![]() Ī hot water solution containing 73.1% (mass) of NaOH is a eutectic that solidifies at about 62.63 ☌ as an intimate mix of anhydrous and monohydrate crystals. Even the n = 3.5 hydrate is difficult to crystallize, because the solution supercools so much that other hydrates become more stable. it often decomposes into solid monohydrate and a liquid solution. When heated, the solid dihydrate might melt directly into a solution at 13.35 ☌ however, once the temperature exceeds 12.58 ☌. However, the solution can easily be supercooled down to −15 ☌, at which point it may quickly crystallize as the dihydrate. įor example, when a solution of NaOH and water with 1:2 mole ratio (52.6% NaOH by mass) is cooled, the monohydrate normally starts to crystallize (at about 22 ☌) before the dihydrate. However, solutions of NaOH can be easily supercooled by many degrees, which allows the formation of hydrates (including the metastable ones) from solutions with different concentrations. The other hydrates, except the metastable ones NaOH♳H 2O and NaOH♴H 2O (β) can be crystallized from solutions of the proper composition, as listed above. The only hydrates with stable melting points are NaOH Įarly reports refer to hydrates with n = 0.5 or n = 2/3, but later careful investigations failed to confirm their existence.
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