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Name: Kavita
Status: student
Grade: 9-12
Location: N/A
Date: March 2008


Question:
What is the analytical importance of weighing crucibles when cool?



Replies:
I think the mass of crucible and contents is mostly finished changing before it cools.

But the scale could not handle heat well:

- it might be damaged by a really hot crucible.

- if the scale is electronic, it does not take much heat to perturb the reading some.

- if rather hot, it might be heating the air around it, causing convection currents, which will make a sensitive (milligram) reading unstable. (Buoyant forces.)

A fourth reason is thoroughly completing a course of treatment on the contents. Suppose you are heating crystals to de-hydrate them. Yes, they will weigh least while hot, and then pick up a little moisture while they cool. But think of it as a black box process, with measurements before and after to be compared. It was room temperature before you "heated-treated" it, and to be exactly comparable it should be room temperature after you heat-treat it.

If it is still hot, moisture can still go in and out fast and easy, and you will not know which direction is happening until you get it on the scale. In a sense it is kind of sad when one cannot put it on the scale and watch the action. But once it is all cool, at least any changes will be slower. For example, if you happen to heat for too short a time, it might actually continue losing mass until it is cool. If it is changing in mass while it is on your scale, which reading will you use? It could be confusing, and it definitely makes it harder to think clearly when analyzing the changes and talking about why. Another example is that you might open the lid, accidentally or to peek inside. If you do this while it is still warm or hot, I suspect the moisture up-take rate will be greater than if it is fully cooled down.

A fifth reason might be safety for you and assorted lab equipment.

Jim Swenson



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