Viscosity and molecule size

[Notmyrealnam]

Is it true that the larger the size of the molecules, the more viscous the substance? (Original version of question, asked by my son: Why is water liquid and not gooey?)

[fo3nix]

Not sure about the size issue, but I think water is a liquid and "slippery" because it has weak intermolecular forces. I'm sure someone will clarify that though, my chemistry isn't brilliant.

[JustSia]

Wouldn�t take what I am about to say as a gospel as I am not a chemist. I�ve been trying to put little bits of information I have together and see if it would make sense. I think it might be true especially when it comes to polymers. Viscous means resistant to shear. Shear occurs when you move a part of that object against its parallel plane and the similarly charged ions are brought against each other, which causes repulsion.
The longer or more complicated the molecules the more intermolecular forces involved in their relationship the more forces there are to counteract the repulsion. Certain forces are stronger than the others so you have to have a balance of the right length and the right building blocks of a molecule to get the perfect result. Can�t wait to hear a proper explanation, your son asks really good questions.

[Notmyrealnam]

Thank you - we homeschool (actually, we unschool), so he gets to ask questions all day long. Looking forward to more answers, but thanks for what you've said so far!

[jake-the-peg]

A better question is why water is a liquid and not a gas

After all it's quite small only 2 hydrogen atoms and an oxygen. Compare with Carbon dioxide CO2 , Nitrogen NO2 dioxide, Methane CH4 all gasses.

Water is rather anomalous in a number of ways there's a number of good articles on it on the web such as here:
http://www.lsbu.ac.uk/water/explan.html
or here:
http://en.wikipedia.org/wiki/Water_(molecule)

One of the reasons is to do with the fact that the molecule is boomerang shaped and you get a slight negative charge at the oxygen atom in the middle so they attract each other a bit like a load of little magnets.

All in all water is a pretty amazing little molecule!

[Gef]

If you compare similar types of molecule then as the molecular size increases so viscosity increases. Petrol, lubricating oil and candle wax are all composed of hydrocarbon molecules of increasing size. The bigger molecules tangle together more so lubricating oil is much more viscous than petrol and in candle wax the molecules are held so firmly in place that it is solid at room temperature.

Viscosity is also affected by intermolecular forces which make molecules "stick" to each other. The strongest of these are hydrogen bonds which are responsible for the unusual properties of water (see jake's links). In fact, without hydrogen bonds life, as we know it, would not exist.