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Tekanan uap suatu cairan bergantung pada banyaknya molekul di permukaan yang memiliki cukup energi kinetik untuk lolos dari tarikan molekul-molekul tetangganya. Jika dalam cairan itu dilarutkan suatu zat, maka kini yang menempati permukaan bukan hanya molekul pelarut, tetapi juga molekul zat terlarut. Karena molekul pelarut di permukaan makin sedikit, maka laju penguapan akan berkurang. Dengan pekataan lain, tekanan uap cairan itu turun. Makin banyak zat terlarut, makin besar pula penurunan tekanan uap.
==Estimating vapor pressures with Antoine equation==
The [[Antoine equation]] <ref name=frostburg>[http://antoine.frostburg.edu/chem/senese/101/liquids/faq/antoine-vapor-pressure.shtml What is the Antoine Equation?] (Chemistry Department, [[Frostburg State University]], [[Maryland]])</ref><ref name=Sinnot>{{cite book|author=R.K.Sinnot|title=[http://books.google.ca/books?id=DJaxUL3numgC&pg=PA331&lpg=PA331&dq=antoine+equation+constants&source=bl&ots=2c0cqzbR0t&sig=YPuvrW2kWnWP2s4QvY9TTpxGgNM&hl=en&sa=X&ei=widrUcu9Cce0qgHq24CgCA&ved=0CGcQ6AEwBzgK#v=onepage&q=antoine%20equation%20constants&f=false Chemical Engineering Design]|edition=4th|publisher=Butterworth-Heinemann|year=2005|page=331|isbn=0-7506-6538-6}}</ref> is a mathematical expression of the relation between the vapor pressure and the temperature of pure liquid or solid substances. The basic form of the equation is:
:<math>\log P = A-\frac{B}{C+T}</math>
and it can be transformed into this temperature-explicit form:
:<math>T = \frac{B}{A-\log P} - C</math>
where: <span style="vertical-align:+12%;"><math>P</math></span> is the absolute vapor pressure of a substance<br>
: <span style="vertical-align:+15%;"><span style="vertical-align:+12%;"><math>T</math></span> is the temperature of the substance</span>
: <span style="vertical-align:+12%;"><math>A</math></span>, <span style="vertical-align:+12%;"><math>B</math></span> and <span style="vertical-align:+12%;"><math>C</math></span> are substance-specific coefficients (i.e., constants or parameters)
: <span style="vertical-align:-30%;"><math>\log</math> is typically either <math>\log_{10}</math> or <math>\log_e</math></span><ref name=Sinnot/>
A simpler form of the equation with only two coefficients is sometimes used:
:<math>\log P = A-\frac{B}{T}</math>
which can be transformed to:
:<math>T = \frac{B}{A-\log P}</math>
Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components in mixtures.<ref name=frostburg/> The Antoine equation is accurate to a few percent for most volatile substances (with vapor pressures over 10 Torr).
== Pranala luar ==
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