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Does Salt Raise The Boiling Point of Water

April 29, 2013

Introduction to colligative property

A colligative property of a solution depends purely on the number of particles dissolved in it, rather than on the chemical nature of the particles.The colligative properties can be regarded as the properties of the solvent in a given solution. Generally, the solute is considered as non-volatile. The various colligative properties are as below :
i. Lowering of vapor pressure of the solvent.

ii.  Elevation of boiling point

iii Depression of freezing point

iv Osmatic pressure

If we take a pure liquid in a closed container, we find that a part of the liquid evaporates and fills the available space with its vapour. The vapour exerts a pressure on the walls of the container and exists in equilibrium with the liquid. This pressure is referred as the vapour pressure of the liquid. When a non-volatile solute is dissolved in the solvent so that a dilute and homogeneous solution results, then again the vapour pressure of the solution will be made up of entirely from the solvent since the solute does not evaporate. This vapour pressure of the dilute solution is found to be lower than the vapour pressure of the pure solvent.

Elevation of boiling point

The boiling point of a pure liquid is the temperature at which its vapour pressure becomes equal to the atmospheric pressure. Since the vapour pressure of a solution is always lower than that of the pure solvent, it follows that the boiling point of a solution will always be higher than of the pure solvent. elevation of boiling point

the upper curve represents the vapour pressure – temperature dependance of the pure solvent. The lower curve represents the vapour pressure – temperature dependance of a dilute solution with known concentration. It is evident that the vapour pressure of the solution is lower than that of the pure solvent at every temperature. The temperature To gives  the boiling point of the pure solvent and T the boiling point of the pure solution. This is because at these temperatures (To, T) the vapour pressures of pure solvent and solution becomes equal to the atmospheric pressure.Elevation of boiling point is found directly proportional to the molality of the solution (or) inturn the number of molecules of solute. Also it is independent of the nature of the solute for a non-volatile solute. Hence, boiling point elevation is a colligative property.

The addition of salt increase the boilling point of water.  This  is because of the above mentioned reason.  The colligative properties.  If a salt like NaCl is added to the water it raises the boiling point of water deponding upon how much of the salt is added to the water.  If the salt is added more then the vapor pressure decrease is more so the solution will boil at very high temperature.  Since lowering the vapor pressure increases the boiling point this is the main reason for increase in boiling point

Due to the colligative property the boiling point of the water is elevated and hence when the number of particle is more there will be more increase in the boiling point

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