How boiling point and freezing point is related to vapour pressure?

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Solution

Boiling point is the temperature at which the vapour pressure of the liquid substance becomes equal to the pressure of surroundings and at this point the liquid changes into vapours.
If we add a non volatile solute in liquid its vapour pressure gets decreased so its boiling point is increased. This is called elevation in boiling point.
Freezing point is the temperature at which the solid and the liquid state of the substance have the same vapour pressure. When a non volatile solute is added to a solvent, the freezing point of the solution is always lower than that of the pure solvent.

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Q. How are vapor pressure and boiling point related?

How does pressure affect the boiling any melting/freezing point of a substance ?

Q. At very low pressure what is the boiling point and freezing point of water?

Properties such as boiling point, freezing point and vapour pressure of a pure solvent change when solute molecules are added to get homogeneous solution. These are called colligative properties. Applications of colligative properties are very useful in day-to – day life.

One of its examples is the use of ethylene glycol and water mixture as anti-freezing liquid in the radiator of automobiles.

A solution M is prepared by mixing ethanol and water. The mole fraction of ethanol in the mixture is 0.9.

Given, freezing point depression constant of water

$(K_{f}^{w a t e r}) = 1.86 K k g m o l^{- 1}$

Freezing point depression constant of ethanol

$(K_{b}^{w a t e r}) = 2.0 K k g m o l^{- 1}$

Boiling point elevation constant of water

$(K_{b}^{w a t e r}) = 0.52 K k g m o l^{- 1}$

Boiling point elevation constant of ethanol

$(K_{b}^{e t h a n o l}) = 1.2 K k g m o l^{- 1}$

Standard freezing point of water = 273 K

Standard freezing point of ethanol = 155.7 K

Standard boiling point of water = 373 K

Standard boiling point of ethanol = 351.5K

Vapour pressure of pure water = 32.8 mm Hg

Vapour pressure of pure ethanol = 40 mm Hg

Molecular weight of water $= 18 g m o l^{- 1}$

Molecular weight of ethanol $= 46 g m o l^{- 1}$

In answering the following questions, consider the solutions to be ideal dilute solutions and solutes to be non-volatile and non-dissociative.

The vapour pressure of the solution M is

Q. How do the freezing point and boiling point of ocean water compare to those of distilled water?

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