Solubility and Solutions

Unit Overview

One of the suggested Science 10 units is titled "Water Quality". That unit deals with environmental issues involving water quality, and with testing aqueous solutions to determine the presence of particular ions. Basic water and waste water treatment is also considered. If students have had this unit in Science 10, it may serve as a jumping off point for an advanced consideration of water chemistry. If the unit was not used in your school, you might consider using some of the ideas from the Science 10 Curriculum Guide to help develop the concepts of this unit in a context familiar to all students.

The emphasis in this unit is on the qualitative description of solubility equilibria. Students should also be able to analyze information from solubility charts, tables, and experimental results. They should be able to determine whether a precipitate is likely to form when two or more different solutions are mixed.

In addition, practice in calculating the strengths of dilute solutions made from solutes and from concentrated solutions is important. Students have been exposed to the use of solutions and the notation for describing their strength during previous courses. The depth with which this topic is considered during this unit will depend on their experiences and understanding.

Factors of scientific literacy which should be emphasized

• A4 ..........replicable
• A5 ..........empirical

• B2 ..........interaction
• B3 ..........orderliness
• B8 ..........quantification
• B9 ..........reproducibility of results
• B11 ..........predictability
• B12 ..........conservation
• B13 ..........energy-matter
• B14 ..........cycle
• B16 ..........system
• B23 ..........invariance
• B28 ..........equilibrium
• B29 ..........gradient

• C1 ..........classifying
• C4 ..........working cooperatively
• C5 ..........measuring
• C6 ..........questioning
• C7 ..........using numbers
• C10 ..........predicting
• C11 ..........controlling variables
• C12 ..........interpreting data
• C13 ..........formulating models
• C14 ..........problem solving
• C15 ..........analyzing
• C16 ..........designing experiments
• C17 ..........using mathematics
• C20 ..........defining operationally

• D3 ..........impact of science and technology
• D4 ..........science, technology, and the environment
• D8 ..........limitations of science and technology

• E2 ..........using natural environments
• E3 ..........using equipment safely
• E4 ..........using audio visual aids
• E5 ..........computer interaction
• E7 ..........manipulative ability
• E9 ..........measuring volume
• E10 ..........measuring temperature
• E11 ..........measuring mass
• E13 ..........using quantitative relationships

• F3 ..........search for data and their meaning
• F5 ..........respect for logic
• F7 ..........demand for verification
• F8 ..........consideration of premises

• G1 ..........interest
• G2 ..........confidence
• G3 ..........continuous learner
• G5 ..........avocation
• G7 ..........vocation
• G8 ..........explanation preference

Foundational Objectives for Chemistry and the Common Essential Learnings

Calculate concentrations of, and prepare, solutions.

• Express the concentration of a solution in moles of solute per litre of solution (M, moles×litre^-1 or mols×L^-1).
• Manipulate the relationship which links the mass of solute, volume of solution and concentration of solution so that given two, the other can be determined.
• Describe how to prepare standard solutions and serial dilutions in the laboratory.
• Manipulate the relationship which links original concentration, volume of diluent and concentration of diluted solution so that given two, the other may de determined.
• Relate concentrations expressed as ppm or ppb to those expressed as mols×L^-1 or g×L^-1.

Understand the principles of qualitative analysis of solutions.

• Use solubility charts to determine the solubility of various substances.
• Describe how to perform tests on solutions to determine which ions or ion groups are present.
• Describe how to separate ions in solution by selective precipitation.
• Describe how the common ion effect influences the solubility of a solute in a solution.
• Investigate the application of the principles of solubility.

Use numbers and numerical data to strengthen understanding of the concept of solubility. (NUM)

• Read and interpret solubility charts and tables.
• Discuss with others the process of estimation.
• Use the concepts of probability and logic to understand how qualitative analysis is done.

Promote both intuitive, imaginative thought and the ability to evaluate ideas, processes, and experiences in meaningful contexts. (CCT)

• Use metaphoric and analogic thinking to build understanding and create insights.
• Generate and evaluate alternative solutions to problems.
• Analyze data to create hypotheses, predictions and estimates.
• Generate and explore rules underlying categories.
• Propose generalizations which explain relationships.
• Explore the concepts of probability and risk as it applies to levels of pollutants.
• Consider all evidence before drawing conclusions and developing generalizations.
• Withhold judgement when the evidence and reasons are insufficient.

Appreciate the value and limitations of technology within society. (TL)

• Understand the impact of molecular species detection technology on our knowledge of what pollutants are in our environment.
• Use probabilistic reasoning in relation to the analysis of risk related to molecular species detection.

Suggested activities and ideas for research projects.

• Write complete instructions for the preparation of 250 mL of 0.15 M sodium sulfate solution.

• Write instructions for the preparation of a 100 mL sample of 0.01 M sodium sulfate solution from a stock 0.15 M solution.

• (for those of you who still have AgNO₃)

  To 100 mL of 0.01 M AgNO₃, add 100 mL of 0.01 M NaCl. Observe the reaction and record a description of it. If the precipitate has started to settle, resuspend it by swirling the container. Divide the suspension into three parts.

  To the first part, add 20 mL of 0.2 M Na₂S₂O₃. A reaction will occur immediately. Describe the reaction.

  Heat the second part gently, either in a flame or a hot water bath. Record observations. What happens if this portion is shaken after it is heated?

  Expose the third portion to either strong sunlight or to an ultraviolet source for five minutes. Record observations.

  Devise explanations for each of the effects observed. Investigate the role of the first reaction in the developing of exposed photographic film. (This activity was adapted from CHEM13 NEWS, #81, page 4, November 1976, based on an idea by L. Sibley, St. Catharines, ON)

• One of the pollution problems which results from potash mining is the spread of the salts from the tailing piles and brine disposal ponds. While the Na⁺ and K⁺ ions are not a problem, the Cl^- ions may be. A simple test for the presence of chloride ion (Cl^-) in water can be based on the observation that AgCl has a very low solubility in water. Suppose that dilutions of a silver ion (Ag⁺) solution are made so that the concentrations of the series are 10^-1 M, 10^-2 M, 10^-3 M, 10^-4 M, 10^-5 M and 10^-6 M. Samples of these dilutions are then used to test water which is suspected of having significant chloride levels in it. What results would lead you to suspect that there was a large amount of chloride in the water? What results would you expect to see if you tested samples of distilled water? How could you estimate the levels of chloride in an unknown sample? What is the negative effect that chloride ions have on the soil and plants?

• What does a report of 8 ppb of PCBs in drinking water mean in terms of absolute amounts present, risk created and possibility of removal before consumption?

• One of the main potash ores in Saskatchewan is sylvanite. Sylvanite is a mixture of halite (sodium chloride - NaCl) and sylvite (potassium chloride - KCl). How is the KCl separated from the NaCl?

• What are the principles which are used in the solution mining of potash at Belle Plaine? Compare solution mining at Belle Plaine with that used at the sodium sulfate mines at Palo, Snakehole Lake or Chaplin?

• Are paints solutions, colloids or suspensions? How are pigments for paints produced? Are paints used by artists different from paints used by house painters? What different solvents are used to produce paints? What types of paints were used by artists during the Renaissance? How are acryllic or latex paints different from alkyd paints? What types of paints were used to do rock paintings?

• At room temperature, prepare a saturated solution of calcium acetate. Gently heat the mixture, over a low flame or in a water bath. What happens to the solution as it becomes hot? Cool the sample and agitate. Record observations and discuss them.

  Cool the calcium acetate to room temperature and mix 5 mL to 10 mL with 9 times the volume of ethanol (ethyl alcohol). Describe the effect. How does varying the 1:9 ratio affect the properties of the gel? Try mixing calcium acetate with other alcohols. (Caution: This gel, similar in composition to products marketed as `canned heat', is very flammable. Small amounts can be burned by supporting them on a screen.) (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 17, based on an idea contributed by C. McNeill, Savannah, GA. He attributed the idea to Denman Evans.)

• Gallstone and kidneystone disease involve formation of insoluble stones which block ducts from these organs. The blockage causes severe pain. Are gallstones and kidney stones chemically similar? What is the source of chemicals for their formation in the human body? How do they form?

• Paper chromatography can be used to separate dyes in food colouring and felt tip markers. What solutions are used as solvents to carry the molecules? What determines how effective a solvent will be? Compare the speed and separating ability of several formulations of solvents, including a 0.1% by weight solution of NaCl_(S) in distilled water. What are the chemical and physical properties of the solvents, solutes and paper which interact to make this procedure work? (This activity is based on an article by Peter G. Markow, West Hartford, CT in CHEM13 NEWS, #184, March 1989, page 11.)

• Why does scum form when soap is used in hard water? Write an equation for a typical chemical reaction involving scum formation.

• How do additives such as Calgon^TM soften the water? Devise an investigation to measure the effectiveness of softening water with Calgon, as compared to a regular household water softener and to distilled water.

• What chemicals are responsible for hard water in Saskatchewan? Do the chemicals found in hard water vary from location to location in Saskatchewan? Why is some water harder than other water? Do some chemicals in the water cause more severe effects than others? (Severe can be defined as a more noticable effect, an effect more harmful to health, produced by chemicals more difficult to remove, or some other criteria. Since severe is a relative adjective, it must be defined to give it some quantitative meaning.)

• How do water softeners work? What chemical reactions are involved? What is the chemical difference between hard and soft water? What is the chemical difference between distilled water, water sold in the stores as demineralized water, and soft water? How does water become hard? What is the chemical composition of the salt used in water softeners?

Sample ideas for evaluation and for encouraging thinking

• Stones don't dissolve in water very quickly. Propose a method for determining whether they dissolve in water at all.

• How do kidney stones and gall stones form? Can they be dissolved to eliminate them?

• Calculate the strength of a solution made by dissolving 8.05 grams of MgCl_2(s) in enough distilled water to make 500 mL of solution. Why is it important to use distilled water for making solutions? Is there a difference between saying "in enough distilled water to make 500 mL of solution" and "in 500 mL of distilled water"?

• How many moles of HCl are there in 25.5 mL of 2.50 mol·L^-1 HCl_(aq)?

• Suppose that each time you rinse a 50 mL graduated cylinder, 1 mL of solution is left on the walls of the cylinder. Use your knowledge of serial dilutions to explain why three rinses of 15 mL each is more effective in removing traces of the previous solution than is one rinse of 45 mL.

• A water testing laboratory measured the [Cl^-] concentration in a sample of well water to be 220 ppm. Estmate this [Cl-] expressed as mol·L^-1?

• Ca²⁺ and Mg²⁺ ions are often found in Saskatchewan well water. Why are they so common? Use solubility charts to determine if there are any negative ions which could be used to precipitate them from water.