verner@inuxh.UUCP (Matt Verner) (01/14/86)
I am looking for a short concise explanation for the noncompressability of liquids. What changes when a gas goes to a liquid phase that causes this inability, even though the original gas was compressable. Are there any liquids that are? Matt Verner UUCP: ...ihnp4!inuxc!verner AT&T Consumer Products Laboratories AT&T: (317) 845-3631 P. O. Box 1008 Indianapolis, IN 46206 "The whole point of this sentence is to clearly explain the point this sentence is making."
cmpbsdb@gitpyr.UUCP (Don Barry) (01/16/86)
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
UUCP: ...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!gitpyr!cmpbsdbentropy@fluke.UUCP (Terrence J. Mason) (01/21/86)
In article <385@inuxh.UUCP> verner@inuxh.UUCP (Matt Verner) writes: > >I am looking for a short concise explanation for the noncompressability >of liquids. What changes when a gas goes to a liquid phase that causes >this inability, even though the original gas was compressable. Are there >any liquids that are? Actually it has been my observation that this is a popular misconception. I have toured Flow Industries (high pressure water jet cutting tools) and they indicated that H2O is actually quite compressible, although not within the scope of everyday experience. The numbers that I remember were that water at about 50,000 psi. was compressed 12% by volume. I was sure surprised to discover that I had been mislead all these years. They are located in Kent WA if you're interested in their side of the story. terrymason (entropy@fluke)