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C-5
C.
Use the finger utility clamp to hold the test tube containing the hydrate, and heat the sample gently at
first. Be sure to continually move the test tube so that the hydrate sample is continually agitated. This
will speed the dehydration process and minimize decomposition. Carefully observe the salt to
determine if it melts and dissolves in its own water of crystallization or shows any signs of
decomposition. If it does become a liquid or starts to decompose, heat the sample very gently at first
to avoid splattering and decomposition. Then, increase the heat moderately, but do not allow the test
tube to become red hot.
You will recognize when the hydrate is dry by noting the following possible changes (a) a color
change, (b) when the liquified sample has completely dried, or (c) when no more changes are
apparent in the dried powder. Stop heating the sample at this point (~ 5 to 10 minutes) and place the
test tube back in the 400 mL beaker to cool.
NOTE: Be sure that there is no condensation at the top of the test tube before stopping the
heating process.
NOTE:
Some hydrates will undergo thermal decomposition from excessive heating. If this
becomes apparent (marked color change or formation of colored vapors),
immediately stop heating, and ask your laboratory instructor how to proceed.
D.
Allow the test tube and its contents to cool for ~10 minutes. When cool, re-weigh the test tube and its
contents carefully, but quickly, to avoid re-absorption of water from the air by the salt.
NOTE: If the mass displayed on the balance fluctuates, then the test tube is too warm. Let it
stand for another 2 to 5 minutes and try again.
After obtaining this mass measurement, you must reheat the test tube and its contents a second
time, allow them to cool, and record the mass again to determine if a constant mass (within +0.0010
g) for the anhydrous sample has been reached. This time, do not heat as strongly or as long as
before. If decomposition occurs, stop heating immediately. Once the anhydrous mass is constant
(within +0.0010 g), the dehydration process is complete.
NOTE: It may be necessary to reheat the sample 3 or 4 times until a constant mass for the
anhydrous salt is obtained.
E.
On your Observations Sheet, calculate the mass percent of water in your unknown hydrate.
(Remember to use the correct number of significant figures.)
