（１０分）化學名---咩叫supercooling同superheating??

1-4)
Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid.

Description
A liquid below its freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. The homogeneous nucleation can occur above the glass transition where the system is an amorphous—that is, non-crystalline—solid.

Droplets of supercooled water often exist in stratiform and cumulus clouds. They form into ice when they are struck by the wings of passing airplanes and abruptly crystallize. (This causes problems with lift, so aircraft that are expected to fly in such conditions are equipped with a deicing system.) Freezing rain is also caused by supercooled droplets.

An equivalent to supercooling for the process of melting solids is much more difficult, and a solid will almost always melt at the same temperature for a given pressure. It is, however, possible to superheat a liquid above its boiling point without it becoming gaseous.

http://en.wikipedia.org/wiki/Supercooled

In physics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its standard boiling point, without actually boiling. This can be caused by rapidly heating a homogeneous substance while leaving it undisturbed (so as to avoid the introduction of bubbles at nucleation sites).

Mechanics
Because a superheated liquid is the result of artificial circumstances, it is metastable, and is disrupted as soon as the circumstances abate, leading to the liquid boiling very suddenly and violently (a steam explosion). Superheating is sometimes a concern with microwave ovens, some of which can quickly heat water without physical disturbance. A person agitating a container full of superheated water by attempting to remove it from a microwave could easily be scalded.

Superheating is common when a person puts an undisturbed cup of water into the microwave and heats it. Once finished, the water appears to have not come to a boil. Once the water is disturbed, it violently comes to a boil. This can be simply from contact with the cup, or the addition of substances like instant coffee or sugar, which could result in hot scalding water shooting out. The chances of superheating are greater with smooth containers, such as brand-new glassware that lacks any scratches (scratches can house small pockets of air, which can serve as a nucleation point).

Rotating dishes in modern microwave ovens can also provide enough perturbation to prevent superheating.

There have been some injuries by superheating water, such as when a person makes instant coffee and adds the coffee to the superheated water. This sometimes results in an "explosion" of bubbles. There are some ways to prevent superheating in a microwave oven, such as putting a popsicle stick in the glass, or having a scratched container to boil the water in.

Scope restriction
Milk and water with starch content do not boil over because of superheating, but rather result in extreme foam buildup. This foam is stabilized by special substances in the liquids and therefore does not burst.

http://en.wikipedia.org/wiki/Superheating

5) 有. 响發電過程(尢其係核能), 發電過程用嘅水就係superheated. 做cooling嘅水就係supercooled.

Shadow.