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My friend and I were debating about if Molarity is constant in a disolution if this one changes volume due to evaporation.

There was this problem that stated that 125 [mL] of sodyum chloride 0.122 [M] evaporates over time and final volume becomes 113 [mL]

I say Molarity keeps being constant, he says no because volume changes.

What do you think?

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    $\begingroup$ $c_1=n/V_1;$ $c_2=n/V_2.$ What do you think the relation between $c_1$ and $c_2$ is when $V_1>V_2?$ Also, Wikipedia. $\endgroup$ – andselisk♦ Jan 25 at 5:35
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    $\begingroup$ If you knew what exactly molarity is, there would be no debate. As you did not, why did you not rather search it up, instead of asking? It would be much faster. // Do people obtain salt by evaporation of seawater, or are they liers ? Does have saturated NaCl solution the same molarity as e.g. 0.1 M NaCl solution ? $\endgroup$ – Poutnik Jan 25 at 6:32
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    $\begingroup$ Presumably this is an aqueous solution. So more fundamental than andselisk's question: (1) Does water evaporate from a sodium chloride solution? (2) Does sodium chloride evaporate from a sodium chloride solution? $\endgroup$ – MaxW Jan 25 at 6:33
  • $\begingroup$ Interestingly, I was reading someone's thesis on corrosion yesterday and airborne salinity is indeed a problem near coastal areas, so in one sense sodium chloride can also be lost in the air too! $\endgroup$ – M. Farooq Jan 25 at 6:42
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    $\begingroup$ @M.Farooq But it s rather dynamic separation of solution into aerosol, eventually evaporated later. I have head you need on some island to work with tap water ventils/regulators every 2 weeks, otherwise they get "frozen" due salt and you cannot turn them anymore. $\endgroup$ – Poutnik Jan 25 at 6:57
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As molarity is defined as the ratio of molar amount of the dissolved solute and solution volume:

$$c=\frac nV$$

molarity increases when solvent ( like water ) evaporates.

Molarity can be considered constant only in context of later refilling the evaporated solvent and mixing the solution. Then molarity returns to the original value. It assumes the solute ( like sodium chloride ) content is constant, not undergoing any reaction.

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