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Q & A: why does liquid freeze at different temps.?

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Most recent answer: 02/02/2015
Q:
why does liquid freeze at different temps.?
- elijah (age 13)
america, AZ, payson
A:

Substances, including liquids, are made of building blocks called molecules. The molecules are bound to each other by bonding. The stronger the bonding, the harder it is to break them apart. So you need more energy to break it.

Since the freezing point (the temperature at which your liquid freezes) and melting point (the temperature at which your solid melts) are the same, and freezing is in fact reverse of melting, let me talk about melting first. Different kinds of solids are made of different molecules, so the bonding strength is different for different solids. Melting the solids means breaking the bonds. So if solid A has stronger bonding than solid B, you need to give more energy to solid A to break its bond than to solid B. Heat is our way to transfer energy. This means that solid A needs more heat to melt and to become liquid. In other words, solid A melts at higher temperature than solid B. (e.g. ice melts at 32 F while iron melts at 2,800 F. The bonding of iron is much stronger!) 

Freezing is the reverse process. If the bonding is weak, a little bit of energy can still break the bonding, preventing the liquid from solidifying. So you have to cool down enough to get rid of energy. For example, liquid iron "freezes" at 2,800 F, so you only see solid iron in the room temperature. On the other hand, you have to cool down liquid water to 32 F to get rid of enough energy that prevents water molecules from forming bonds with each other. 

So in our example, the liquid iron and the water freezes at different temperatures because their bonding strength is different. This reasoning applies to other liquids as well.

SHC.


(published on 02/02/2015)

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