Physical Science Institute

Summer 2013

Just Add Water-Teacher Guide

MATERIALS

(1) balance, top loading

(1) bottle, wide mouth, 4oz w/ cap labeled “magnesium sulfate” with ~10 g of Epsom salt

(1) bottle, wide mouth, 2 oz w/ cap labeled “coffee creamer” with ~10 g of non-dairy coffee creamer

(1) bottle, wide mouth, 2 oz w/ cap labeled “copper acetate”with ~10 g of copper acetate

(1) bottle, wide mouth, 2 oz w/ cap labeled “citric acid” with ~10 g of citric acid

(1) 20-oz container of sugar

(1) bottle, dispensing w/ flip-top 2oz (60 mL), labeled “ammonia” with ~20 mLhousehold CLEAR ammonia solution

(1) bottle, dispensing w/ flip-top 8oz (250 mL), labeled “purified water” filled with purified water

(1) bottle, dispensing w/ flip-top 8oz (250 mL), labeled “ethyl alcohol” filled with ethyl alcohol

(7) test tubes, 20x150mm

(1) test tube rack

(3) stoppers, rubber #2

(1) funnel

(5) sheets filter paper, 12.5 cm

(1) graduated cylinder, 100-mL

(2) weighing dishes

(1) pipet, Beral type (disposable)

(1) microspatula

Sugar
Non-dairy coffee creamer
Ammonia, clear
Water, deionized
Magnesium sulfate (epsom salts)
Copper(II)acetate, 100g / Fisher / S25276A
Ethyl alcohol, 4L / Flinn / E0013
Citric acid, 100 g / Fisher / S25254
Bottle, 2oz w/ flipcap, boston round / US Plastic / 60138
Bottle, 8oz w/ flipcap, boston round / US Plastic / 60140
Bottle 2oz/60cc Wide Mouth / US Plastics / 66732
Bottle 4oz/120cc Wide Mouth / US Plastics / 66733
Cap w/ liner / US Plastics / 66508
Filter paper, 12.5 cm / Fisher / 09-803-6F
Pipet, diposable / Fisher / S304679
Test tube 20x150mm / Fisher / S00184
Test tube rack / Fisher / S99439
Stopper, rubber, solid #2 / Fisher / 14-130D
Weighing dish / Fisher / S67090A
Labeling tape, white, 1" / Flinn / AP1372
Microspatula / Fisher / S50821
Balance, top-loading / Fisher / S94792K
Funnel, utility, 3 1/2" / Flinn / AP3201
Graduated cylinder, 100-mL / Fisher / S31857

NOTES

  • Make sure to use CLEAR household ammonia; don’t use SUDSY ammonia, it has soap mixed with it.
  • Demonstrate how to fold a piece of filter paper before beginning Part 1 procedure.
  • When looking for clarity and shades of the mixtures in the test tubes, participants can use the filter paper as a white background.
  • When the instructions say “shake until all the solid dissolves” in step #11, the participants may see some undissolved impurities in their test tubes once all the copper acetate dissolves. They can ignore these contaminants.
  • When making the saturated solution of copper acetate in the 10 mL of water (step#12), the mixture may have foam at the top that traps some of the undissolved solid. While the participants are letting the solid settle out, they may have to gently tap the test tube to break up the surface tension so that the solid can sink.
  • Use Flinn Scientific disposal method #26b for all the mixtures in this activity.

QUESTIONS

10)

Mixture / Is the mixture cloudy (scatters light)? / Can mixture be separated by filtering? / Does the mixture settle out over time?
Test tube #1 / YES / YES / YES
Test tube #2 / YES / NO / NO
Test tube #3 / NO / NO / NO

Questions:

  1. Is the blue color evenly dispersed—uniform—for each solution? Yes
  2. We can use how dark or light (the shade) the solution is as an indicator of the concentration of copper acetate in solution. Which solution is more concentrated, i.e., has a darker shade?

The test tube with the 10 mL of water is more concentrated.

  1. Which solute produced a saturated solution when 1 gram was mixed with the water?

Neither, there was no undissolved solid in either test tube.

  1. Which solute produced a saturated solution when 1 gram was mixed with the ethyl alcohol?

The 1 gram of sugar produced a saturated solution; there was undissolved solid in the test tube.

  1. Which solute, citric acid or sugar, has a greater solubility in ethyl alcohol?

Citric acid has a greater solubility than sugar in ethyl alcohol.

  1. Which test tube contains the greatest mass of undissolved solid?

The test tube with 0.50 grams of copper acetate in 10 mL of water contains the most undissolved solid.

  1. What limited the mass of copper acetate that would dissolve in the water?

The amount of water is one factor that affects the amount of solute that will dissolve.

  1. Look at the shade of each solution above the undissolved solid. Are the concentrations noticeably different?

Because both solutions are saturated, the concentrations, and hence the shades, should be equal.

  1. What visual cue can be used to determine if a solution is saturated?

After the mixture is adequately stirred, there is undissolved solid remaining.

  1. Which of the diagrams in Fig 5.1 represents the concentration of copper acetate in solution when both 10 mL and 20 mL of water were saturated with 1.8 grams (0.5 g + 1.3 g) of copper acetate?

Diagram A, the concentrations for both saturated solutions will be equal.

  1. Some people say that sugar “melts” when it is added to water. How would you describe the difference between the processes of melting and dissolving?

In the process of melting the particles making up a substance are given a higher average kinetic energy by raising the temperature. Eventually the kinetic energy of the particles overcome attractive forces and forms a liquid state. In dissolving, the attractive forces between the solute and the solvent overcome the attractive forces between the solute particles. Or…, melting involves one substance and dissolving involves two or more substances.

  1. Suppose that 200 mL of a saturated solution of copper acetate was left standing in an open beaker for two weeks. During this time half of the water evaporated.
  2. Would the mass of copper acetate dissolved in the solution change?

Yes, as the amount of water decreased, then there would be less copper acetate dissolved.

  1. Would the concentration of the copper acetate solution change during the two weeks?

No, the solution started out saturated. As water evaporates, more copper acetate crystalizes out of solution to keep the concentration constant.

13. A saturated solution is one in which no more solute will dissolve. Our kinetic model of matter says that molecules are in constant motion. How can a solute “stop dissolving” if the molecules of the solute and solvent are in constant motion and always interacting with one another?

The two opposing processes of dissolving and crystallization are occurring at the same rate giving the appearance that no more solute is dissolving.

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