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When a solute is added to a solvent, the volume of the solution should be equal to the sum of the individual volume of the solute and solvent (i.e total volume), right? But why is there a decrease in the total volume when sodium hydroxide (NaOH) dissolves in water (H2O), given sodium hydroxide (NaOH) does not react chemically with water (H2O)?

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When dissolved into water, $\ce{NaOH}$ dissociates into $\ce{Na+}$ and $\ce{OH-}$. Charged ions interact strongly with water via ion-dipole interactions, which are stronger than the hydrogen bonds characteristic of water intermolecular interactions. These strong interactions pull water molecules into relatively dense localized hydration complexes.

The water in these complexes are denser than pure water, decreasing the volume of the water. Adding $\ce{NaOH}$ to water does increase the volume of the solution compared with the pure water, because the ions occupy some volume, but the total volume of the solution is less than the sum of volumes of pure water and solid $\ce{NaOH}$ due to ion hydration.

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There is no law for volume conservation when dissolving a compound in another.

If "hetero-intermolecular" bonds are tighter than the weighted average of "homo-intermolecular" bonds, it leads to the overall volume contraction when they get dissolved. The same occurs for homo/hetero-intermolecular bond strengths and the energy outcome of mixing.

Analogical effects occur for ion-ion and ion-dipole interactions for ionic compounds being dissolved or formed.

Typical cases are dissolution of strong acids and alkali hydroxides. Another case is dissolution of ethanol and water.

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    $\begingroup$ (+1) I recall the demonstration of this effect using ethanol and water from my high-school chemistry class. $\endgroup$
    – njuffa
    Aug 29, 2022 at 23:08
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You can see it from the lens of intermolecular interactions. As answered earlier NaOH when dissolved in water creates $Na^{+}$ and $OH^{-}$ ions. Earlier it was just water which where hydrogen bonding (3rd strongest) was the force of attraction. After introduction of ions into water the force of attraction is ion-dipole (2nd most strongest force) which is much stronger as compared to the previous one.

Now higher intermolecular attraction means molecules tend to be near and more molecules exist in the same space (compared to earlier). Which mean higher density and lower volume.

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