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I am a chemistry super noob and I was watching a MIT video where the professor states that:

"Certain molecules become charged when they come in contact with water in the natural state"

Can someone give me an example of one or more such molecules ?

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It depends on the context how he meant it. I could imagine two scenarios:

  • Certain materials can produce ions when they dissolved in water. A typical example is table salt, NaCl. Table salt is solid form has a lattice that contains Na+ and Cl+ ions, and when you put it in water, the water molecules can help releasing these ions into the solution, where you now can find the ions. Note, that in this case the ions are atomic entities but molecules with charge are also ions, like $\ce{MnO4-}$ or $\ce{SO}_4^{2-}$.

  • Another scenario when the charged entities are formed as a reaction with water by e.g. giving an electron (oxidation) to water molecules or to other dissolved components e.g. oxygen in water. As you can imagine, it happens generally with very reactive materials (materials that are very sensitive to moisture), but essentially the rusting of iron is a similar reaction.

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    $\begingroup$ An addition: amino acids often gain or lose charge in water depending on the pH. $\endgroup$ – lemon Aug 5 '14 at 8:27
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Terrible phrasing. Molecules by definition have no overall charge. Ions have overall charge. Not molecules. Molecules becoming charged become ions.

If you want to get into the nitty gritty you can try looking up more information about zwitterions, which are actually not ions in the strict sense as they possess no net charge.

This is the IUPAC definition of molecule:

An electrically neutral entity consisting of more than one atom (n > 1). Rigorously, a molecule, in which n > 1 must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.

This is the IUPAC definition of ion:

An atomic or molecular particle having a net electric charge.

This is the IUPAC definition of zwitterion:

Neutral compounds having formal unit electrical charges of opposite sign. Some chemists restrict the term to compounds with the charges on non-adjacent atoms. Sometimes referred to as inner salts, dipolar ions (a misnomer). E.g. H3N+CH2C(=O)O− ammonioacetate (glycine), (CH3)3N+–O− trimethylamine oxide.

http://goldbook.iupac.org/M04002.html

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  • $\begingroup$ So, betaine is a molecule and not an ion? $\endgroup$ – LDC3 Aug 5 '14 at 5:32
  • $\begingroup$ Yes, it would. It is also a "zwitterion" since it has formal charges of opposite signs on its constituent ions. See here for the IUPAC definition of molecule - it does not contradict anything I said. goldbook.iupac.org/M04002.html $\endgroup$ – Dissenter Aug 5 '14 at 13:35
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    $\begingroup$ Good point, it is better be careful about this detail. Interesting that the adjective form "molecular" is still used in the other sense, irrespective of charge. $\endgroup$ – Greg Aug 5 '14 at 14:56
  • $\begingroup$ Well that's an unfortunately loose application of terms. $\endgroup$ – Dissenter Aug 5 '14 at 15:31

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