Why ony pure water boils at 100 degrees and freezez at 0 degrees? why do impurities if in water increase the Boiling point and lower the freezing point?

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Solution

Boiling point of liquids and melting point of solids are unique for a substance and for water they are 100°C and O°C. At 100°C, the vapour pressure of gaseous water (steam) equals the atmospheric pressure and thus is the boiling point of water.

Boiling point of a substance is the temperature where the vapour pressure of the liquid above the liquid equals the external atmospheric pressure. When a solute(impurity) is added to water the vapour pressure is less than that of the pure water. So, the solution of the impurity in water needs to be heated upto a higher temperature so that it equals the atmospheric pressure. This elevates the boiling point of the water with impurities.

In simple terms, for a solution of impurity in water, part of the heat supplied is utilized by the particles of the impurity and is not available for the water molecules to convert into gaseous form. This decreases its vapour pressure.

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Similar questions

Q. Write a Python program to convert temperature in degree Celsius to degree Fahrenheit. If water boils at 100 degree C and freezes as 0 degree C, use the program to find out what is the boiling point and freezing point of water on the Fahrenheit scale. (Hint: T(°F) = T(°C) × 9/5 + 32)

Q. As per Celsius scale, the freezing point of water is ________ degree and the boiling point is ___ degree Celsius.

Q. Statements : Water boils at 100

^{\circ}

C.
Water freezes at 0

^{\circ}

C.
Conclusions : At low pressure, water boils at lower temperatures.
The conclusion drawn is :

Q. An aqueous solution freezes at

272.4 K

, while pure water freezes at

273.0 K

, while pure water freezes at

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. Determine
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(ii) boiling point of the solution
(iii) lowering of vapour pressure of water at

298 K

(Given,

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