Butanol
Vapor pressure is a manifestation of the tendency of a liquid or solid to become gaseous.
The vaporization of a liquid is called
. The vaporization of a solid is called
. For any pure substance there are certain conditions of temperature and pressure at
which the tendency of the substance to change state in one direction (e.g., liquid to gas) is
exactly balanced by its tendency to change state in the opposite direction (gas to liquid). Under
these special conditions, a state of equilibrium exists between the two phases. For a pure
substance, such as water, there is a set of pressure-temperature points at each of which there is
equilibrium between the gaseous and liquid phases. Since the liquid and gaseous phases coexist
at these points, they are called
liquid-gas coexistence points.
The lines connecting coexistence
points are called
(see Figure 10.1 on the following page). Each of the points
along a liquid-gas coexistence curve is a boiling point; that is, the liquid will boil when the
prevailing pressure is less than the equilibrium vapor pressure at that temperature. The particular
boiling-point temperature that corresponds to an equilibrium vapor pressure of exactly 1
atmosphere is called the
normal boiling point.
For pure water this temperature is exactly