CHEMISTRY NOW: Separating Salt Water, an Aqueous Solution (Grades 5-8)

Objective:


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 and debate effective experimental design.


Introduction Notes:


CHEMISTRY NOW: Separating Salt Water, an Aqueous Solution

 

 

Title:

 

 

Separating Salt Water, an Aqueous Solution

 

 

Subject/Topic:

 

 

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 and debate effective experimental design.

 

 

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).

 

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)

-          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 a mixture 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; heat and air movement cause evaporation to occur.] 

 

Part 2

 

Work with students to develop an experiment in which they make a mixture (saltwater 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 can describe an experimental procedure you think can get the salt out of an aqueous solution the fastest? Explain what 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?

 

1. 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  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 into smaller groups to allow for diversity in procedures and approaches. 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.

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.

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 open in a warm space overnight works well.

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 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 specify what number of grams of salt their experiment retrieved in what period of time (grams/time).

 

Also have students discuss how they developed or 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.

 

 


 

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:

__________________________________________________________________

 

__________________________________________________________________

 

__________________________________________________________________

 

__________________________________________________________________

 

__________________________________________________________________

 

__________________________________________________________________

 

__________________________________________________________________

 

__________________________________________________________________


Data Table(s):

 

Object

Measurement

Mass of empty container

 

Mass of dry solute (salt) in container

 

Mass of dry solute (salt)

 

Temperature of H2O

 

 

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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