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I believe I may have misunderstood water solubility this entire time.

I assumed that water soluble meant that it would literally dissolve in water. It would break down into atoms. A sugar molecule would break its bonds and become something else...

However I must be mistaken because I'm reading about why DNA is spiral shaped and it says that it is to keep the hydrophobic bases inside the spiral to protect them from the water, but keeping the hydrophilic phosphate and sugar molecules on the outside. I would assume that because these molecules are hydrophilic they would disassociate from each other but this has to be incorrect. What am I misunderstanding about water solubility and what would happen if the bases were exposed to water?

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    $\begingroup$ Water solubility only means that a substance is soluble in water. Usually it will not decompose (break into single atoms) in this step, as dissolving is a purely physical process. $\endgroup$ – Chris♦ Apr 3 '15 at 20:21
  • $\begingroup$ So why is phosphate a "water soluble" molecule but yet it stays as "phosphate" in water? It doesn't dissolve in water it stays phosphate? $\endgroup$ – Paze Apr 3 '15 at 20:27
  • $\begingroup$ @BjarniJóhannsson see my answer below for an accessible explanation. If you are having trouble with the English words, you may want to look them up in your native language. You are confusing dissolve with hydrolyze. $\endgroup$ – MattDMo Apr 3 '15 at 20:31
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    $\begingroup$ You might be thinking of simple ionic compounds like sodium chloride, which when dissolved do separate into their constituent ions (e.g. sodium cation and chloride anion). $\endgroup$ – jerepierre Apr 3 '15 at 20:32
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This question really belongs at Chemistry.SE, but I'll give you a quick answer. A substance is soluble in water when its solid form (such as a sugar cube) completely dissolves in water to become a sugar solution. The sugar molecules themselves are unaffected, essentially - instead of all being bound to one another in a crystal, they are now floating around in the water. The proof is in the taste - solid sugar and sugar solutions taste pretty much the same. If the sugar had been broken down into component hydrogen, oxygen, and carbon, it would taste much different!

On the other hand, hydrolysis is what happens when water actually participates in a chemical reaction to break one or more molecular bonds within a certain molecule, breaking it down to two new molecules (or one molecule and an atom, or whatever). While sugar can be hydrolyzed, it requires either very high heat, or the presence of a catalyst such as an enzyme.

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    $\begingroup$ And it's more: If you remove the water, the substance will be the same as before. Dissolving is a purely physical process which will not change the molecule. This is only done during chemical reactions. $\endgroup$ – Chris♦ Apr 3 '15 at 20:44
  • $\begingroup$ Downvoter care to explain? $\endgroup$ – MattDMo Apr 3 '15 at 21:22
  • $\begingroup$ Except for ionic compounds which do dissociate when dissolved $\endgroup$ – WYSIWYG Apr 4 '15 at 9:24
  • $\begingroup$ It's about sugar and DNA. It is fine here at Bio.SE. Well answered too. +1 $\endgroup$ – AliceD♦ Apr 4 '15 at 9:48

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