Supersaturation

Name: Shane Berning, Hope Fluegel, Elizabeth Morrison

Title of Lesson: Supersaturation

Date of Lesson: Spring 2007

Length of Lesson: 50 minutes

Description of Class: High school (10^th - 12^th grade) chemistry

Source of Lesson: Addison-Wesley. Chemistry. “Laboratory Manual” Teacher’s Ed.

TEKS Addressed:

(1) Scientific processes. The student, for at least 40% of instructional time, conducts field and laboratory investigations using safe, environmentally appropriate, and ethical practices. The student is expected to:

(A) demonstrate safe practices during field and laboratory investigations; and

(B) make wise choices in the use and conservation of resources and the disposal or recycling of materials.

(12) Science concepts. The student knows the factors that influence the solubility of solutes in a solvent. The student is expected to:

(A) demonstrate and explain effects of temperature and the nature of solid solutes on the solubility of solids;

(B) develop general rules for solubility through investigations with aqueous solutions; and

(13) Science concepts. The student knows relationships among the concentration, electrical conductivity, and colligative properties of a solution. The student is expected to:

(A) compare unsaturated, saturated, and supersaturated solutions;

The Lesson:

I. Overview:

This lesson is designed to teach the students about supersaturation. They will learn how temperature affects the solubility of solute in a solvent and how this affects the supersaturation of the solution. Students will make observations and draw conclusions about the mechanism at work.

II. Performance or learner outcomes:

· Define supersaturation, saturation, unsaturation, and solubility

· Draw conclusions based on observations made from the experiment

· Analyze how temperature can affect the supersaturation of a solution

III. Resources, materials and supplies needed

Per student:

Safety goggles

Lab notebook

Per group (2 or 3 students each)

100 mL beaker

centigram balance

medium test tube

test tube rack

test tube holder

10-mL graduated cylinder

gas burner

sodium sulfate decahydrate

ice

distilled water

IV. Safety

All students must wear eye protection during lab exercises. Do not touch hot equipment. Exercise care when working with an open flame. Have the students tie back hair and loose clothing. Instruct the students to heat the bottom and sides of the test tube evenly.

V. Supplementary materials, handouts:

Lab procedure

VI. Advance Preparation:

Set up materials at each lab table before class.

Engagement:

Teacher does:

Student does:

Show the students some pictures of caverns and geysers from around the world. Ask the students how the caverns were formed. How are stalactites and stalagmites formed? How do the deposits around the geysers get there? Remind the students about solubility of solutions and how a solution can become saturated once it contains the maximum amount of solute for given solvent.

“There is also a property of solutions in which the solvent is holding more solute than is theoretically possible, without the solute precipitating. This is called supersaturation, which we will explore today.”

Explain that the formations in caverns are a result of supersaturated water that flows into the caverns. The deposits from geysers result because the solutes are no longer soluble in the water at the lower temperature outside of the geyser.

Students listen and join in discussion about caves and geysers.

Exploration:

The teacher has all of the students split into their lab groups (2 or 3 students each) and head back to the lab table. Instruct the students to wear goggles at all times during all lab exercises.

The procedure is as follows:

Place 5 g of Na₂SO₄*10H₂O in a clean medium-sized test tube and add 10mL water.
Hold the test tube in a test-tube holder and heat it over a flame, agitating the mixture gently until all solute dissolves.
Place the test tube in a test-tube rack and add 1 crystal of Na₂SO₄*10H₂O to the warmed solution and gently agitate. Record your observations.
Place the test tube and contents in a beaker of ice water to cool. If crystals begin to form, reheat the the to redissolve the crystals and cool the tube again.
When solution is cold, gently remove from bath and place in rack.
Add one small crystal of Na₂SO₄*10H₂O. Describe what happens

The students split into groups and obtain lab goggles.

The students follow the lab procedure and make observations.

Explain:

What happened to the first extra crystal that was added to the test tube? (it dissolved)

What happened to the extra crystal that was added to the cold solution? (it caused the solute to crystalize)

Why is it necessary to heat the mixture in step 2 of the process?

Is the solution unsaturated, saturated, or supersaturated at the end of step 2?

Is the solution unsaturated, saturated, or supersaturated at the end of step 3?

At the end of step 4, when crystallization is complete, what is the state of solution?

Students answer: It should have dissolved; caused the solution to crystalize

In order to get the solute to dissolve.

It is unsaturated because the added crystal dissolved.

It is supersaturated because it contains more solute than it can theoretically hold at 0^o C.

It is saturated.

Elaborate:

The temperature of the solution has a large effect on the solubility of the solute. This is called Henry’s Law when it applies to the solubility of gases and pressure. Henry’s Law states that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid. Basically, the solubility increases as the pressure increases. How might the solubility of gases in lake or river water be important for living organisms?

Students listen and give input

Evaluate:

What is supersaturation?

How does temperature affect supersaturation?

What is crystallization?

Students answer based on what they’ve learned.