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Could someone please explain why an aqueous solution of NaCl conducts electricity? I've Googled this up but couldn't get a satisfactory answer. This has an answer on Physics.SE but that's too complicated. My textbook doesn't answer my question.

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  • $\begingroup$ Essentially you understand that it is because it dissociates, and your follow-up question is why it dissociates, which, as someone else pointed out, is another question. $\endgroup$ – DHMO Oct 8 '16 at 11:10
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NaCl is an electrolyte. When in solution it dissociates into Na+ and Cl-. When you put electrodes in the solution, the cations are drawn to the cathode and the anions to the anode. This movement produces a current and that is why NaCl solutions can conduct electricity.

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  • $\begingroup$ Why does it dissociate? $\endgroup$ – user29731 Oct 7 '16 at 19:29
  • $\begingroup$ I think this answer needs some elaboration. Does this only work with AC? If not, what happens with DC when the bulk of the ions have already migrated? $\endgroup$ – bpedit Oct 7 '16 at 19:34
  • $\begingroup$ With a DC current, you'll probably get some electrolysis of water and possible oxidation of the chloride (which helps by removing negative charge from the positive electrode). $\endgroup$ – Zhe Oct 7 '16 at 20:58
  • $\begingroup$ @Zhe Agreed. I was just trying to prompt the psoter into fleshing out his answer for the OP. $\endgroup$ – bpedit Oct 7 '16 at 21:21
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NaCl is strongly ionic compound. It gets completely ionized and dissociates in Na+ and Cl-. Both Na+ and Cl- are surrounded by water molecules, Na+ is surrounded with O of H2O facing towards Na+, similarly Cl- is surrounded by H2O molecules with H facing towards Cl-. Presence of +ve and -ve ions helps in conduction of electricity.

When electrode are inserted and current is passed through these electrode, movement of ions in opposite direction creates current.

After passing current for a long time (especially DC current), the concentration of Cl- ions will decrease. The concentration of OH- will increase gradually and there will be competition between Cl- and OH-.

Electrolysis of NaCl

Anode: 2Cl- ---> cl2 +2e- E = -1.36V

Cathode: H2O +2e- ---> H2 + 2OH- E = -0.83V

Electrolysis of NaOH

Anode: 4OH- ---> O2 + 2H2O + 2e- E = -0.40V

Cathode: H2O +2e- ---> H2 + 2OH- E = -0.83V

After Sufficient long time there will be competition between Cl- and Oh- to get oxidized on Anode, whereas there is only one possible reaction on cathode. Reaction on Anode depends upon the concentration and reduction potential of Cl- and OH-. Actually on cathode there is also possibility of reduction of Na+ but reduction potential is -2.7 which is difficult than H2O so only one reaction takes place on cathode.

Hope this helps!

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$\ce{NaCl}$ dissociates into $\ce{Na+}$ and $\ce{Cl-}$ when it dissolves into water. Hydration stabilizes the ions formed. There is $\delta+$ charge on hydrogens of water and $\delta-$ charge on the atoms of oxygen. When $\ce{NaCl}$ is dissolved into water, the hydrogens are attracted to the $\ce{Cl-}$ ions and oxygen atoms to the $\ce{Na+}$ ions. Thus water molecules surround and separate the ions. enter image description here

Hence, in an aqueous solution of $\ce{NaCl}$, there are positive and negative ions available to move freely(of course with some resistance due to other molucules in vicinity). When a potential difference is applied, the $+ve$ ions are attracted to the $-ve$ terminal and vice versa. Thus, a current is estabilished.

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  • $\begingroup$ Can u please till why it dissociates or is the reason that the Na + ions are attracted to the OH - ions thats why it dissociates? $\endgroup$ – user29731 Oct 8 '16 at 15:25
  • $\begingroup$ Anyway great answer ! $\endgroup$ – user29731 Oct 8 '16 at 15:25
  • $\begingroup$ @Abcd See this video on YouTube. link $\endgroup$ – Apoorv Potnis Oct 8 '16 at 15:31
  • $\begingroup$ I do not think that actual $\ce{OH-}$ ions are attracted to $\ce{Na+}$ because the dissociation constant of water is quite low, $pK_w = 14$. So, I don't think that free $H+$ and $OH-$ ions are available. A water molecule as a whole orients itself in a particular way and surrounds the ions. $\endgroup$ – Apoorv Potnis Oct 8 '16 at 15:49
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    $\begingroup$ The simple answer is that water molecules have a dipole. The hydrogen atoms are slightly positively charged and the oxygen atoms slightly negatively charged. So, as you can see in the diagram above, water molecules around positively charged sodium orient themselves such that the oxygen is closer to sodium than the hydrogens. This is known as an ion-dipole interaction. The opposite is true for chlorine. @Abcd $\endgroup$ – orthocresol Oct 8 '16 at 16:23

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