Rambam: (Hilchot Tum'at Ochalin Beg Chap 7)"A column of liquid being poured is not considered as joined etc. If one who pours cold liquids into hot liquids, the liquids are considered as joined? Because the vapors of the hot liquids ascend like a pillar of smoke." 

 

Three ways in the reason of the law (of cold liquids into hot liquids, the liquids are considered as joined) and their explanation in Avodat HaAdam

 

http://www.chabad.org/library/article_cdo/aid/1525386/jewish/Chapter-7.htm

 

Halacha 1

A column of liquid being poured is not considered as joined, neither to an impure entity nor to one that is pure.

 

What is implied? If one was pouring pure liquids into an impure container or even on the carcass of a crawling animal itself, the column of liquid being poured is pure. If one would collect some of the liquids that are being poured while they are in the air, what he collects is pure. Needless to say, that the liquids in the container from which one is pouring are pure.

 

Halacha 2

When does the above apply? When one is pouring cold liquids to cold liquids, hot liquids to hot liquids, or hot liquids to cold ones. If, however, one pours pure cold liquids into impure hot liquids, the column of liquids being poured is considered as joined. The liquids are considered as a primary derivative of impurity. They impart impurity to the container in which they are held.

 

Why did the Sages say that when one who pours cold liquids into hot liquids, the liquids are considered as joined? Because the vapors of the hot liquids ascend like a pillar of smoke. It becomes intermingled with the column of liquid being poured and the liquid in the upper container, causing it to become impure. For the vapor ascending from the hot liquid is also considered as liquid.