Separating Salt Water, an Aqueous Solution
TITLE:
Separating Salt Water, an Aqueous Solution
SUBJECT/TOPIC:
To make a solution (homogeneous mixture) and separate it
back into its parts
GRADES:
5–8
STANDARDS
ALIGNMENT:
Science As Inquiry
Properties and Changes in Properties of Matter
TIME ALLOWANCE:
50 minutes
OVERVIEW AND PURPOSE / OBJECTIVE(S)
(information, concepts to be learned):
Students will be able to discuss formation of a mixture.
Students will be able to discuss separation of a solution by evaporating the water.
Students will be able to describe solubility and solutions.
Students will create a solution (mixture) and separate the mixture by evaporation.
Student will be able to discuss effective experimental design.
Students will debate the their experimental designs.
Vocabulary
Aqueous solution—a solution with water as the solvent.
Evaporation—when a liquid turns to a gas.
Homogenous mixture—a mixture that appears uniform throughout; a solution.
Mixture—two or more substances that are physically combined.
Pure substance—an element or compound that has particles that are all the same.
Soluble—when a substance will dissolve in a solvent (salt is soluble in water).
Solute—a substance dissolved in a solution (there is less of this substance than of the
solvent).
Separating Salt Water, An Aqueous Solution (Middle School level)
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Solution—a homogeneous mixture of two or more substances in which one substance is
dissolved in another, appearing uniform throughout.
Solvent—substance dissolving the solute(s) in a solution (there is more of this substance
than of the solute).
Materials (per group of 4):
(this is a student-designed lab; quantities may vary)
-
-
water
salt
large test tubes with stoppers to fit
evaporating dish or beaker to heat solution in
variety of safe heat sources (hot plates, oven, sunshine, fan, warm rooms, are all
examples of what can be used and incorporated into class discussions when
designing experiments or formulating hypotheses)
pot holders or tongs to handle hot glassware or other potentially hot items
balances
graduated cylinders
thermometer
Activity/Engagement
(reinforcing lesson, making real-world connection)
Part 1—Class discussion
Introduce/review mixtures versus pure substances. Discuss what a solution is, what solute
and solvent mean, and what an aqueous solution of salt is. Use diagrams when
appropriate. Leading questions include the following:
 What is a mixture?
 How does it compare to a pure substance?
 How can we prove a mixture is a mixture and not a pure substance?
(It can be separated based on the physical properties of the substance,
differences in boiling point for example; pure substances cannot be separated
by physical means.
 What is a solution? (A homogenous mixture in which one or more substances
are dissolved in another substance.)
 What is an aqueous solution? (One in which water is the solvent.)
 What is a solute? A solvent? A solute is the substance dissolved (less of this);
a solvent is the substance dissolving the solute (more of this).
 How can we make a mixture of water and salt?
 How can we separate this mixture? Or, what is needed for evaporation to
occur? (Many possibilities and ideas for extracting salt from water are
possible. Heat and air movement cause evaporation to occur.)
Separating Salt Water, An Aqueous Solution (Middle School level)
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Part 2
Work with students to develop an experiment in which they make a mixture (salt water
solution) and then figure out how to separate this mixture back into its components.
Guide the students toward a procedure that will allow them to create an aqueous solution
of table salt and then separate this mixture to yield the water and salt again.
Divide the students into cooperative groups. Tell each group they are part of an industry
that extracts salt from water. Ask each group some of the following leading questions to
start their thought processes:

Who would like to describe an experimental procedure you think can get the salt
out of an aqueous solution the fastest? Describe the supplies you would need and
how you would do it.

Who can think of an experiment that separates salt from water that is different
from the ones discussed?
2. Encourage students to experiment even with methods that might not actually work,
such as filtration; this will help them to understand that a solute is dissolved in a solution
and its particles are small enough that they will not stay in the filter paper.
3. Have students consider surface area to aid in evaporation. Using varying sizes of watch
glasses or other dishes may accelerate evaporation.
3. Have students record their hypotheses and their experimental procedures on their data
sheets. Feel free to break students up into groups that allow for a diversification of
experimental procedures. At the end of this lab students can debate their procedures and
argue why their methods for separation work best. For example, it would be interesting to
see which experimental design yielded the most salt in the shortest period of time.
WARNING: Make sure students follow all safety precautions, especially if they work
with a heat source. They should wear goggles and use protective gloves (oven mitts are
fine) or tongs to handle hot glassware. Do not let them place any hot glassware onto a
balance as it is easy to ruin electronics or even the plastic balance pans.
Sample procedure
Obtain a large test tube and stopper that fits it.
Obtain an evaporating dish or beaker. Measure and record its mass.
Carefully measure an amount of water that will fill the test tube about 2/3 full; record this
temperature and volume (volume of solvent).
Obtain a sample of salt (NaCl).
Gradually add about 5 g salt to the test tube of water, stopper it, and shake to dissolve the
salt.
Carefully pour all of the solution into your beaker or evaporating dish.
Separating Salt Water, An Aqueous Solution (Middle School level)
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Heat the solution to evaporate off the water (solvent) and leave the salt (solute) in the
dish. This is best done slowly, so leaving the containers on a low heat source overnight
works great.
Once the salt is completely dry, weigh and record the mass of the salt and the container.
Subtract the weight of the container to find the mass of the salt alone (solute). The final
mass should be equal to the original mass of salt added.
Concluding discussion/activities
During the experiment encourage students to make detailed observations of what is
happening. Guide discussion with and among the students about their process and about
mixtures and pure substances. You can begin by using the following leading questions:



What did the two substances look like before mixing? (Salt is a crystalline
solid that is white in color; the water is a clear liquid.)
What happened when the two substances were mixed? (The salt crystals
“disappeared” as they dissolved in the water leaving a clear liquid again,
looking just as the pure water did.)
How can we get the salt back? (Allowing the water to either evaporate or be
boiled off will leave the solid, white salt behind. Placing a cold beaker over
the boiling water will allow students to see the condensation of the water
vapor being driven off of the solution.)
Once all students have completed an effective experiment have them prepare a group
presentation to the class about their results. Their main conclusions should say that with
their experimental design they were able to retrieve X grams of salt in X period of time
(grams/time).
Also have students discuss why they chose their experimental designs. Ask them about
issues that affect efficiency or use of energy to extract the salt. Did they have to use a
heat source that consumes electricity or did they use the sun to cause water to evaporate?
In the end students can debate which experimental design was the best and for what
reasons.
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Student Worksheet for Separating Salt Water
Name: _____________________________________
Date: ______________
Materials available:
- salt (NaCl)
- water
- heat sources and other items that cause accelerated evaporation, e.g., a fan
or breeze, sunny window, greenhouse, hot plate
- beakers or evaporating dishes of varying sizes
- large test tubes and stoppers
- graduated cylinder
- balance
- thermometer
Procedure:
Design an experiment to first make and then separate a mixture of water and salt.
Lab safety equipment should be used, and safety protocols followed.
Student
Hypothesis:______________________________________________________
_____________________________________________________________________
Student Procedure:
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
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Data Table(s):
Object
Mass of empty container
Mass of dry solute (salt) in container
Mass of dry solute (salt)
Temperature of H2O
Measurement
Observations:
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Conclusion:
How did you make a mixture?
_____________________________________________________________________
How did you separate your mixture?
_____________________________________________________________________
What physical property allowed you to separate your mixture?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
How many grams of salt were extracted from your sample? How long did it take?
__________________________________________________________________
What can you do to improve your experimental design?
____________________________________________________________________
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