Beacon Lesson Plan Library

Salt and Water Divorce by a Physical Change

Rosemary Wilson
Santa Rosa District Schools

Description

The students vaporize water from a solution of salt and water leaving the NaCl behind and showing the separation by physical means (change of state of the water).

Objectives

The student knows the difference between a physical and chemical change.

Materials

- Bunsen burner and tubing
- Evaporation dish
- Watch glass
- Striker or matches
- Ring stand
- Iron ring
- Tongs
- NaCl (salt)
- 100 mL beaker
- Stirring rod

Preparations

1. Have all the equipment for the lab experiment arranged on a cart for easy access by the students.
2. Have the lab apparatus set up for the demonstration.
3. Purchase inexpensive sodium chloride (NaCl), so as not to use reagent-grade.

Procedures

1. Start the class by reiterating the definitions of physical properties or changes and chemical properties or changes stressing the fact that changes of state are physical changes.

2. Begin the lab by requiring students to wear approved safety goggles and aprons. When the students have all donned these items, ask the students to come to the demonstration table or area. Have the lab equipment set up so that the students can see how they are to set up the equipment for their own labs. When the students are all present, show them the different items arranged on the table and explain the names and uses of each piece of equipment.

3. Demonstrate the lab to the students so that they know what to expect. Mix some NaCl and water in the beaker, stirring the solution with the stirring rod to show the dissolving of the salt. Decant some of the solution into the evaporation dish. Place the watch glass on the top of the evaporation dish using the tongs. Remind the students to use tongs or forceps to pick up hot objects. Tell them that one can't always determine if some items are hot or cold just by observing them. Show them how to test and see if an object is hot by placing the back of the hand above the item to see if waves of heat can be detected.

4. Turn on the gas supply and light the Bunsen burner using the striker or matches. Place the Bunsen burner flame underneath the evaporation dish so that the solution begins to boil and the water vaporizes leaving the NaCl. Point out the “popping” of the salt as the process proceeds and explain the use of the watch glass for the purpose of retaining the popping crystals.

5. At the completion of the demonstration, using the tongs, remove the watch glass turning it upside down on the demonstration table. Using the tongs again, show the students the salt in the evaporation dish. Point out the fact that the crystals of salt now appear as a powder and not as the nicely shaped crystals they were.

6. At the completion of the demonstration, allow the students to perform the lab experiment on their own following the lab instructions provided in the Associated File.

Assessments

1. (Multiple choice) Which of the following is not a physical change?
a. burning wood
b. steam condensing on a mirror
c. breaking chalk
d. melting ice

2. (Multiple choice) Which of the following is an example of a homogeneous mixture?
a. walnut brownies
b. nuts and bolts
c. salted water
d. students in a classroom

Rubric for Laboratory Reports (See Associated File)

Extensions

Lecture on the differences between chemical and physical changes and between homogeneous and heterogeneous mixtures before this lab.

Web Links

Web supplement for Salt and Water Divorce by a Physical Change
Separating salt from saltwater

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