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Name: Maggie
Status: student
Grade: 9-12
Country: Malaysia
Date: Fall 2011


Question:
Why does crystalline solid has to be in the lowest energy state?



Replies:
Maggie,

My first thought upon reading your question was: "... lowest energy state - relative to what?"

The problem is that yes, crystalline solids have lower energy states compared to the same substance at higher temperatures and different states, but it may also be of a higher energy state relative to other substances at the same temperature.

So, maybe let us look at what "crystalline solid" means - and then you can answer the question based on whatever parameters you want to apply.

By "solid" we mean that the atoms that make up the substance do not have translational motion. Meaning to say, they may vibrate in place, but on average stay in the same location. This means that there is a higher level of order (lower level of entropy) in solids compared to other phases (liquids and gases) that do have translational mobility. For this reason, when compared to other states, we say solids - everything else being equal - have a lower energy state.

By "crystalline" we mean that the atoms that comprise the solid are very regularly spaced. This means that solids that are not crystalline have more disorganization (and likely more translational mobility) than crystalline solids. It is likely than that there is less stored energy in crystalline solids than in noncrystalline solids.

Of course, all these statements rely on a careful statement of what is held constant in the comparison. So, you need to carefully state that for yourself when making such comparisons.

Greg (Roberto Gregorius) Canisius College


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