1
Experiment 3: Separation of a Mixture
Pre-lab Exercise
Name: _____________________
The amounts of sand, salt, and benzoic acid that will dissolve in 100 g of water at
different temperatures:
Temperature
sand
salt (NaCl)
benzoic acid
0 °C
0g
36 g
0.17 g
20 °C
0g
36 g
0.29 g
40 °C
0g
37 g
0.42 g
60 °C
0g
37 g
1.2 g
80 °C
0g
38 g
2.8 g
100 °C
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
Experiment 3: 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:
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 100 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 to 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.
Experiment 3: Separation of a Mixture
Name: ____________________________________
Report Sheet
Lab Partner: _____________________________________
Desk: _____________ Date:_____________
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 in mixture
% of sand in mixture
Show work:
________ g
________ %
Mass of salt in mixture
% of salt in mixture
Show work:
________ g
________ %
Mass of benzoic acid in mixture
% of benzoic acid in mixture
Show work:
________ g
________ %
Total % of sample recovered
Show work:
________ %
6
Separation of a Mixture
Post-Lab Exercise
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 / non-soluble] in water at any temperature.
How many grams of benzoic acid will dissolve in water at 60°C? __________
What will happen to this benzoic acid if you let the water cool to 40 °C?
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?