This made Dorothy so very angry that she picked up the bucket of water that stood near and dashed it over the Witch, wetting her from head to foot.
Instantly the wicked woman gave a loud cry of fear, and then, as Dorothy looked at her in wonder, the Witch began to shrink and fall away.
"See what you have done!" she screamed. "In a minute I shall melt away."
"I'm very sorry, indeed," said Dorothy, who was truly frightened to see the Witch actually melting away like brown sugar before her very eyes.
"Didn't you know water would be the end of me?" asked the Witch, in a wailing, despairing voice.
"Of course not," answered Dorothy. "How should I?"
"Well, in a few minutes I shall be all melted, and you will have the castle to yourself. I have been wicked in my day, but I never thought a little girl like you would ever be able to melt me and end my wicked deeds. Look out--here I go!"
-The Wonderful Wizard of Oz, L. Frank Baum (1900).
Imagine a race of humanoid, intelligent aliens who display the same characteristics as the Wicked Witch of the West, but aren't magical. How could I explain why such aliens literally "melt" when splashed with a bucket of room-temperature pure water?
The ideal answer would provide a plausible reason for pure water to cause this effect based on known chemistry and biology with minimal handwaving.