One of the basic, bedrock principles of chemistry is that pure water has a pH value of 7. pH is a scale used by scientists to describe the acidity of a substance. If the liquid has a ph 7, it’s alkaline. Pure water (by definition) was given a value of 7. The scale is logarithmic, meaning that a difference of 1.0 pH unit meant a 10x increase. So, something with a pH of 4 is 1000 times more acidic than pure water (10x10x10). The belief that pure water has a pH of 7 is ingrained in science, and has never been challenged; it forms the foundation of all of chemistry’s explanations of acid-base behavior.
Well, all that may now have to change. In a surprising turn of events, chemists from Israel have described in a recent copy of the Proceedings of the National Academies of Science that pure water may actually have an acidic “skin” – the layer of water molecules on the very top of the water’s surface. Even more surprising, this acidic layer was found to have a pH of approximately 4, meaning the water in that layer was 1000x as acidic as the rest of the water – almost as acidic as orange juice. How can this be?
The explanation lies in the nature of hydrogen bonds. Water has such a high boiling point because it can form an extended network of hydrogen bonds. H-bonds are bonds which form between hydrogens and electronegative elements such as oxygen, and they are quite strong. However, if a water molecule becomes protonated – meaning it becomes acidic – the oxygen atom in that particular water molecule is no longer capable of forming hydrogen bonds. Instead of trying to incorporate this “defective” molecule into the otherwise well-organized hydrogen-bonded network that exists between normal water molecules, the acidic water molecule is ostracized and pushed towards the surface of the water droplet. The three hydrogens on the protonated molecule point down into the surface of the water and participate in hydrogen bonding, while the protonated oxygen puts towards the air at the air-water interface, which means that it doesn’t interfere with the regular packing of the rest of the structure. This is the phenomenon responsible for the skin of the droplet becoming so acidic – all of the (naturally occurring) acidic molecules in pure water (which are normally balanced by alkaline water molecules) congregate on the surface.
This discovery has many implications. Atmospheric chemistry is the study of chemical interactions between pollutant molecules and water droplets found suspended in the air. Up until now, it’s been assumed that the gaseous water molecules have a pH of 7. The discovery that the droplets are actually fantastically acidic means that the models designed to describe these atmospheric interactions have to be changed; acid-catalyzed reactions are much different from reactions taking place at neutral pH levels. Hopefully this will reconcile some of the more puzzling results from atmospheric observations that our theories, to this point, have not been able to explain.
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