Answer to Question #81705 in General Chemistry for sierra

Question

would the mass of the crucible and cover after step 1 be too large, too small, or the same?

Answer 1

Each crucible and lid pair should be heated to redness for 15 min. That is, you should heat until

you see all of the ceramic in the crucible glowing red, and then wait for 15 min past that point, it will take a lot longer to heat to redness if the crucible is not being exposed to the light blue part of the flame.

Then cool each crucible and lid for 30 min in a desiccator (to minimize the adsorption of water vapor onto the crucible and lid) and weigh each crucible and lid independently on an analytical balance. Next, repeat the above process: 15 min of heating to redness, 30 min of cooling in a desiccator, and weighing on an analytical balance. Your two weighings should agree to within 1.0 mg for each piece. If not, you should repeat the process once more. (Usually the last weight is the most accurate.) When you are done weighing your crucibles and lids, store them in a desiccator and put them away in your cabinet for use next week. Make sure the lid of the desiccator seals tightly.

The mass of the crucible decreases with each roasting, because the moisture evaporates from the pores of the crucible and it becomes smaller.

Δm1 = m(before heating) – m(after 1 heating)

Δm2 = m(after 1 heating) – m(after 2 heating)

Δm1 > Δm2