Material:
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Just like glass, set transmission to 1, and use pure white color.
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Decrease the indice of refraction IOR down to 1.33 (1.31 for boiling liquid) - lookup correct value.
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Add Volume absorption in case the liquid is colored and not fully transparent.
Because of a bug in blender, water should slightly overlap the glass material. Otherwise, refraction is incorrect (thin layer of air betwen the glass and the liquid).
Make sure that face orientation is OK (mesh not inside out).