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Name: Andres
Status: other
Grade: other
Country: USA
Date: Spring 2014


Question:
Does moisture affect the melting point of a compound? (compound is wet)



Replies:
Andres,

If the compound is solid water, then the presence of liquid water does not affect the melting point of the ice. The presence of liquid water may aid in the heat transfer and speed up the melting process, but the ice would still melt at 0 degC under standard conditions.

If the compound is something other than water, and liquid water is able to dissolve into the compound or the compound dissolves in liquid water, then the melting is lower than without the liquid water. The effect is from one of the so-called "colligative properties" - specifically, freezing point depression. Essentially, what is happening is that at the melting point, the rate of solid becoming liquid and liquid becoming solid are equal. By definition, the melting point is when this equilibrium happens. The presence of some other compound (in this case, water) inhibits the rate of the liquid becoming solid - you can think of it as physically getting in the way of the liquid molecules reattaching to the solid phase (although the rationale is more complex than that). Because of this rate inhibition, the system has to go to a lower temperature in order for the equilibrium to be established (in the presence of the water molecules). So the melting point is at a lower temperature.

Note that the inhibition mentioned in the previous paragraph only happens if the water molecules are able to dissolve in the liquid phase of the compound. If the water is not soluble in the liquid phase of the compound, then the rate inhibition does not happen, and there is no freezing point depression.

Dr. Roberto Ma. Gregorius, Assoc. Prof.


Hi Andres,

Thanks for the question. In some circumstances, moisture can affect the melting point of a compound. In general for this to occur, the moisture must be trapped in the material and the material must be miscible (at least partially) with water. Water will affect the melting point of certain compounds such as aspirin (acetylsalicylic acid). This effect is often noticed by my students who make aspirin in their chemistry courses. Moisture that is trapped in the aspirin crystals will cause the melting point to be depressed and broadened. For example, a compound that normally melts at 165.2-165.7 degrees might melt at 124.4-128.9 degrees.

I hope this helps. Thanks Jeff Grell


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