When a compound reaches boiling/melting poin

[SephirothXR]

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Does the whole thing vaporize/melt immediately or does it slowly melt/evaporate?

 

[vayntraubinator]

Does the whole thing vaporize/melt immediately or does it slowly melt/evaporate?

They're in equilibrium between the two phases:

at boiling point there is an equilibrium b/w vapor and liquid.

at melting point there is an equilibrium b/w solid and liquid.

How fast it changes phase refers to the kinetics and rate, which depends on different factors, mainly Temperature, Pressure and Energy change of the system.

 

[SephirothXR]

So what if water is at 102 degrees Celsius. Because it's above the equilibrium, can you then say with certainty if it will all immediately vaporize or you'd still need more information?

 

[salim271]

So what if water is at 102 degrees Celsius. Because it's above the equilibrium, can you then say with certainty if it will all immediately vaporize or you'd still need more information?

At atmospheric pressure, it will vaporize. For vaporization and melting, both enthalpy and entropy are positive.

If you look at the equation above, if both enthalpy and entropy are positive, whether or not the reaction will run is completely dependent on the temperature. When a liquid's vapor pressure exceeds local atmospheric pressure, it will boil, and temperature is the factor that increases vapor pressure.

Conversely, for condensing and freezing, both entropy and enthalpy are negative, and therefore are spontaneous reactions as long as temperature (again the dependence on temperature) is not high enough to stop it from happening.

Also just to mention it, vaporization and evaporation are not the same thing, although evaporation is dependent on the vapor pressure in comparison to pressure above the surface of the liquid. Evaporation happens when the liquid molecules at the surface have a high enough vapor pressure to break from the surface, and as long as the pressure directly above the liquid is low enough, that momentary burst of kinetic energy is enough for the water molecules to escape the liquid (as long as you aren't covering the top of the jar or something.) The liquid doesnt boil when is evaporates, it boils when it vaporizes.

 

[exquisitemelody]

This sounds almost like a question I saw where it was talking about putting a heat source under a sample that was almost nearly at its melting point for only a second and asking whether or not the whole sample would melt. The answer is no - the source of energy wasn't there long enough to make the whole sample melt, even though the melting point of the compound was reached.

 

[salim271]

This sounds almost like a question I saw where it was talking about putting a heat source under a sample that was almost nearly at its melting point for only a second and asking whether or not the whole sample would melt. The answer is no - the source of energy wasn't there long enough to make the whole sample melt, even though the melting point of the compound was reached.

That's because melting and vaporizing arent processes that take place within a single second, the temperature has to maintained. If the temperature drops from 102 to 99 for the entire liquid, then at atmospheric pressure it obviously wont boil anymore, again following the delta G equation. Temperature is everythingggggggg 🙂

Edit: Oh and actually, as soon as the temperature of vaporization is reached (such as 100 degrees C) then the temperature of the compound wont increase anymore until it has completely vaporized as all the heat entering the system is going towards disrupting the intermolecular forces (i.e h bonding, dipole-dipole) that are holding the liquid in its condensed form. So the liquid wont actually reach 102 degrees until after is a gas, unless the local atmospheric pressure is high enough that 102 is the true boiling point and not 100.

 

[Isoprop]

the above answer (edit) is correct.

 

[Axon hillock]

the above answer (edit) is correct.

Awesome image.