Heating a Crucible to ConstantWeightby Oliver SeelyThis web page and accompanying photos room in the general public domain and also may it is in copiedwithout restriction.Your an initial exercise teaches friend some an abilities on the proper use of the laboratory burner (in thiscase calleda Tirill Burner), the mediate of the flame and also the ideal placement the a crucible i beg your pardon is to beheated to consistent weight.You must make certain at the outset the the crucibles girlfriend use have actually indelible identifying markson them. Sometimes there are indentations left throughout the produce of a crucible i beg your pardon willallow you to identify one indigenous another. More likely, there will be a letter or number writteninto the next by a previous student, such together the one shown at the left. Keep in mind the "A" scratched intothe glossy surface. This have the right to be done with the diamond pencil obtainable in the lab, as presented at theright.The hottest component of the fire is in ~ the top of the glowing cone the oxidation. The zone beneaththe sharp allude of the cone is filled with a climbing mixture that cool gases not yet in the procedure ofcombustion. Although the rim approximately this region is rather hot, the gases within are essentially atroom temperature. Check out that location on the left. Such aplacement is not recommended note the exactly placement the the crucible in the photo atthe right.The crucible may be inserted right side up, together in the left photo, or on its side, as in the rightphoto. Heating it top top its next is preferred because the all at once strain on the ceramic material at thehigh temperatures used is somewhat less.The crucible need to be heated to incandescence for 5-10 minutes (left). In ~ the end of thistime, eliminate the fire and enable the crucible come cool top top the stole triangle for a couple of minutes (right)When moving a crucible come the dessicator tongs should be used. If the crucible has actually beenallowed to cool, you may use ceiling tongs (left), however if you have to transfer a hotcrucible, make sure that girlfriend cover the end of the tongs with aluminum silver paper (right) as the repaint onthe tongs might melt onto the crucible and change its weight. Make sure you take the crucible onthe edge as shown. Don"t try to choose up a crucible through squeezing the tongs top top the external rim bothbecause the crucible might slip from your grip fan to that slick walls. Over there is likewise the possibilityof break a warm crucible that way.Place the cool crucible into one the the holes in the plate in her dessicator and cover till ithas reached room temperature. At the time you may remove it for weighing.If a crucible is not at the temperature of the balance compartment it will likely show a weightdifferent from the it would present were it at the same temperature. If that is hotter the weight will beslightly less, if cooler slightly much more owing to the manufacturing of convection currents i beg your pardon affectthe obvious mass. Right here are some photos to prove the point. (Please note that two separateanalytical balances had to be used for this succession of photos therefore as not to interfere v studentuse. The final agreement is within the which is frequently observed for two separate balances.) Adry cold crucible is weighed to the nearest 0.0001 g.The crucible is then heated come incandescence for 5 minutes, as presented at the left. The fire isextinguished and also the crucible is permitted to cool for 5 minutes.Its obvious weight is much less by an ext than 4 milligrams.It is permitted to cool for secondary five minutes.Its showed weight is still 0.0003 g less than that of the cold crucible, above.Finally, after an additional five minute of cooling, the crucible exhibits a load slightly greaterthan that of the cold crucible, above. The commitment here is within that which is routinelyobserved because that two separate balances.
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This cooling sequence take it place exterior the dessicator. The lot of time crucial to accomplish thermal equilibrium v the environment will likely besomewhat much longer when you let the crucible cool in the dessicator.