Basic help to solve physics homework problems – Understanding of vapor pressure

1. B AS I C H E L P TO S O LV E P H Y S I C S H O M E W O R K
P R O B L E M S – U N D E R S TAN D I N G O F VAP O U R
P R E S S U RE
Vapour pressure

2. What is Vapour pressure ?
 Vapour pressure is defined as the pressure exerted
by a vapour in thermodynamic equilibrium with its
condensed phases (solid or liquid) at a given
temperature in a closed system.
 Vapour pressure is nothing but the tendency of
particles to escape from the liquid (or a solid).
 At normal temperatures, substance with a high
vapour pressure is often referred to as volatile.

3. Pictorial illustration
 With the increase in the
temperature, the vapour
pressure becomes
sufficient to overcome
atmospheric pressure and
lift the liquid to form
vapour bubbles.
Source: Wikipedia

4. Calculation of vapour pressure
Antoine equation -This equation relates the vapour
pressure and the temperature of the substance.
log P = A-{B/C+T}
where:
 P is the absolute vapor pressure of a substance
 T is the temperature of the substance
 A, B and C are substance-specific coefficients

5. Antione coefficients
 The same substance have separate sets of Antoine
coefficients for Sublimations and vaporizations.
 Each parameter set for a specific compound is only
applicable over a specified temperature range.
 Temperature ranges are generally chosen to maintain the
equation's accuracy.
 For various volatile substances, different sets of
parameters are available.

6. Boiling point of water
 Water boils when its vapor pressure reaches its surrounding
pressure.
 The boiling temperature of water for atmospheric pressures
can be approximated by the Antoine equation:
 log10P = 8.07131 - {1730.63/233.426 + Tb}
 where the
 Temperature Tb is the boiling point in oC
 Pressure P is in Torr.
Vapor pressure is very helpful to understand physics concepts