Mercury Beating Heart

26 April 2014

If you were lucky enough to have a cool science teacher you may have seen the mercury beating heart in class. If not, here it is for you!  It is an electrochemical redox reaction which causes a drop of mercury in liquid to oscillate – and it looks (at times at least) like a beating heart.

In fact you may have guessed there is a little more to it than just mercury and liquid.  When the mercury is placed on a glass surface (in this case it looks like a watch glass or something similar) the liquid used is sulphuric acid.  This is an electrolyte (something that acquires a negative or positive charge by gaining or losing electrons when it is mixed with water).  It contains an oxidizing agent, which is something that can accept electrons from another source. 

The source in question is the slither of iron which has been placed, almost touching the mercury.  The oxidizing agent in the sulphuric acid oxidizes the mercury which forms a layer of mercury oxide. This layer reduces the surface tension on the mercury droplet which means it flattens out and comes in to contact with the iron.

The mercury sulfate oxidizes the iron to the iron and is reduced back to metallic mercury. When it is all metallic again the drop rounds back up and the contact with the iron is lost. Then the process begins all over again.

People have been having fun with the mercury beating heart since it was first observed by a no doubt startled Carl Adolf Paalzow in 1858. It’s probably just as well it wasn’t a century or two earlier – this may have looked a little too much like witchcraft for anyone foolish enough to announce their observation to have survived for long!

Still interested? Then here is an experimental sonification (the use of non-speech audio to convey information or perceptualize data) of the redox reaction.

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