The following questions all relate to the normal boiling point of water where th

Question

The following questions all relate to the normal boiling point of water where the atmospheric pressure is defined to be 1 atm

A. What is the boiling point of pure water? =100 C

B. If a nonvolitle, insoluble substance is mixed with water, will the boiling point of the mixture be above, below or the same as the boiling point of water?

C. If a nonvolitle substance is dissolved water, will the boiling point of the mixture be above, below or the same as the boiling point of water?

D. What is the temperature of the vapor above a sample of boiling water?

E. If a nonvolitale, insoluble substance is mixed with water, will the temperature of the vapor above the boiling mixture be above, below or the same as the boiling point of water?

F. If a nonvolitale substance is dissolved in water, will the temperature of the vapor above the boiling mixture be above, below or the same as the boiling point of water?

Explanation / Answer

A) boiling point of pure water=100 degree C

B)If a nonvolitle, insoluble substance is mixed with water, the boiling point will not change but remain the same as the insoluble substance will precipitate or settle down at the bottom,and not stay at the surface of water.Thus ,surface energy (equal to kinetic energy acquired by the molecules) required by water molecules to escape from water-air interface remains the same.So, boiling point ,which is proportional to kinetic energy of molecules remains the same.

C) If a nonvolitle, soluble substance is mixed with water, the boiling point will increase and be above the boiling point of water.This happens because the surface of the solution (interface between vapor phase and liquid phase ) in a solution has solute molecules in addition to the water molecules.So,

1) due to less surface density of water molecules,and

2) ionic interactions between ions of the soluble salt(salt) and polar water molecules

more energy is required to break apart the water and ions and set them free into gaseous molecules.Thus,boiling point is increased thereby increasing the energy required for vaporisation

D)The temperature of the vapor above a sample of boiling water is equal to the normal boiling temperature of water =100 degree C

Boiling point is the temperature at which the vapor pressure of volatile liquid becomes equal to the atmospheric pressure

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