Say I mixed together 20 mL of HCl solution with 20 mL of NaOH solution.

$$\ce{HCl (aq) + NaOH (aq) -> NaCl + H2O}$$

Strictly speaking, would the water produced from the neutralisation process cause the volume of the overall mixture to slightly increase? So, would the volume of the final mixture (strictly speaking) be 40 mL + the volume of water produced from the reaction?

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    $\begingroup$ Yes but, a quick calculation shows that using 0.1M solutions, which is relatively strong for many applications, the amount of water produced would be 0.036g of water, which is less than 0.1% of the initial volume. This is in the same order as the thermal expansion of about 10 degree temperature rise at roughly room temperature. Does that context help? $\endgroup$ – Spontification Feb 17 '16 at 12:21
  • $\begingroup$ @spontification why not just write an answer? :) $\endgroup$ – N A Feb 17 '16 at 13:16
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    $\begingroup$ The volume may change slightly, but not for that reason. After all, these water molecules did not appear out of thin air. Instead, they formed from some particles ($\ce{H+}$ and $\ce{OH-}$) that were in the the initial solutions and certainly did occupy some volume there. $\endgroup$ – Ivan Neretin Feb 17 '16 at 13:33
  • $\begingroup$ a good question to me, I think I felt I would be making more out of the question than is actually there. Strictly speaking the answer to the OP is "yes" but my comment was "practically speaking this is almost always irrelevant". $\endgroup$ – Spontification Feb 17 '16 at 13:36

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