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From: cmpbsdb@gitpyr.UUCP (Don Barry)
Newsgroups: net.physics
Subject: Re: compressabiltity of liquids
Message-ID: <1265@gitpyr.UUCP>
Date: Thu, 16-Jan-86 11:59:27 EST
Article-I.D.: gitpyr.1265
Posted: Thu Jan 16 11:59:27 1986
Date-Received: Sat, 18-Jan-86 00:37:16 EST
References: <385@inuxh.UUCP>
Distribution: net
Organization: Georgia Institute of Technology
Lines: 55

Actually, the statement that "liquids are incompressible" hinges on the
fact that there is a distinguishable phase change for most substances
at room temperature and pressure between what is called a liquid and
solid state.  When we speak of a gas, such as air, we typically see 
nearly ideal behavior at standard conditions (near room temperature
and pressure).  By ideal behavior, we mean that the gas acts as a
collection of individual atoms exhibiting no attraction towards one
another, but interacting through point contact.  This approximation 
works very well for most gases at room temperature, but this is because
the kinetic energy of a gas molecule is sufficient at this temperature 
and pressure to overwhelm the relatively minute Van der Waals attractions
which manifest themselves only on a scale of 2-5 Angstroms.  The molecules
spend the greatest portion of their time more distant than this from their
neighbors, so the non-interacting model works well.  

When we move to other domains of temperature and pressure, things change.
Condensation occurs for most "average" gases as the kinetic energy falls
to the point that the "tail" of the Boltzmann distribution, which describes
how many particles are found in a given energy state, shows very few 
particles present with energy above that necessary to escape from the
Van der Waals energy well, therefore, molecules get "stuck" near their
neighbors, and the gas condenses as a phase in which molecules float about
at all times within a few radii of their neighbors, well within the 
Van der Waals energy well.  To compress this "liquid", one must reduce 
the spacing between molecules, but as they are already "touching", so to
speak, one must work against the so called "Born Repulsion" of the
atoms, which is a 6th power force.  Thus normal liquids are considered to be
incompressible.

At higher temperatures and pressures, the distinction between a liquid 
and gas becomes more sublime.  Every substance that exhibits a liquid
phase has a so-called critical temperature, above which attempts to liquify
the substance in the gaseous phase through application of pressure will
fail because no discernable change in the substances' properties is
observed.  For example, we know that carbon dioxide is liquifiable at
room temperature through application of pressure.  If we seal aa sample
of CO2 in a strong glass sphere with sufficient pressure that a mixture
of liquid and gas is present, and heat the mixture, we will observe the
pressure to increase.  If the initial pressure is adjusted properly, the
meniscus between liquid and vapor will not move, because the tendency of
the liquid to evaporate will be counterbalanced by the increase in the
pressure of the gas phase.  At the critical temperature, the miniscus
will suddenly disappear.  At this temperature, there is no distinction
between gas and liquid, and the former "liquid" has taken on properties
of a gas, namely, it is somewhat compressible, and the "gas" has some
properties of a liquid, namely, it exhibits considerable deviation from
the ideal gas law.  


-- 

Don Barry (Chemistry Dept)          CSnet: cmpbsdb%gitpyr.GTNET@gatech.CSNET
Georgia Institute of Technology    BITNET: CMPBSDB @ GITVM1
Atlanta, GA 30332      ARPA: cmpbsdb%gitpyr.GTNET%gatech.CSNET@csnet-relay.ARPA 
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