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Typically, water experiences melting point depression when it is impure. The more impure it is, the lower the melting point, up to a point.

I understand you could raise the melting point by increasing pressure, as discussed here: How to RAISE the melting point of water?

However, are there any impurities known that increase the melting point of the water. Ideally, whilst still being mostly water.

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    $\begingroup$ I understand you could raise the melting point by increasing pressure It is rather converting ice to more compact ice structures with higher melting points. But the needed pressure is not convenient. :-) // Impurities generally decrease melting point of major substance. Many salt hydrates have high melting points, rather dissolving themselves in own crystal water, but I guess it is not what you want. $\endgroup$
    – Poutnik
    Jul 14 '21 at 8:35
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    $\begingroup$ When this kind of topic comes up, I like to cite tetrabutylammonium hydroxide 30-hydrate. Consisting of 67.5% water by weight, it can be isolated as a pure solid with a melting point around 27 °C. You could interpret this as an impurity which raises the melting point of ice by almost 30 °C. This is the hydrate with the highest water content I know that is a solid at room temperature, but others may exist with even more water. $\endgroup$ Jul 14 '21 at 11:11
  • $\begingroup$ When the pressure is about 1 bar, the melting point decreases when increasing the pressure, This tendency is reversed if the pressure is extremely high, and higher thant 1000 bar, which is not easy to obtain, $\endgroup$
    – Maurice
    Jul 14 '21 at 12:01
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    $\begingroup$ The more D and T atoms in water, the higher melting point. $\endgroup$
    – Mithoron
    Jul 14 '21 at 16:34
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    $\begingroup$ chemistry.stackexchange.com/questions/150905/… $\endgroup$
    – Mithoron
    Jul 14 '21 at 16:45

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