# Why is the solubility of sodium acetate trihydrate so much lower than anhydrous sodium acetate?

According to Wikipedia:

Solubility of anhydrous sodium acetate in water at 0 $^{\circ}$C is 119 g/100 mL (AKA 1.495 mol/100mL)

Solubility of trihydrate sodium acetate in water at 0 $^{\circ}$C is 36.2 g/100 mL.

This makes no sense to me because it should take MORE of the trihydrate sodium acetate (about 1.495 * 136.08 g) to dissolve the same amount of sodium acetate.

So this begs two questions:

1.) Is my understanding of hydrates incorrect? Are their bonds somehow harder to solvate than the anhydrous version? I think this can't be correct because the melting point of sodium acetate trihydrate is 58 $^{\circ}$C compared to the anhydrous melting point of 324 $^{\circ}$C.

2.) Is my understanding of solubility incorrect? I thought solubility is a measure of the quantity of solute that could be dissolved in a solvent. In other words, it seems that MORE sodium cations and acetate anions can be dissolved in 100 mL of water if they came from an anhydrous crystal rather than a trihydrate crystal... How can that be true?

• See fr.wikipedia.org/wiki/Ac%C3%A9tate_de_sodium there's 362 not 36.2 - someone put decimal point in wrong place. – Mithoron Aug 7 '16 at 22:27
• The figures in the English Wikipedia for the solubility on anhydrous sodium acetate in water are most probably wrong. – aventurin Aug 7 '16 at 23:09
• @Mithoron That still doesn't make sense because that is literally three times the amount than anhydrous can be dissolved...@aventurin I can't seem to find anywhere another source of the solubility of anhydrous sodium acetate – Nova Aug 8 '16 at 0:51
• The trihydrate already has it's water of crystallisation in the crystal lattice so it would take longer to dissociate in water , it would be less hygroscopic . – Technetium Aug 8 '16 at 2:08
• @Mithoron The values in French Wikipedia are given per liter, so they're the same. – vapid Aug 8 '16 at 9:40

## 1 Answer

The values of solubility for these salts are $100\%$ correct. Your understanding of solubility is also correct. What you didn't know (probably) is that solutions can exist in a non-equilibrium state. In case of anhydrous sodium acetate, it dissolves in water forming so-called supersaturated solution, which is metastable. Eventually, crystals of sodium acetate trihydrate will precipitate out of the solution, forming a system which is in the thermodynamic equilibrium.

• Crystals of sodium acetate trihydrate would precipitate out of solution (of anhydrous sodium acetate) until it reaches the solubility of the normal sodium acetate trihydrate solution?? That is incredible. Then what is the point of naming both solubilities when one will eventually equilibrate to the other? – Nova Aug 8 '16 at 23:51
• @Nova Why name it otherwise? It is solubility. All substances dissolve in water to the point where they reach the equilibrium of dissolution and crystallization processes. Most substances dissolve starting from zero concentration and reach the saturation. There are some substances, however, that can exceed the saturation concentration, then reach the equilibrium from the other site - by crystallizing out of the solution. Sodium acetate is a classical example. There is nothing incredible, really. – vapid Aug 9 '16 at 7:03
• @Nova You can picture it that way (it is a bit of simplification): when you dissolve the anhydrous sodium acetate you get a solution which is saturated in respect to the anhydrous salt and supersaturated in respect to the trihydrate. You will eventually get a saturated solution of trihydrate and excess of crystalline trihydrate. – vapid Aug 9 '16 at 7:11