I have a very clever player that uses Prestidigitation for many useful things and one of the questions he asked me the other night is if he could use Prestidigitation to clean the salt out of the water. I wonder if water that has salt in it would be considered dirty. If I were to allow the cantrip to clean water in such a way am I forgetting another spell that would be tossed to the side because of this cantrip?
[RPG] Can Prestidigitation “Clean” Salt Water
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Best Answer
No, Prestidigitation only alters the temperature or flavor.
Prestidigitation is clear in its limits (PHB, 267):
This is not altering the properties of the sea water, it is merely flavoring it so that you couldn't taste the salt that's actually there.
Soiled Object
If you are trying to "clean" the salt water to make it potable, this is the section that you are trying to use is:
If water is considered an object, which some do not, then you can try to apply the above aspect of Prestidigitation.
But you should still find an obstacle here in that you can't 'clean' the water of its salt. While the salt may spoil its use for potable drinking water, it does not mean the water is soiled even if you do count water as an object (which you probably shouldn't). The salt isn't something that represents being soiled, it represents something that in high doses would be a poison.
Sodium Poisoning
Should you want to purify that water in order to remove the salt and make it potable, you would use Purify Food and Drink (PHB, 270), which states (emphasis mine):