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After heating an ampoule of tellurium around its melting point in a furnace for about a day, I've noticed drops of the element on the upper inside surface of the ampoule, as if it condensed there. I've also noticed similar behavior with other intermetallic compounds. Is it common for something like tellurium to condense and resolidify in drops when an ampoule of it is heated (like water as shown here)?

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This is a general phenomenon. All substances at any temperature above $0\ K$ will shed a few atoms from the solid or liquid, forming a gas however dilute. The tendency for this to happen varies with temperature, and it can be measured by determining the equilibrium pressure of a substance inside an evacuated chamber. The key concept behind this is vapour pressure. There are several previous Chem.SE questions regarding vapour pressure, so you may find something useful, such as perhaps this.

As a rule of thumb, the higher the boiling/sublimation point of a substance, the lower its vapour pressure at any given temperature (in fact, the definition of the boiling/sublimation point is the temperature where the vapour pressure equals the pressure exerted by the surroundings onto the solid/liquid substance, which is in many cases equal to $1\ bar$). Tellurium is a relatively low boiling-point solid, with a vapour pressure of $1\ bar$ at $988\ °C$. According to this article, the vapour pressure of tellurium at its melting point ($450\ °C = 723\ K$) is approximately $70\ Pa$. This means there is a fair amount of tellurium vapour present inside the ampoule at this temperature. It is entirely conceivable that some of this vapour could deposit in small droplets on the upper surface of the glass ampoule, outside the main body of the liquid. Note that theoretically the deposition of tellurium on the upper surface should also happen at room temperature, though the equilibrium vapour pressure of tellurium at $25 °C$ is only about $5 \times 10^{-4} Pa$, which would make the transfer of a macroscopically visible amount of tellurium an extremely slow process.

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  • $\begingroup$ I think this best answers the question, but I am still confused about whether the top surface of the ampoule would have to be cooler than the rest of the ampoule. I heated things slowly, and the furnace was big and the ampoule reasonably small, so I'm guessing that temperatures would be roughly the same throughout the ampoule. I wonder if the light from the heating elements heated the tellurium before the glass, causing the condensation, and if shielding the ampoule from the light would stop the condensation from occurring. $\endgroup$ – user1704042 Mar 7 '15 at 19:53
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    $\begingroup$ @user1704042 No, even if the entire ampoule were in perfect thermal equilibrium, you could still expect to see matter transfer upwards. Once you have the dilute tellurium vapour inside the ampoule, it is in dynamical equilibrium with the solid. The solid will continue to sublime slightly, and the vapour will continue to deposit. Even though it is kinetically easier for the vapour to deposit on the surface of the solid tellurium (which works as a good nucleation surface), there's no reason some of it can't deposit on the ampoule glass instead and begin nucleating tellurium crystals there. $\endgroup$ – Nicolau Saker Neto Mar 7 '15 at 20:03
  • $\begingroup$ @user1704042 If you feel like testing it out, you can experiment with pure naphthalene. The solid has a larger vapour pressure at a much more accessible temperature (melts around 80°C). If you pack some pure naphthalene into the bottom of an ampoule or a tightly closed vial, and heat it homogeneously in your furnace to 60-70°C, you should be able to see small crystals form above the previous level of the solid. Give it enough time and it should coat the entire interior surface of the container. $\endgroup$ – Nicolau Saker Neto Mar 7 '15 at 20:36
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The melting point of tellurium is approximately 450 °C; hence, I guess, the hot part of the melt could have a temperature of roughly 500 °C. At this temperature, the vapour pressure of tellurium is roughly 100 Pa (0.1 % of the normal atmospheric pressure). By way of comparison, the vapour pressure of water at room temperature is 2–3 kPa. Thus, a small but noticeable amount of tellurium could evaporate during the period of one day if the tellurium vapour is continuously removed by condensation on colder parts of the ampoule. The actual transport rate depends on the temperature gradient, the pressure of other gases in the ampoule, and the geometry of the ampoule.

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