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We were doing an experiment about hydrated crystals and more precisely how to determine $n$ in $\ce{CuSO4.nH2O}$. After we heated the crucible we were to cool it down using a Desiccator. Then this question came to my mind: Why must we cool it down? By doing researches in the Net I were only able to come up with these suggestions, but none seem very applicable for me:

Using Einstein's formula:

His famous equation:$$E = mc^2$$ This means that matter is able to be converted to energy. Thus, more energy should mean more mass because $C$ is constant. However, I argue that since $C$ is a very large number, normal and observable-in-normal-labs changes in energy aren't able to modify the net change of the mass dramatically (to a degree that the balance is able to detect)

A gut feeling, no backup research:

Anyone sane, and with regards to the value the balance has in the lab, would give slightest chances that the hot container would harm the surface of the balance and therefore, damage it. But chances are that I'm wrong too.

So, why must we cool the crucible in order to measure the mass of the sample in it using a balance?

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  • $\begingroup$ I'm confused why you accepted this answer. The below answer is good and your answer is not bad. I don't want to tell you which answer is better and You can accept which one you want. $\endgroup$ – Berne Jan 23 '15 at 17:24
  • $\begingroup$ Fred's answer is very informative, but it adds points to what ron and I said. And ron's answer was simple and short, while I went on telling grandma stories! :) $\endgroup$ – M.A.R. Jan 23 '15 at 17:37
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A container that is not at ambient temperature will generate air currents around it. If you place such a non-ambient container in a balance, air currents will develop around the container as it heats or cools to ambient. These air currents will cause the balance to read incorrectly.

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The other two answers here are fine; it's true that as the hot object you're trying to weigh warms the air around it, the air will rise, creating air currents that will give you an unsteady reading.

I'd like to add three things they didn't mention:

  • The balance pan will heat up, too, causing metal parts in the balance to expand. This will also contribute to an error in your reading, and as the metal cools, the reading will change.

  • Another contribution to the error is that the density of a hot sample is less than the density of a cold sample, so its buoyancy is different. The object will appear to gain a tiny amount of weight as it cools!

  • Don't worry about $E=mc^2$ in chemistry unless you've got some serious nuclear reactions going on.

And a fourth thing: if you put a hot object into an air-tight dessicator, the air inside will contract as the object cools---and you might have some trouble getting the lid off again!

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Your gut feeling is right: Very hot crucibles can cause damage to the balance. However, this isn't the main reason for why chemists cool their samples down before measurement.

We are all familiar with convection currents: When air is heated the molecules will collide more than its previous state; resulting in an increase in either volume or pressure depending on the "flexibility" of the environment of effect. Thus, the density of hot air decreases, and it goes up and "replaces" the cold air. If the process of heating the air continues, hot air will continuously go up in what we call a "convection current".

The system of the crucible is thermodynamically open, so in contact with air it will lose some of its heat to its surrounding gas, which is normally atmosphere air. If we put the hot crucible for measuring its mass, convection currents will occur around the crucible, where it's in contact with the surrounding air. The motion of the hot air upwards forces your crucible upwards, but in small amounts.

The balance has to be precise enough to detect these differences, though. Most balances in the labs can measure up to 0.01 or 0.001 gram, and will sense the difference. Since you usually need (or are made to write) exact measurement results up to 1 miligram, these differences are going to be irking pests in measurement.

This is a very useful source from which the answer came from. It also has useful recommendations on practical problems in the lab.

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You must let the crucible cool before measuring it because the heat from the crucible warms the surrounding air, which rises, then that air cools down and falls. This rise and fall of surrounding air is called a convection current and will give you an unsteady reading that is rising and falling. These currents will make it hard to find the weight of the anhydrate(CuSO4), thus making it hard to find n, the moles of water per mole of CuSO4.

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    $\begingroup$ This was already told in other answers. $\endgroup$ – Mithoron Sep 4 '17 at 21:32
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While these are all very good answers, I have a much simpler one. These far overthink the question. The crucible has to cool in the desiccator to avoid adding moisture to it. The experiment you're doing deals with hydrates, therefore getting as much moisture out as possible is important as it will reduce your percent error.

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    $\begingroup$ I'd agree with that in general, but the question is asking why it must be cooled, not why it must be cooled in a desiccator. $\endgroup$ – orthocresol Nov 17 '16 at 2:53

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