Expert: Henry Boyter - 9/2/2006

Question
Hello, im a Sec.4 student from Singapore taking the Cambridge 'O'levels this year.

The question i am asking has no relation to schoolwork. It is a question out of my own curiousity.

I read that impurities lower the melting point of a substance. I think this is due to nucleation right? But how does impurities increase the boiling point of substances?

If nucleation results in lowering the melting point, what phenomenon causes the b.p.t to increase?

Thanks for your time...

Answer
Let me give you a very simple explanation and you can read about Raoult's Law on your own.

http://members.aol.com/profchm/raoult.html

Boiling happens when the vapor pressure equals the outside pressure. Adding things to solutions lowers the vapor pressure so you have to add more energy.  That is Raoult's Law.

The simple reason is this.  When you add salt, for example, to water, the water molecules interact with the salt ions.  In fact, we write

NaCl (s) =  Na+ (aq) + Cl- (aq)

Note the aq indicates that the salt has changed, but it also indicates the water has changed.  In fact (in most cases), it has taken energy from the surroundings to dissolve the salt.  The salt-water interacted species are now more stable than just the water alone.  Now when you try to turn the water into vapor, these interations have to be broken down (the same reason water has a high boiling point than expected due to hydrogen bonding).  Breaking down these interactions takes energy, so the boiling point increases because you are now having to separate the water from the salt AND also vaporize it.