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For many salts there are solubility curves as a function of temperature that are smooth (don't have any kinks). Sodium sulfate, however, has a kink in the solubility-T curve at 30 degrees as shown below:

enter image description here

What happens when the temperature is about 30 degree?

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"Sodium sulfate" is a mixture of hydrated phases in shifting equilibrium given temperature and concentration.

Na2SO4.(H2O)x phase diagram, 1 Na2SO4.(H2O)x phase diagram, 2

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The phase diagram is also interesting here. Notice the convergence of three lines at about 30C and 30 wt% NaSO4, right near where the kink in the solubility happens.

enter image description here

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  • $\begingroup$ Do you still remember the source where you got this phase diagram? $\endgroup$ – Thomas Jungers May 28 '18 at 12:01
  • $\begingroup$ No, sorry. Wish I had cited it. $\endgroup$ – wordsforthewise May 28 '18 at 19:35
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    $\begingroup$ In case anyone needs it: the figure is from Angeli et al., 2010 [1], adapted from Hougen et al., 1954 [2]. [1]M. Angeli, R. Hébert, B. Menéndez, C. David, and J.-P. Bigas, ‘Influence of temperature and salt concentration on the salt weathering of a sedimentary stone with sodium sulphate’, Geological Society, London, Special Publications, vol. 333, no. 1, pp. 35–42, Jan. 2010. [2]O. A. Hougen, K. W. Watson, and R. A. Ragatz, Chemical Process Principles. Part I. 2nd edn. Wiley, New York, 1954. $\endgroup$ – Thomas Jungers Mar 12 at 14:57

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