Chapter 6 Laboratory: Separating Mixtures 117POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.- Set up your ring stand, support ring, clay triangle, and
burner. Heat the crucible and cover gently for a minute or
two to vaporize any moisture. - Remove the heat and allow the crucible and cover to cool
to room temperature, which may require 10 or 15 minutes. - Weigh the crucible and cover, and record their mass to
0.01 g on line A of Table 6-7. - Transfer about 5.0 g of copper sulfate pentahydrate to
the crucible. (The copper sulfate pentahydrate should
be in the form of fine crystals. If it is in the form of large
lumps, use your mortar and pestle to crush it into finer
crystals.) - Reweigh the crucible, cover, and contents and record the
mass to 0.01 g on line B of Table 6-7. Subtract the mass of
the empty crucible and lid from the mass of the crucible
with the copper sulfate and record the initial mass of the
copper sulfate pentahydrate on line C of Table 6-7. - Place the crucible and cover on the heat source and
begin heating them gently. As the crucible warms up,
increase the heat gradually until it is at its highest setting.
Continue heating the crucible for at least 15 minutes. - Remove the heat and allow the crucible, cover, and
contents to cool to room temperature, which may require
10 or 15 minutes. - After you are sure the crucible has cooled, reweigh
crucible, lid, and contents. Record the mass to 0.01 g
on line D of Table 6-7. Subtract the initial mass of the
crucible and lid (line A) from this value, and record the
mass of the anhydrous copper sulfate to 0.01 g on line E
of Table 6-7. Subtract the mass of the anhydrous copper
sulfate (line E) from the mass of the copper sulfate
pentahydrate (line C), and record the mass loss to 0.01 g
on line F of Table 6-7.
deliquescence.) Conversely, the water molecules in some
hydrated compounds are so loosely bound that the compound
spontaneously loses some or all of its water of hydration if
left in a dry environment, a property called efflorescence.
Some compounds may be either hygroscopic or efflorescent,
depending on the temperature and humidity of the environment.
For example, anhydrous copper sulfate exposed to a humid
atmosphere gradually absorbs water vapor and is converted
to the pentahydrate form, and copper sulfate pentahydrate
exposed to warm, dry air gradually loses water and is converted
to the anhydrous form.
Because the water of crystallization in a hydrate can be driven
off by heating the hydrate, a hydrate is actually a mixture of an
anhydrous salt with water rather than a separate compound.
(Recall that a mixture is a substance that can be separated
into its component parts by physical means, such as heating,
as opposed to a substance that can be separated into its
component parts only by using chemical means.) Because the
number of water molecules in a hydrate are in fixed proportion
to the number of molecules of the compound, it’s possible
to determine that fixed proportion by weighing a sample of
a hydrate, heating the compound to drive off the water of
crystallization, weighing the resulting anhydrous compound, and
using the mass difference between the hydrated and anhydrous
forms to calculate the relationship.
In this laboratory, we’ll heat hydrated copper sulfate to drive
off the water of crystallization and use the mass differential
to determine how many molecules of water are associated with
each molecule of hydrated copper sulfate. We could have used
any number of common hydrates, but we chose copper sulfate
because the hydrated and anhydrous forms have distinctly
different appearances. In hydrated form, copper sulfate forms
brilliant blue crystals; in anhydrous form, copper sulfate is a
white powder (see Figure 6-7).
CUTIOA nS
This laboratory uses strong heat. Use extreme care
with the heat source and hot objects. A hot crucible looks
exactly like a cold crucible. Wear splash goggles, gloves,
and protective clothing.z
FIGURE 6-7: Copper sulfate samples before (left) and after
heating (right) to remove water of crystallizationdISoALp S : Retain the anhydrous copper sulfate for
use in later experiments. you can store it in a sealed
bottle, or allow it to remain exposed to air until it
rehydrates to form blue crystals.