1
Separation of a Mixture
In-Class Prelab
NAME___________________
The amounts of sand, salt, and benzoic acid that will dissolve
in 100 g of water at different temperatures:
Temperature
0 oC
20 oC
40 oC
60 oC
80 oC
sand
0g
0g
0g
0g
0g
salt (NaCl)
36 g
36 g
37 g
37 g
38 g
benzoic acid
0.17 g
0.29 g
0.42 g
1.2 g
2.8 g
100 oC
0g
40 g
7g
1. Discuss with other students what “solublity in water” means. Write down a
brief explanation using your own words.
2. Use the table above (and common sense) to answer the following questions:
Is sand soluble in water? _____
Is salt soluble in water? _____
How does the temperature affect the solubility of salt in water?
3. Suppose you have a mixture of sand and salt in water. Using common sense,
come up with a simple way, using only the equipment available in the lab, to
separate the sand from the salt.
4. Suppose you had a mixture of salt and benzoic acid in water. Come up with a
way of separating the salt from the benzoic acid. Studying the table above may
help you come up with an idea.
2
Separation of a Mixture
Introduction
In this experiment you will separate a mixture of three substances:
silicon dioxide (sand),
benzoic acid, and
sodium chloride (NaCl; table salt).
Separation will be achieved by utilizing the differences in their solubility in water.
Solubility is defined as the amount of a substance (the solute) that will saturate a
given amount of solvent. In other words, it is the amount of a substance that can
be dissolved in a specific amount of the solvent. The degree of solubility of a
substance in water depends on the temperature of the water and the chemical
properties of the substance. Sand is completely insoluble in water, benzoic acid
is slightly soluble in water, and NaCl is quite soluble even in cold water. The
table below shows the solubility of these three compounds as a function of
temperature.
Solubility of each compound at various temperatures.
Amount of the compound that will dissolve in 100 g of water at given temperature:
o
o
o
o
o
o
0 C
20 C
40 C
60 C
80 C
100 C
--------------------------------------------------------------------------------------------------------------Sand
0g
0g
0g
0g
0g
0g
Benzoic acid
0.17 g
0.29 g
0.42 g
1.20 g
2.80 g
7.00 g
NaCl
36.0 g
36.0 g
37.0 g
37.0 g
38.0 g
40.0 g
---------------------------------------------------------------------------------------------------------------
Separation methods used in this experiment
1. Decantation—a process of separating a solid residue (precipitate) from a
liquid (the supernate) by gently pouring off the liquid without disturbing the solid
that has settled to the bottom.
2. Filtration—a process of separating a solid from a liquid by using a filter
through which the liquid, but not the solid, can pass. The filter may be made of
paper or other porous material. (A paper coffee filter is an example—water
passes through the filter, but the solid coffee grounds do not.) In order to speed
up the filtration process, suction may be used to draw the liquid through the filter.
This is called vacuum filtration. During vacuum filtration, a paper filter is placed
in the bottom of a special funnel called a Buchner funnel. The mixture to be
separated is poured into the funnel, and suction is used to draw the liquid
through the filter and out of the funnel into a separate container. The solid does
not pass through the filter, but remains behind in the funnel (it has been filtered
out of the liquid).
3
Separation of a Mixture
Procedure
1. Weigh out ~5 grams of the mixture of sand, benzoic acid, and salt, and record
the exact mass (all digits) on the report sheet. Determine and record the mass of
a clean, dry 400 mL beaker. Transfer the mixture into this beaker and add 50 mL
of distilled water.
2. Determine and record the mass of a clean, dry 250 mL beaker. Set the
beaker aside for now. Prepare a beaker of wash water by half filling a 150 mL
beaker with distilled water and placing it on a hot plate to boil. Prepare an ice
bath by half-filling a 600 mL beaker with ice and a little distilled water.
3. Stir the mixture in the 400 mL beaker thoroughly with a glass stirring rod and
put it on a hot plate to heat until it boils. Avoid breathing the fumes from the hot
liquid. While waiting for the liquid to boil, place the 250 mL beaker nearby so you
will have it for the next step. Also obtain a “hot hands” for grasping hot beakers
from the cabinet in your lab bench.
4. As soon as the liquid begins to boil, use the hot hands to remove the beaker
from the hot plate. Briefly stir again to ensure that all soluble material is
dissolved, and then immediately decant the liquid into the 250 mL beaker,
leaving behind all the solid residue (sand) in the original beaker.
Pour a small amount of the boiling water from the 150 mL beaker over the sand
in order to rinse it, swirling the water over the sand to achieve good rinsing. This
will wash any benzoic acid and NaCl residue off of the sand. Pour this rinse
water into the 250 mL beaker with the rest of the liquid that was decanted. This
liquid contains dissolved benzoic acid and NaCl.
5. Place the entire 250 mL beaker of liquid into the ice bath so that the outside
surface of the 250 mL beaker is surrounded by slushy ice. Leave it there for ~15
minutes. During this time you will be able to see the benzoic acid becoming
insoluble, forming crystals as it comes out of solution. While waiting, use a wax
pencil to label the 400 mL beaker (containing the sand) with your name, and
place the beaker in the oven for drying. To ensure that the sand is completely
dry, leave it in the oven for at least 20 minutes, until the end of the lab period
when you are almost done with the experiment.
6. Obtain a Buchner funnel and filter flask from the stockroom. Assemble the
funnel and flask for vacuum filtration as follows. The Buchner funnel should be
placed on top of the filter flask and the flask clamped to a ring stand so that it
does not tip over. Put a piece of filter paper into the Buchner funnel and wet it
with a little distilled water so that it sticks in place. Connect the arm of the filter
flask to the water aspirator using a vacuum hose (a heavy hose which can be
found in the cabinet of your lab bench). Ask the instructor to check your setup
before proceeding to the next step.
4
7. Turn on the water aspirator to generate suction through the Buchner funnel.
Remove the 250 mL beaker containing the benzoic acid crystals from the ice
bath. Swirl the beaker to suspend the crystals in the liquid and then pour the
crystals into the Buchner funnel. Some crystals will probably remain stuck in the
beaker. Use a very small amount of ice water from the ice bath to rinse the
crystals out of the beaker and into the Buchner funnel. After all the liquid has
passed out of the Buchner funnel and into the filter flask, allow the water
aspirator to run for a few minutes more—this will draw any remaining traces of
liquid through the funnel and away from the benzoic acid crystals, drying the
crystals. After a few minutes, detach the hose from the filter flask to break the
vacuum, and then turn off the water aspirator.
8. Remove the Buchner funnel from the filter flask and turn it upside down on a
paper towel. Set it aside where it can’t be knocked over or broken. You will
remove the crystals from the funnel in the next step.
Clean and dry the 250 mL beaker. Pour the liquid from the filter flask (which
contains dissolved NaCl) into the 250 mL beaker. Weigh out ~0.5 grams of
boiling chips and record the exact mass on the report sheet (record all digits).
Put the boiling chips into the beaker of liquid and place the beaker on a hot plate.
Heat it until all the liquid has evaporated. Remove the beaker from the hotplate
immediately when all the liquid has boiled away. While waiting for the liquid to
evaporate, proceed to the next step below.
9. Remove the beaker of sand from the oven and allow it to cool for a while—but
if the sand is not dry, leave it in the oven longer. While waiting for it too cool,
carefully remove the benzoic acid crystals and filter paper from the Buchner
funnel by using a micro-spatula to lift the filter paper. Place the crystals and filter
paper between paper towels and press gently to dry the crystals. Then
determine the mass of the crystals by scraping them onto a piece of weighing
paper that has first been tared (“zero-ed”) on the balance. Record the mass of
the benzoic acid crystals on the report sheet.
After the beaker has cooled to room temperature, determine the mass of the 400
mL beaker containing the sand and record the mass on the report sheet. Then
calculate and record the mass of the sand alone.
10. When all the liquid has evaporated from the 250 mL beaker on the hotplate,
allow the beaker to cool. While waiting, work on the postlab questions. After it
has cooled to room temperature, determine the mass of the beaker and all its
contents (salt and boiling chips), and record this on the report sheet. Then
calculate and record the mass of the salt alone. Complete the remaining
calculations on the report sheet. Ask the instructor for help if you are not sure
how to do the calculations.
5
Separation of a Mixture
Report Sheet
Name _________________
Primary Data
Mass of mixture
________ g
Mass of 400 mL beaker
________ g
Mass of 250 mL beaker
________ g
Mass of boiling chips
________ g
Mass benzoic acid crystals
________ g
Mass of 400 mL beaker + sand
________ g
Mass of 250 mL beaker + boiling chips + salt
________ g
Calculations
Mass of sand
Show work:
________ g
Mass of salt
Show work:
________ g
% of benzoic acid in mixture
Show work:
________ %
% of sand in mixture
Show work:
________ %
% of salt in mixture
Show work:
________ %
Total % of sample recovered
Show work:
________ %
6
Separation of a Mixture
Postlab
Name ____________
1. Using the table on page 1, answer the following questions.
NaCl is [more / less] soluble at high temperatures than at low temperatures.
Sand is [soluble / slightly soluble / nonsoluble] in water at any temperature.
How many grams of benzoic acid will dissolve in water at 60oC? __________
What will happen to this benzoic acid if you let the water cool to 40 oC?
2. Was your percent recovery greater than 100% or less than 100%? ________
Give some possible reasons why you didn’t achieve exactly 100% recovery. Be
specific—don’t just say “human error.”
3. Define the following terms:
Solubility—
Decantation (to decant)—
4. What is the name of the piece of equipment (a special type of funnel) used for
vacuum filtration?
5. In step 3 you heat the mixture; in step 4 you quickly decant the liquid while it
is still hot and use hot water to wash the sand. What is the purpose of heating
the mixture and then decanting while it is still hot (rather than letting it cool before
decanting)?
6. Why do you put the liquid in an ice bath in step 5 and use ice-cold water
(rather than warm water) for rinsing in step 7?