|MadSci Network: Chemistry|
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. H / (-) O (+) \ H 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 substance.) 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 : | H-O-H..O-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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