What's the scientific reasoning behind the principle?

Question

How come the crystallization temperature is determined during cool-down and not as soon as the substance dissolves?

For example, if I'm trying to find the crystallization temperature of salt or sugar, how come I'm supposed to use the temperature at which it crystallizes when cooling down instead of just taking the temperature at which it fully dissolves?

Isn't that the same difference as ice freezing/melting? I'm guessing there's some good reasoning here, but I'm just not quite sure what it is ;-)

Answer 1

Dissolving and crystallizing are not the same as freezing and melting. For instance, you could dissolve sugar in warm water, and chill it all the way to freezing without the sugar crystallizing.

I have never heard of a crystallization temperature. In my opinion, it depends on the amount of solute you have dissolved.

A 50% by weight sugar solution could crystallize at room temperature, while a 0.1% solution may never crystallize. And you should look up "supersaturated solution," very interesting stuff.

Answer 2

Many salts & organic compounds are exothermic ( give off heat) when dissolving in solvents. In this case you will get an unreal supersaturated solution because of the energy released during dissolution. Thus an accurate crystallization temp. can only be found after it starts to cool.

Answer 3

Well, since nobody has replied i tried to search online, but the example it stated was for cloud formation.

See exerpt below:

The initial formation of cirrus clouds normally requires that cooling take place to saturation, and to have temperatures near –40°C. Under these conditions, water droplets are first formed, but most of them immediately freeze. The resulting ice crystals persist as long as the humidity remains near saturation with respect to ice. There is some evidence that the speed of the cooling, and the kind and abundance of freezing nuclei, may have an important effect on the form and occurrence of cirrus clouds. Slow ascent starts crystallization at humidities substantially below saturation; this is presumably the case in extensive cirrostratus clouds associated with warm frontal altostratus clouds. If slow ascent occurs in air that has insufficient freezing nuclei, a widespread haze may result, which at –30° to –40°C is predominantly composed of water droplets. In the case of more rapid cooling, there is a tendency for the initial condensation to contain a higher proportion of water droplets, which leads to a “mixed cloud’ that will convert to ice or snow in time. Presumably, dense cirrus, fine cirrus, cirrocumulus, and anvil cirrus clouds are of this type. It is assumed that fine cirrus clouds (proper) are formed in shallow layers that are undergoing rapid convection due to advection of colder air at the top of the shear layer.

Here's the link: http://www.tpub.com/content/aerographer/14010/css/14010_73.htm