Editing Partial pressure (section)

==Vapor pressure==
{{main|Vapor pressure}}
[[Image:vapor_pressure_chart.svg|thumb|right|A log-lin vapor pressure chart for various liquids]]

[[Vapor pressure]] is the pressure of a [[vapor]] in equilibrium with its non-vapor phases (i.e., liquid or solid). Most often the term is used to describe a [[liquid]]'s tendency to [[evaporate]].  It is a measure of the tendency of [[molecule]]s and [[atom]]s to escape from a liquid or a [[solid]]. A liquid's atmospheric pressure boiling point corresponds to the temperature at which its vapor pressure is equal to the surrounding atmospheric pressure and it is often called the [[normal boiling point]].

The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point of the liquid.

The vapor pressure chart displayed has graphs of the vapor pressures versus temperatures for a variety of liquids.<ref>{{cite book |editor1-last=Perry |editor1-first=R.H. |editor2-last=Green |editor2-first=D.W. |title=Perry's Chemical Engineers' Handbook |edition=7th |publisher=McGraw-Hill |year=1997 |isbn= 978-0-07-049841-9 |title-link=Perry's Chemical Engineers' Handbook }}</ref> As can be seen in the chart, the liquids with the highest vapor pressures have the lowest normal boiling points.

For example, at any given temperature, [[methyl chloride]] has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point (−24.2&nbsp;°C), which is where the vapor pressure curve of methyl chloride (the blue line) intersects the horizontal pressure line of one atmosphere ([[Atmosphere (unit)|atm]]) of absolute vapor pressure. At higher altitudes, the atmospheric pressure is less than that at sea level, so boiling points of liquids are reduced. At the top of [[Mount Everest]], the atmospheric pressure is approximately 0.333&nbsp;atm, so by using the graph, the boiling point of [[diethyl ether]] would be approximately 7.5&nbsp;°C versus 34.6&nbsp;°C at sea level (1&nbsp;atm).