What's happening to the electrical resistance at a molecular level when the temperature of a water with a bit of salt is increased? I noticed that the resistance decreases but in metals it is totally different and increases. I don't understand why is it different in metals and in solutions?
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2 Answers
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Salty water is conducting because it salt ionizes in water, breaking into Na$^+$ and Cl$^-$ ions. The solubility of NaCl increases with increasing temperature (though not very strongly), as you can see from this plot. This means that at higher temperature more ions will be formed and therefore the resistivity will decrease (i.e. the conductivity will increase).
In a metal, resistivity is caused by scattering of the electrons with defects and vibrations (phonons) in the crystal lattice. If temperature is increased, the density of phonons and defects is increased, therefore the scattering events are more frequent. As a result, resistivity increases.
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1$\begingroup$ What about the conductivity of water at a constant concentration of NaCl? $\endgroup$ Commented Mar 13, 2018 at 17:28
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$\begingroup$ @BertBarrois I am assuming that the concentration is constant. It is the solubility (how much of NaCl ionizes) that changes with temperature. $\endgroup$– valerioCommented Mar 13, 2018 at 17:43
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1$\begingroup$ NaCl is not a weak electrolyte. It is completely ionized when in solution. I think I have a clue to the increasing conductivity: The viscosity of water decreases dramatically as the temperature increases. This means that the aggregates are smaller. $\endgroup$ Commented Mar 13, 2018 at 19:41
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$\begingroup$ The same is observed independent of concentration $\endgroup$ Commented Mar 13, 2018 at 19:49
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$\begingroup$ Maybe it has something to do with the mobility of the ions that increases with increase in temperature of the water? Well, I can't explain why the mobility increases but I am sure it does. $\endgroup$ Commented Mar 14, 2018 at 7:32
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The increase in temperature in water makes the separation of the nuclei in water greater. O is farther apart from H as temperature increases. The respective electron clouds now occupy more space between the nuclei and less space elsewhere. Therefore the nuclei are more visible and the partial charge $\delta$ increases. The water molecules now attract each other more strongly and come closer together. Density thus increases.
Add salt. Now more partial charge $\delta$ in the water dipole means more interaction with salt atoms. Solubility thus increases.
The calculation sketch. Use Hartree-Fock to obtain the electron orbitals of water molecule while minimizing the energy by adjusting nuclei separation. obtain the charge distribution from the calculated wavefunctions and then the partial charge space-distribution.
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$\begingroup$ While the mechanism described here may contribute to the increased conductivity, your text does not explain the increased conductivity. $\endgroup$ Commented Feb 20, 2022 at 23:42