Adding Cold Water on Shabbos

Q. May I add cold water to a hot cup of tea on Shabbos?

A. In general, raw food may not be placed in a klei rishon on Shabbos, but it may be placed in a kli sheini (a vessel which was filled with liquid from a kli rishon). Raw food will not cook in a kli sheini, because the heat is less intense. However, the Talmud states that certain foods which are “kalei ha’bishul” (that cook easily) will cook even in a kli sheini. In practice, we are uncertain which foods are kalei ha’bishul, and therefore we are generally strict and do not place any raw food in a kli sheini (see Mishnah Berurah 318:42). However, it is clear from the Talmud and poskim that water is not in the category of kalei ha’bishul (see Mishnah Berurah 318:86), and therefore, cold water may be added to a kli sheini.

Still, there may be another concern with adding cold water to a hot cup of tea on Shabbos. The Mishnah Berurah (318:48) cites the opinion of the Chayei Adam that if the hot water in a kli sheini is at the temperature of yad nichveis bo (scalding hot) then the vessel retains the status of a kli rishon, and cold water may not be added on Shabbos. Rav Belsky zt”l estimated that temperatures exceeding 180ºF are considered scalding hot. Igros Moshe (OC 4:74, Bishul §4) was asked if the custom is to follow the stringency of the Chayei Adam. He responded that it is uncertain, but he seemingly recommends being stringent. However, Rav Moshe states that if the hot water is transferred to a kli shlishi (a third vessel), even the Chayei Adam would allow adding cold water.

In summary, cold water may be added to a cup of tea if the water is not scolding hot. However, if the tea is transferred to a third vessel, cold water may be added under all circumstances.

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The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.