MadSci Network: Chemistry

Re: Wouldn't acid be a better universal solvent than water?

Date: Thu Jan 20 14:48:41 2000
Posted By: Sarah Earley, Grad student, CU Boulder
Area of science: Chemistry
ID: 947344981.Ch

Hi Karin,

 Water (H2O) can dissolve more substances than any other solvent 
(including acids), so it is called the universal solvent.  It is so good at 
dissolving many different substances for two reasons:

1. Water is a non-ionic, polar molecule.  ("Non-ionic" means that it does 
not have an overall charge, and "polar" means that there is a charge 
difference throughout the molecule.)  In a water molecule, the oxygen atom 
has a slight negative charge, whereas the hydrogen atoms each have a slight 
positive charge.  
                             (-) O    (+)

Since "like dissolves like", water will dissolve other polar, non-ionic 
substances, such as sugar.  In this case, water does not change the 
chemical structure of the molecule at all.  

Furthermore, the slight charges enable water molecules to attract to (and 
therefore dissolve) both positively and negatively charged substances, such 
as the sodium and chloride ions that make up table salt.  Here, the salt 
molecule actually dissociates into the two ions that make it up. (Water 
will not dissolve olive oil, because olive oil is a non-polar, non-ionic 
2. Water can hydrogen bond. Hydrogen bonds are weak bonds that form between 
a hydrogen atom and an oxygen, nitrogen, or fluorine atom.  The hydrogen 
must already be covalently bound to O, N, or F, so that the hydrogen ends 
up being sandwiched between two of the other atoms.  Here is a very rough 
illustration of hydrogen bonding among water molecules: 

                           H-O-H     H
                           :         |

The dashes (-) represent covalent bonds between atoms, whereas the dots 
(..) represent hydrogen bonds between atoms.  Covalent bonds are strong 
bonds that hold atoms together within molecules, whereas hydrogen bonds are 
much weaker bonds that hold atoms together between molecules.  (By the way, 
the angle in a water molecule is actually about 104.5 degrees, not 180 
degrees or 90 degrees as I've shown in this illustration.)

 Some acids might dissolve many substances, but there aren't any acids that 
dissolve as many substances as water.  Plus, acids will react chemically 
with substances, thereby changing them into different species rather than 
simply dissolving them.  

 Complicated, eh?  You might want to check out a general chemistry textbook 
in order to learn more about this (you might get some more helpful 
pictures!).  I hope that this explanation helps you out, though. 

Sarah Earley
CU Boulder    

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