I believe you are thinking of the "if you put boiling water into an ice cube try and put it into your freezer it will freeze faster than tap water in your ice cube tray and placed in you freezer" observation. This is true depending on where you put the tray and it is a physical rather than chemical reason. When you put an ice cube tray into a freezer, you usually put it on top of a solid surface (often the solid metal cooling element) within the freezer which generally has a layer of frost. Tap water does not melt the surrounding ice and the ice cube tray rests on top of the ice crystals which are present on the top of the ice; only the very top of these
crystals are in contact with the base of the tray. However, when you put an ice cube tray containing boiling water into a freezer it melts the ice it comes into contact with underneath the ice cube tray; this causes the ice cube tray to sink into the ice and become engulfed within the ice, thus more of the vessel is in direct contact with the ice surface therefore the water cools a lot more quickly as the heat is more effectively removed from the base of the ice cube tray. If you levitated the two trays in the middle of the freezer (therefore eliminating the effects of contact with solid surfaces) the tap water one would freeze first.
Air pressure also has an effect. For example up Everest water boils at about 70 degrees and under atmospheric conditions it is, as we all know, 100 degrees. So to freeze water from boiling whilst up Everest you only need to remove 70 degrees worth of heat to get it to freeze, at atmospheric conditions you need to remove 100. The actual temperature water freezes at does vary from 0 under the two conditions but this is only by a very small amount (a fraction of a degree).
