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### Equation Choice

Posted: Fri Feb 14, 2020 1:48 pm
What defines a phase change and how does that relate to which equation you use?

### Re: Equation Choice

Posted: Fri Feb 14, 2020 2:05 pm
A phase change is when something goes from a solid to liquid (fusion), liquid to gas (vaporization), solid to gas (sublimation), or the reverse of these.
In terms of equations to use, I assume you are talking about finding the q value, or amount of heat, associated for that phase change. In this case you would use the equation q = m*deltaH, for which m = mass and deltaH = change of enthalpy for the phase change (ex: deltaH of fusion).

### Re: Equation Choice

Posted: Fri Feb 14, 2020 2:11 pm
Phase change occurs when a molecule/reaction goes from solid to liquid, liquid to gas, etc. This denotes fusion, vaporization, in terms of change in enthalpy. We learned equations: Hvap=Hvap-Hliq and Hfus=Hliq-Hsol and Hsub=Hvap-Hsol which relate to phase changes.

### Re: Equation Choice

Posted: Fri Feb 14, 2020 2:38 pm
Remember that if the phase change involves the atoms becoming more free and moving faster (i.e. solid to liquid, liquid to gas) the process is endothermic because the system is gaining heat and energy, meaning the q value is positive. The opposite is true for exothermic processes.

### Re: Equation Choice

Posted: Sun Feb 16, 2020 9:59 pm
Phase changes: solid --> liquid (fusion), liquid --> gas (vaporization), solid --> gas (sublimation) or vice versa : melting, condensation, deposition.