[kenny1999]

when reflux condenser is used in distillation to separate the desired chemicals (distillate) from the mixture. It is said that water (as coolant) has to drive from the lower fitting to the upper fitting, but I really don't understand why after spending quite a good period of time researching on web, maybe I am too stupid but I hope someone could explain the reason to me clearly. I want to understand the reason behind. Thank you every one

[enahs]

If you fill from the bottom, it has to fill up the entire condenser first before exiting.

If you fill from the top, it will just go from entrance to exit directly (path of least resistance), and not fill the entire condenser up, thus not cooling properly. Hook it up backwards and watch.

[Doc Oc]

In addition to what enahs said, if you fill from the bottom of the condenser the coolest water is always entering closest to the flask where the vapors are at a maximum.  If you cool from the top all the coolest water is at the top of the condenser where the least amount of vapor is.

[discodermolide]

when reflux condenser is used in distillation to separate the desired chemicals (distillate) from the mixture. It is said that water (as coolant) has to drive from the lower fitting to the upper fitting, but I really don't understand why after spending quite a good period of time researching on web, maybe I am too stupid but I hope someone could explain the reason to me clearly. I want to understand the reason behind. Thank you every one

Filling from the top stands a good chance of cracking the glass ALWAYS fill from the bottom.