Solubility

 

Name: Shane Berning, Hope Fluegel, Elizabeth Morrison

 

Title of Lesson: Examining solubility of different substances

 

Date of Lesson: Spring 2007

 

Length of Lesson: 50 minutes

 

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

 

Source of Lesson: Addison-Wesley. Chemistry. TeacherÕs Ed.

 

TEKS Addressed:

(c)  Knowledge and skills.

(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 will teach students the general principles of solubility.  Students will conduct experiments with two different substances to determine their approximate saturation point in water.  They will then discuss the mechanism of solubility and factors that effect solubility.

 

II.            Performance or learner outcomes:

á      Define solubility, saturated, unsaturated, solute, solvent, solution, and weight percent

á      Examine what factors affect solubility

á      Make generalizations about solubility

á      Analyze data and make observations

 

III.         Resources, materials and supplies needed

Per student:

      Safety goggles

      Lab notebook

 

Per group (2 or 3 students each)

      250 mL beaker

      100 mL beaker

      salt

      sugar

      stirring rod

      water

      centigram balance

 

IV.          Safety

All students must wear eye protection during lab exercises.  Do not ingest any substances used in the lab.

 

V.            Supplementary materials, handouts:

Lab procedure

 

VI.          Advance Preparation:

Make sure lab tables have all necessary items before class starts.

 

Engagement:

Teacher does

Student does

ÒWhat is a solution?Ó

 

ÒWho can give me some examples of solutions that we come across everyday?Ó

 

ÒDo all solutions contain only liquids?Ó

 

A homogeneous mixture; when two substances are mixed together; etc

Students respond: answers may vary

 

 

No, some solutions can contain gases, solids

 

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.

  • Each group will be given a 250 mL beaker and salt. 
  • One person from each group will measure out about 100mL of tap water into their beaker.  Have the students measure the temperature of the water.
  • The students will then measure out 5 g of salt and pour it into the beaker of water and stir.  Ask them to record their observations.  (all of the salt should dissolve).
  • Have the students continue to add 5 grams at a time, stirring each time, until no more salt dissolves in solution.  This is the saturation point for salt at this temperature.  (It should be at about 30 to 40 grams of salt).
  • Have the students record the approximate saturation point for salt.
  • The students will repeat the procedure for sugar using only 50 mL of water.
  • They will record the approximate solubility for each as:

      grams solute/mL water X 100% (which is the weight percent)

  • Every group will then record their data on the chalk board and a class average of solubilities will be calculated.

Have the students clean up their work area and prepare for discussion.

The students split into groups and obtain lab goggles.

 

 

 

The students follow the lab procedure.

 

 

 

 

 

 

 

 

 

 

 

The students record the approximate saturation point for salt.

 

 

 

 

 

 

 

 

 

 

 

 

The students record the approximate saturation point for sugar.  They will then calculate the weight percent of the solution and find the average solubility for the class.

 

Explain:

The students will share their findings of which substance is more soluble in water.

ÒSolubility is a lot like population density.  The solvent can only hold so much solute suspended in solution before it canÕt hold any more.  The point at which it canÕt hold anymore is the saturation point and is the calculated solubility of solute in solution.  Several other factors determine solubility of a substance.  What factors affect solubility?  How do you think it would have changed your findings if we had used freezing water? Boiling water?  What if we had used cubes instead of granulated?  What if we didnÕt stir?  Solubility is therefore affected by temperature, surface area, and agitation of the solution.  What about an insoluble substance?  Is everything soluble in water?

Students will report their findings with the class.

 

 

 

 

 

 

 

The students answer: it would have been less soluble; more soluble

Cubes would not have dissolved as quickly as the granules.

The solution would not have dissolved as quickly without stirring.

 

Elaborate:

One other substance that dissolves in water is CaCO3.  Does anyone have an idea as to where we can find calcium carbonate?  This is the principal component of limestone, which is what our aquifers are made of.  The Ca2+ ions get into our water supply, producing Òhard water.Ó  If the water has too much Ca2+ concentration, it deposits on the pipes and causes the ring around a bathtub after it mixes with detergents in the water.

Students listen.

 

Evaluate:

What is happening chemically to make the substances dissolve? (the polar water molecules isolate and surround individual ions and molecules from the solute)

Why does the solution become saturated? (there are only so many water molecules to surround the solute.  Once they have all been used up, the solute precipitates out of solution)

What does saturated mean? (contains the maximum amount of solute for the given amount of solvent at a constant temperature)

Why did we measure the solution in weight percent?  (we are able to determine what percentage of the solution is taken up by solute)

Why is the sugar more soluble than the salt?

Which was the solvent and which was the solute? (the sugar and salt are solutes and the water is the solvent)

What kind of molecule is water and how does that affect solubility? (water is polar meaning it associates with other polar molecules)

Students answer based on what they have learned.