14.3. Gas Mixtures http://www.ck12.org
chemical makeup of the individual particles. However, not all properties are like this. For example, the particles of
different gases have different average speeds, even if the samples are kept at the same temperature. In this lesson,
we will examine how molecular speed is determined for a given sample and use this information to investigate what
happens to gas behavior when molecular speeds vary.
Gas Mixtures
Gases are frequently encountered as mixtures. For example, the air we breathe is a mixture of several gases. It
is mostly nitrogen and oxygen, but water vapor, argon, carbon dioxide, and various other gases are also present in
small amounts. Suppose we place a sample of air in a rigid container. If we were to visualize what is happening
at the molecular level, we would see various gas particles mixed together in a homogeneous fashion. Each gas is
evenly distributed throughout the entire container, just as they would be if there were no other substances present.
The following simulation will help us visualize this setup: http://phet.colorado.edu/en/simulation/gas-properties.
Based on your observations of the above simulation, would you say the mixture of heavy and light gases was
homogeneous or heterogeneous? Were the collisions elastic? What are the similarities and differences between the
heavy gas particles and light gas particles?
Dalton’s Law of Partial Pressures
In our hypothetical gas mixture, we could measure the total pressure exerted by the mixture using a barometer or
other instruments. However, we cannot directly measure the pressure exerted by each individual component. How
can we determine this information? The concept ofpartial pressureswas first described in 1801 by John Dalton
(1777-1844).Dalton’s law of partial pressuresstates that if two or more gases are mixed in a container, each one
will exert the same amount of pressure as it would if it were by itself in a container of the same size. As a result, the
total pressure of the mixture will be the sum of the partial pressures. For example, here is an image that shows what
happens at the molecular level when two different gases are mixed.