Soluble or Not?

The physical laws of liquids are a little complex yet a general understanding of them is important to understanding the mechanics of cleaning chemistry in the removal of both soluble and not-so-soluble contaminants from parts.  The general concept was discussed in the blog Chemistry – Solvent Characteristics. This blog will take things a bit further.

Question Solubility –

We generally have acquired the notion that everything is “soluble” (at least a little bit) in anything else.  This applies to not only liquids but also gasses and solids.  We have been told (thanks Culligan man), that iron, sulfur, calcium, chlorine and a number of other gasses, liquids and solids are “dissolved” in our water supplies to our homes which may cause odors, stains and other undesirable consequences.  But, in fact, iron, sulfur, calcium and chlorine are, for all practical purposes, NOT soluble in water!

I can see you scratching your head. Oh, they’re there all right, but not as iron, sulfur, calcium and chlorine.  If you are unfortunate enough to live in a location where there is a high concentration of iron in the water supply, you are well aware that the consequence is rusty stains on plumbing fixtures and laundry.  In fact, that IS rust you are seeing, not iron.  Although iron, per se, is NOT soluble in water, compounds that are formed when air, water and iron mix are soluble in water and therein lies the answer.  This is supported by the fact that you can not collect “iron” that has been dissolved in water with  a magnet because it is no longer iron but, rather, rust (non magnetic).  The case is similar with calcium, and chlorine although chorine is slightly soluble in water.  A small amount of chlorine gas goes into solution with water but the vast majority ends up as water soluble compounds formed as the chlorine and water mix.  In the case of sulfur, of course, the objectionable odor is the familiar “rotten egg” smell which we all know is Sulfur Dioxide and not Sulfur.

So what is soluble?  Think of things like common table salt and sugar.  In these cases, the dissolved solids do not lose their identity through chemical reactions.  Salt water tastes salty and sugar water tastes sweet.  If you dissolve away the water by boiling or evaporation, you get back the salt or sugar.  This is as opposed to the case or iron in water where you get back rust, not iron.  The fact that “calcium” deposits on faucets and shower heads are actually calcium chloride and do not dissolve without chemical assistance (the popular product CLR for example) is further evidence to debunk water solubility.

Solubility Limits –

In most cases there is a limit to the amount of one material that can be dissolved in another. If you add salt or sugar to water, for example, you eventually reach a point where any more material added will remain as cloudiness or granules and not be dissolved.  This is called the solubility limit and is exhibited by liquids, solids and gasses.  Air, for example, is soluble in water to a level of 30mg of air to 1 liter of water.   Often the solubility limit is related to temperature and, generally, solubility in liquids increases with temperature.  In the case of gasses, solubility generally increases with pressure – think of the bubbles of CO2 that come out of solution when you reduce the pressure in the can by pulling the tab.  I’m sure you can also relate to the fact that this release becomes more intense if the can of soda has been left in the trunk of the car without refrigeration all afternoon.

Applications to Cleaning –

I’m sure that those of you who read the blog regularly are starting to put the pieces together.  Contaminants that we consider insoluble may become “soluble” in the presence of water or cleaning chemistry.  But, there is a limit to solubility.  Once the solubility limit is reached (and this is not always obvious) further cleaning will not be possible.  In an interesting flip/flop, the solubility of gasses in liquids generally decreases as temperature is increased while the solubility of solids in liquids increases as temperature is increased.  This often leads to a bit of a balancing act when it comes to selecting the optimum temperature for cleaning – especially with ultrasonics.

 JF