Expert: Profile Closed - 1/7/2010

Question
Hello, it wouldn't let me ask a followup so i am asking another question once more... I am curious as to how the number of bonding sites in H20, NH3, and HF will affect the meniscus. I was told that water has the highest boiling point of all as it is able to hydrogen bond more times than NH3 and HF. HOw many times is each compound able to bond?

Answer
Hi there, thank you for the follow-up.

Firstly, the relative hydrogen bonding abilities of the three compounds are not absolute; there is not a definite figure to place by each one; it is a relative not an absolute thing. What you need to do is consider what causes hydrogen bonds to form and therefore how likely each of the compounds you mentioned is to form a cohesive hydrogen bonding network. You won't come up with any figures, just a "compound x is better at forming h-bonds than compound y, and compound y is better than compound z".

The compound which forms hydrogen bonds most easily will have the highest surface tension. The higher the surface tension, the further the meniscus will be able to rise in the capillary tube, becasue (as the last answer mentioned) the water will rise until gravity balances out the rising meniscus. So the better the compound is at forming hydrogen bonds, the higher it will rise.

This only applies to compounds where at least some hydrogen bonds are formed; in substances where no h-bonding takes place (as in liquid mercury), there will be an inverted meniscus and behave very differently to H-bonded compounds.

Hope this helps.