Say that a person had a container with holy water and the water within it froze. Would that frozen holy water still retain its blessing and be efficacious in protecting against evil, be able to be used in baptism, etc?
Changes of state are accidental changes, not substantial changes—viz., frozen water is still water, despite it changing temperature, density, having a different amount of air dissolved in it, etc.
- Si in hieme Sanguis congeletur in Calice, involvatur Calix pannis calefactis: si id non proficeret, ponatur in ferventi aqua prope Altare, dummodo in Calicem non intret, donec liquefiat.
If in the winter the Blood freezes in the Chalice, wrap the Chalice with warm cloths: if this does not work, put [the Chalice] in hot water near the Altar, such that the it does not enter the Chalice, until [the Blood] liquefies.
It does not say that frozen Blood of Christ ceases to be His Blood. Similarly, frozen holy water is still holy water.
Would frozen holy water still retain its blessing?
The short answer is: No.
Generally, things that are blessed retain their blessing so long as they are “intact” and recognizable as the original substance.
Spilled holy water quickly evaporates or is absorbed and can no longer be recognized. Boiling (minus what evaporates) or freezing holy water technically doesn’t render it unrecognizable (I’m thinking of holy water stoops at the doors of churches in winter, that can get frozen over).
Diluting holy water — greater than adding equal volume — renders it ‘doubtful’, and at some point contaminated (dirt, algae) water no longer looks quite like water. - Christopher Nowak
I disagree in one point with Christopher Nowak. Although "freezing holy water technically doesn’t render it unrecognizable (I’m thinking of holy water stoops at the doors of churches in winter, that can get frozen over)." It is nevertheless not the same substance. Water is water and ice is ice. Water is not ice and ice is not water.
For a priest to create holy water, he must perform the prescribed prayers and signs (of the cross) over ordinary water. In the Extraordinary form of the Mass not only is holy water is exorcised, it is also mingled with exorcised salt. It is not magic. As the saying goes: You do not make holy water by boiling the hell out of it. Water is no longer water if it is evaporated and turned into steam. In the same way, it is no longer holy water if it is frozen. It is in fact not the same substance.
Let us consider what the Catholic Encyclopedia has to say in regards to Baptismal Water:
The remote matter of baptism, then, is water, and this taken in its usual meaning. Theologians tell us consequently that what men would ordinarily declare water is valid baptismal material, whether it be water of the sea, or fountain, or well, or marsh; whether it be clear or turbid; fresh or salty; hot or cold; colored or uncolored. Water derived from melted ice, snow, or hail is also valid. If, however, ice, snow, or hail be not melted, they do not come under the designation water. Dew, sulfur or mineral water, and that which is derived from steam are also valid matter for this sacrament. As to a mixture of water and some other material, it is held as proper matter, provided the water certainly predominates and the mixture would still be called water. Invalid matter is every liquid that is not usually designated true water. Such are oil, saliva, wine, tears, milk, sweat, beer, soup, the juice of fruits, and any mixture containing water which men would no longer call water. When it is doubtful whether a liquid could really be called water, it is not permissible to use it for baptism except in case of absolute necessity when no certainly valid matter can be obtained.
Thus ice is not the same substance as water and the blessing has been lost. This would apply when the holy water has completely frozen over. Care should always be undertaken that this situation be avoided and is in itself a reason why I prefer holy water that has been blessed according to the Rituale Romanum according to Pope St. Pius V: Salt water remains liquid at lower temperatures.