Seeing Forces

Try asking your physics teacher if this is possible – they will probably give you all sorts of (boring) physics examples – for example, gravity pulling down on a book or a boat floating or an airplane flying.

But I can guarantee that none of them well tell you to look at a cup of full water.

If you set the following example up correctly, it can be really visual and impressive plus it can teach you a little about Hydrogen bonds – as well as letting you see them in action.

Now, I can hear your complaints from here … what has a cup of water and Hydrogen bonds got to do with forces? The link is in the Hydrogen bond as they are found in water and are the strongest example of an intermolecular force (the others being London dispersion forces and permanent diploe – dipole forces).

Source: Wikimedia

If you remember, the Hydrogen bond is formed because of the electronegative Oxygen atom being bonded directly to the Hydrogen atom. The Oxygen exerts a strong pull on Hydrogen’s two electrons, this exposes the Hydrogen nucleus and therefore exposes a positive charge (one reason why water has a permanent dipole as well). The consequence of all of this is that there is a relatively strong force of attraction between one Hydrogen nucleus and another Oxygen atom in another water molecule – which we call the Hydrogen bond!

So, back to the big question, how do you see this force?

Well, you will need a tall lass jar – if you are in school, a gas jar is perfect. If you are at home, something like a glass jam jar would be good. Glass seems to work better than plastic. Fill the jar to the top with water – make sure the water level is flat (flush) with the top of the jar.

Next, get some coins – yes, you heard correctly! And just add the coins, one at a time to the jar. Preferably, by allowing the coin to slide down the edge of the jar to the base. And that’s it …. Except keep adding those coins. After a while, if you get down to eye level you can see a noticeable bulge where the water is rising out of the jar. Now we call this service tension but what you are actually seeing is the force of attraction of the Hydrogen bond, holding the water together. If you are lucky, the water will rise out of the jar around 3 mm or so – it is quite a site!