Anonymous

# Vapour Pressure Boilint Point etc.?

Could someone explain to me the whole deal with vapour pressure boilint point saturated vapour pressure and stuff ? dont really get it

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• Anonymous

Vapor pressure is the tendency of a substance to move from the liquid phase into the gaseous phase. The boiling point is when the vapor pressure of the internal environment is equal to the external environment.

For example, you have a solution and you apply energy to it. The molecules in the solution will not break free of the liquid solution and move into the gaseous phase until the vapor pressure of the liquid is equal to the external pressure exerted on the solution.

Also, one of the simplest ways to think about vapor pressure and boiling points in general is to always think of them as having an inverse relationship. As vapor pressure increases, boiling point decreases and vice versa.

Some other things can affect this relationship though. Usually, if you have a nonvolatile impurity (i.e. a substance that doesn't go into the gaseous phase), it will lower the vapor pressure of the total system because the nonvolatile impurity does not have a pure vapor pressure.

For example, if you have pure water, it boils at 100C. If you have a solution of sugar (nonvolatile impurity) and water, the vapor pressure of the sugar water solution will be decreased due to sugar not having a pure vapor pressure (Raoult's and Dalton's Laws apply here).

When you apply this situation to Raoult's law you have Px=P0x*Nx. This says that the partial pressure of one of the components is equal to the pure vapor pressure of the component plus the mole fraction of the component in the system. So, if you have sugar, the pure vapor pressure is 0, and 0 times anything is 0. That means that the partial pressure contributed by that component to the system is 0.

When you apply this to Dalton's law, you have Ptotal=Px+Py+Pz.....etc. This equation says that the vapor pressure of the total system is equal to the sum of the partial pressures.

If sugar has 0 partial pressure, and water's partial vapor pressure is already a fraction (from the term Nx), it shows you that the vapor pressure of the total system is decreased and from the inverse relationship with boiling points, the boiling point is increased.

Another case to consider is when you have immiscible solutions together. Immiscible means that the two solutions do not mix.

For example, take a high boiling organic compound that does not mix with water.

In this instance, the term Nx from Raoult's law does not apply because the two components are not in solution with one another (they are not an idea solution); therefore, the term Nx does not apply. The vapor pressure of the total system then becomes an additive effect from the pure vapor pressures of both components in the mixture (applying Raoult's and Dalton's laws). The additive effect means that vapor pressure is increased and the boiling point of the system is decreased.

That was probably way more than you wanted to know, but that's everything I know about it.

• Anonymous

vapour pressure is nothing but the pressure involved in the vapour state of a substance

boiling point - its nothing but the level of heating a substance and its resistance,.

ex.. water has a Bp of 100' c (max)

this tells that water can be boiled to 100'c ie.. it becomes vapour after that!!!!!!

• Anonymous