Expt 036 -- Temperature Dependence of Salt Solubility

The solubility of a water soluble salt is determined by studying salt solutions saturated at elevated temperatures that are cooled so that some salt precipitates out of the solution.

Introduction

The amount of s substance that can dissolve in a solvent forming a solution is called the solubility. Solubilities are often very dependent upon temperature. Usually, but not always, salts are more soluble in warm solvents than in cool solvents.

Safety

• Burns present the largest hazard in this experiment. Hot object and cold objects look alike. Hold a finger near an object for a few moments before touching it to sense its temperature. Handle the hot tubes removed from the hood with great care.
• Some salts studied may be toxic; do not ingest the chemicals. Wear goggles and apron. Wash spills with water. Wash hands after the experiment.
• If mercury thermometers are used and a thermometer breaks during the experiment, notify the instructor immediately.

Procedure

1. Set up a clamp on a ringstand that will hold a large test tube. (Use an empty test tube to set the clamp so that, when the tube is available, it can be clamped quickly.)
2. Use a hot pad. Remove a tube containing saturated salt solution and a plastic transfer pipet with a large diameter stem from the hot bath under the hood, place it in a small Styrofoam cup, and carry it to the work station. Use the hot pad to handle the tube. Quickly move the tube to the clamp and clamp the tube in place. Place the tube in a beaker of warm water.
3. Carefully place a thermometer in the tube.
4. Squeeze the pipet bulb several times to fill it with liquid and expel the liquid. Continue this throughout the experiment; it maintains the equilibrium between the saturated solution and undissolved salt.
5. Remove the pipet and quickly add enough solution to the first well of a 12-well strip so as to just fill that well. Return the pipet to the tube, and continue squeezing and expelling. Repeat this operation at about 10-degree intervals.

   !!!Click here to See Movie.

6. When cooling slows at about 70 ºC, remove the beaker of warm water and allow the tube to cool in air.

   !!!Click here to See Movie.

7. When the temperature reaches 40 ºC, lower the clamped test tube into a beaker of cooling water -- at about 10 ºC -- in order to be able to complete the experiment in a satisfactory period of time. The last sample should be removed when the temperature is around 20 ºC.

   !!!Click here to See Movie.

8. Place the 12-well strip (with 6 wells filled with chemical) in a shallow ice tray. Cool the strip for 2 minutes. Remove the strip from the bath, gently tap it on the table a few times, and stir each well with a toothpick. Return the strip to the bath. Continue cooling for several minutes, stirring each well occasionally with a toothpick.
9. Remove the well. Rate the amount of solid in each well using a scale of 0 for no solid to 10 for the amount of solid in the well with the largest amount of solid. (Alternatively, the height of solid can be measured.)

   !!!Click here to See Picture.

   !!!Click here to See Picture.

10. Clean the strip. Remove solid to a disposal beaker or jar set aside for this purpose.

Data Analysis

Prepare a graph of the rating (0-10) as a function of the temperature (¼C).

Questions

1. In general terms, what is the relationship between temperature and solubility for the salt studied.
2. Suppose no solid is observed at the bottom of the tube at the start of the experiment when the tube is first removed from the bath and brought to the workstation. Is that solution saturated?
3. When the experiment is over, compare the concentration of salt of the solution in well-2 to that in well-5.
4. Cite any evidence available from personal experience to compare general rules for salt solubility with those for gas solubility.
5. Use the accompanying graph describing the solubility of KClO₃ in water to predict the number of grams of KClO₃ that dissolve in 50 mL of water at 80 ºC.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 036 Temperature Dependence of Salt Solubility

Watch the movies.

Estimate the solid height from the pictures.

Temperatures Your Estimate(0 to 10)

Temperatures	Your Estimate (0 to 10)
80
70
60
50
40
30

Prepare a graph of the rating (0-10) as a function of the temperature (¼C).

Answer the questions.

Curriculum-

Use this experiment when solutions, solubility, saturation, or water are discussed.

Safety-

• Burns present the largest hazard in this experiment. Hot object and cold objects look alike. Hold a finger near an object for a few moments before touching it to sense its temperature. Handle the hot tubes removed from the hood with great care.
• Some salts studied may be toxic; do not ingest the chemicals. Wear goggles and apron. Wash spills with water. Wash hands after the experiment.
• If mercury thermometers are used, have a commercial mercury spill clean-up kit available in case a thermometer breaks. Warn students of the hazards of mercury, and have them tell the instructor as soon as a thermometer breaks. Mercury spills should be cleaned up by the instructor.
• Some salts studied may be toxic; do not ingest the chemicals. Wash hands after the experiment.

Time-

Teacher Preparation: 15 minutes

Class Time: 50 minutes

Materials-

• 25 g KCl or KClO₃ or KNO₃
• ice
• large test tube 15-25 mm x 150-200 mm
• 250-mL beaker
• ring, ringstand, gauze
• 12-well strip
• flat tray
• plastic transfer pipet
• disposal jar for each solid studied.
• mm ruler
• thermometer (prefer alcohol or metal thermometer; possible mercury spill cleanup kit)

Disposal-

The chemicals used in this experiment may be safely discarded at the sink.

Lab Hints-

• Half-fill each large tube with solid. Add water to a level 5 cm above the solid. Heat to near boiling in a large water bath on a hot plate. Allow about 30 minutes to achieve equilibrium. Agitate several time during this period.
• Use a transfer pipet to agitate each solution often. Crystallization is less likely to block a pipet stem when wide stem transfer pipets are used.
• Tubes for these salts have been stored successfully over multi-year periods.
• Either KCl or KClO₃ may be used. KClO₃ is a strong oxidizing agent which presents some safety hazards. However, KClO₃ exhibits a larger change with temperature then KCl. KCl exhibits enough change for measurement.

Answers-

Q1. In general terms, what is the relationship between temperature and solubility for the salt studied.

A1. In general, the salt solubility increases as the temperature increases.

Q2. Suppose no solid is observed at the bottom of the tube at the start of the experiment when the tube is first removed from the bath and brought to the workstation. Is that solution saturated?

A2. The solution may be saturated -- meaning that added solute would not dissolve, but chances are that it is unsaturated.

Q3. When the experiment is over, compare the concentration of salt of the solution in well-2 to that in well-5.

A3. Since these wells are at the same temperature and each is has solution in contact with undissolved solid, the concentrations in these wells are identical.

Q4. Cite any evidence available from personal experience to compare general rules for salt solubility with those for gas solubility.

A4. Bubbles formed in water from the tap as it warms or in carbonated beverages suggest that gases are less soluble in liquids as the temperature increases.

Q5. Use the accompanying graph describing the solubility of KClO₃ in water to predict the number of grams of KClO₃ that dissolve in 50 mL of water at 80 ºC.

A5. The graph shows the solubility of KClO₃ in water at 80 ºC to be 70 g KClO₃/100 g water. The expected solubility for 50 mL of water, therefore is 35 g.

Handout Ans.-

Picture data at 10 ¼ intervals is about:

Temperatures	Your Estimate (0 to 10)
80	10
70	7
60	5
50	3
40	2
30	1

CoopLearn-

Working in pairs is a good team size for the experiment. Assign as many as three teams per salt. Suitable salts include KClO₃, KNO₃ and KCl.

Literature Data-

Sample data were obtained from equations from "Solubility in Inorganic Two-component Systems", M. Broul, J. Nyvlt, & O. Sohnel, Elsevier, Amsterdam, 1981. The data are plotted below.

The equation for the mole fraction of solute is of the form log x = A + B/T + C log T where x is the mole fraction of solute, T is the absolute temperature, log is the base-10 logarithm function, and A, B, and C are parameters specific to the solute.

	KNO3	KCl	KClO3
A	59.93325	6.75911	23.11212
B	-3821.8834	-604.3346	-2195.314
C	-19.523261	-2.357042	-7.134051

Reference-

Early versions of this experiment were developed by Randy Emry, Lincoln, NE and Leonard Allgood, Papillion, NE. It was developed at an EESA workshop at Lincoln East HS, Lincoln, NE, in 1988.

Key Words 1-

solutions, solubility, saturation,

Elements-

K Cl O N