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A kid asked me this question just the other day, and I said that the positive part of $\ce{H2O}$ attracts the negative $\ce{Cl}$ ion and the negative part of the $\ce{H2O}$ attracts the positive Na ion.

Then he asked me several questions which stumped me:

  1. Why doesn't $\ce{NaCl}$ dissolve water instead (break water into $\ce{H}$ and $\ce{O}$)?
  2. If $\ce{H2O}$ is electrically neural, why would it attract any ions?
  3. Why doesn't $\ce{NaCl}$ stick together, how can one know if one molecule will attract the ions hard enough that the ions breaks away?
  4. Why is Na positive and $\ce{Cl}$ negative?

Hopefully someone can help me answer the above questions as I have not done chemistry in years!

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1.) The covalent OH bond strength in water is slightly stronger than the NaCl ionic bond strength.

2.) Water might be neutral but each atom exhibits a partial charge. Oxygen sucks up most of the electron density away from the hydrogen atoms leaving O with a partial negative charge and hydrogen with a partial positive charge. This gives rise to the famous hydrogen bonding behavior of water.

3.) You would have to analyze the energies of each of the interacting components to know.

4.) Cl is more electronegative than Na due to the larger number of protons in the nucleus. This means that Cl will suck electron density away from Na as opposed to the other way around.

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The main reason about why water breaks salt is related to the relative permittivity $\epsilon_r$ of water compared to its value in air. In fact,$\epsilon_r= 80$ in water, while its value in air is $\epsilon_r= 1$. Now, if we look at the formula of the Coulomb force, which is responsible of the electrostatic attraction between ${\rm Na}^+$ and ${\rm Cl}^-$ : $$f = \frac{q.q'}{4\pi \epsilon_0\epsilon_r r^2}$$ (Where q and q' are the charges of ${\rm Na}^+$ and ${\rm Cl}^-$, and r is the distance between the two punctual charges).

We notice that, this force is 80 times bigger in water than it is in air. So, the attraction between the two charges is 80 times weaker in water than in air.C

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