From today's OU Kosher Halacha Yomis
Q. May one pour cold water into hot soup to cool it down?
A. A hot pot of soup is a kli rishon (primary source of heat) and one may not add cold water to the pot, since this will cook the water. However, a bowl of soup is a kli sheini (secondary source of heat), and water does not cook in a kli sheini. It would therefore seem that one may add cold water to a bowl of soup. Still, the halacha is not so obvious, since there are also pieces of meat or vegetables in the soup. A solid food transferred to a secondary vessel retains the status of a kli rishon according to some poskim. Perhaps we need be concerned that the hot pieces of vegetable or meat may cook the water. The Pri Megadim (253: Aishel Avrohom 32) writes that it is possible that all agree that hot pieces of meat that are submerged in liquid in a kli sheini have the status of a kli sheini and cannot cook. This appears to be the consensus of many poskim (see Pischei Teshuva YD 94:7 and Kitzos Hashulchan 124:39). Therefore, one may add cold water to soup.