A baraita[1] states that on Shabbat, one may not crush snow to get water, although he can put snow in a cup of water and have it melt into the water. There are three main explanations of the prohibition.[2] Rashi[3] says that it resembles a melacha, as one actively creates a new object form. The Sefer HaTerumah[4] asserts that the resulting water is nolad (lit. born, referring to a type of muktzeh). The Rambam[5] groups this action with a Rabbinic prohibition related to squeezing fruit for juice.
A practical difference between the explanations comes about when one warms congealed gravy so that it becomes a liquid. The Sefer HaTerumah forbids this as nolad, as well. However, the Beit Yosef presents the Sefer HaTerumah as a minority opinion and thus rules against in the Shulchan Aruch.[6] The Rama,[7] in contrast, writes that the minhag follows the stringent opinion, while noting that one can be lenient in a case of need. The same disagreement should apply to putting ice in a warm place (not hot enough to be considered cooking) to melt.[8]
Most poskim maintain that this machloket also applies to the question of freezing water. If transforming from ice to water is considered changing an object, why should transforming from liquid to solid be any different? Accordingly, for Sephardim, who follow the Shulchan Aruch, it should be permitted to make ice cubes on Shabbat, and for Ashkenazim, who follow the Rama, it should be permitted only in a case of need.
However, some poskim distinguish between the cases in different ways. The Dovev Meisharim,[9] for one, posits that the reason to allow melted ice is that ice that is slated to turn into water is considered a form of water. In contrast, in relation to water, ice is a new entity, which one may not create on Shabbat. However, most poskim[10] reject this distinction and maintain that those who permit melting ice cubes also permit placing water in the freezer to make ice cubes.
It is also possible that the Rama, who is ambivalent regarding melting fats, might allow making ice cubes outright. The Tzitz Eliezer[11] points out that there are two ways to explain the Sefer HaTerumah’s ruling, which is the source of the Rama’s stringency. One explanation[12] is that the water resulting from the melting is muktzeh, and it makes no difference what process produced it. The other explanation[13] is that the process of putting fat near a fire is considered semi-actively turning solid into liquid. If the latter is the only problem, we could consider that placing water in a freezer, where the process of freezing does not begin in earnest for a while, is an act too far removed from the eventual formation of ice to be forbidden. Certainly, the Rambam’s logic regarding crushing snow – that it is similar to squeezing – does not apply to turning liquid into solid.[14]
Some raise the possibility that freezing water is similar to making cheese, which is forbidden as an extension of building,[15] but a consensus of poskim rejects this idea for various reasons. First, building likely does not apply to water.[16] Also, ice lasts only as long as it is kept cold, and thus one has not built anything stable.[17]
In summary, Sephardim can freely make ice cubes in a freezer. Ashkenazim have ample reason to be lenient, and certainly when there is significant need (although recent poskim nuance this compromise differently[18]). If one does not plan to use the ice cubes on Shabbat, the problem of muktzeh does not apply, which would seem to make the case for leniency stronger. However, under those circumstances, there would usually be a problem of hachana (preparations for after Shabbat). Certainly, one may put a drink in the freezer to quickly cool it, despite the possibility that he will forget about it there, considering the preponderance of opinions that maintain that even purposely freezing is permitted.[19] (Automatic ice makers, where electrical systems are a factor, are beyond our present scope.)