Materials Included In Kit

Additional Materials Required

Safety Precautions

Wear chemical splash goggles. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Discard any unwanted sodium chloride in the trash according to Flinn Suggested Disposal Method #26a. Flush all salt solutions down the drain according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• This kit contains enough materials for 30 students working in pairs (15 student groups).
• Perform a demonstration prior to lab to show that the nature of the solute/solvent pair affect whether or not a substance is soluble. For example, try dissolving several solutes (such as iodine, urea, sodium chloride, and cupric sulfate) in several solvents (such as water, ethyl alcohol, and hexane). You will find that the iodine is soluble in alcohol and hexane, but not in water. Both urea and sodium chloride are soluble in water and alcohol, but not hexane. Cupric sulfate is soluble only in water.
• Perform a demonstration prior to lab showing the solubility curves for several salts. For example, the solubility curve for sodium chloride is almost constant at all temperatures (35.7 g/100 mL at 0 °C and 39.12 g/100 mL at 100 °C), while the solubility curve for potassium nitrate increases rapidly with temperature (13.3 g/100 mL at 0 °C and 247 g/100 mL at 100 °C).
• This lab can be performed several different ways depending on your students and your teaching style.
  1. Provide students with the background material given on the following page and have them follow the procedure provided.
  2. Provide students with the procedure only (no background) and have them determine the effect of each factor on the rate of dissolving using their experimental data.
  3. Have students design their own procedure for determining the effect of these factors on the rate of dissolving for a completely open-ended activity.
• In Part B, students will determine that agitating the solution by inverting it helps to dissolve the salt. If the salt in the uninverted solution was allowed to dissolve completely, it would take about an hour. Therefore, for Parts C–E, the tubes are all agitated in addition to varying the particle size, temperature or already dissolved solute. Because all tubes are inverted (at the same rate), students can still see the effect of each factor individually. They will measure the time to dissolve in terms of the number of inversions required to dissolve. Therefore, if one tube requires three inversions and a second tube requires six inversions, then it can be inferred that the salt in the first tube dissolves twice as quickly.
• Make sure that students add equal volumes of water and equal masses of salt to each tube.
• Make sure that students invert all tubes at the same rate.
• A hot plate or Bunsen burner can be used to heat the water for Part C. If hot tap water is >60 °C, it can be used also.

Sample Data

Data Table 1. Effect of Stirring on the Rate of Dissolving

Answers to Questions

1. What is the effect of stirring on the rate of dissolving? Explain.

   Stirring increases the rate of dissolving. Stirring helps to bring fresh solvent in contact with the solute. As the surface layer of the solute dissolves, the solution surrounding the solute crystal tends to have a high concentration of solute, which tends to slow down the rate of dissolving. By stirring the solution, the dissolved solute is transferred to other parts of the solution more quickly and fresh solvent is available so that the next surface layer on the undissolved solute crystal can now dissolve.

2. What is the effect of temperature on the rate of dissolving? Explain.

   Increasing the temperature increases the rate of dissolving. When the temperature of a solution is increased, the average kinetic energy of the molecules and ions in solution is also increased. This means that both the solvent and dissolved solute particles are travelling from one region of the solution to another faster as the temperature is raised. This increased movement increases the rate at which fresh solvent is brought into contact with the undissolved solute crystals. Just as with stirring, bringing fresh solvent in contact with the undissolved solute at a faster rate increases the rate at which the solute crystal will dissolve. In addition, the solvent particles have more energy to remove particles from the surface layer of the solute.

3. What is the effect of surface area on the rate of dissolving? Explain.

   The rate of dissolving increases as the surface area is increased. Because dissolving occurs at the surface of a crystal, the more surfaces that are in solution, the faster the concentration of dissolved solute will increase.

4. What is the effect of already dissolved solute on the rate of dissolving? Explain.

   The rate of dissolving is slower if there is already some dissolved solute in solution. The water surrounding the solute can only accept a certain number of grams of solute in a given volume. Using the analogy from the Background Section, imagine that the water has a specific number of seats in which dissolved solute particles can sit. As these seats become filled, it is harder for an undissolved particle to find an empty seat, and it takes that particle longer to find an empty seat before it can dissolve.

5. Why is solvation a surface phenomenon?

   Solvation is a surface phenomenon because it is those molecules or ions at the surface of the solid, not those in the interior, or bulk, of the solid, that interact with the surrounding solvent and dissolve in that solvent.

6. Give three examples of solutions you encounter on an everyday basis. List the solute and solvent for both examples.

   Answers will vary. Some possibilities include

   {11829_Answers_Table_1}

References

Handbook of Chemistry and Physics, 69th ed.; Weast, R. C., Ed.; CRC: Boca Raton, FL, 1988.

Introduction

Why is it that some soluble solids take so much longer to dissolve than others? What factors affect the rate of dissolving? Understanding solutions is important when studying chemistry since solutions are used to perform so many chemical reactions. In this laboratory activity, you will test several factors and observe how each factor affects the rate of a solute dissolving in a solvent.

Concepts

• Solutions
• Solubility
• Rate of dissolving

Background

Solvation Is a Surface Phenomenon
The process of a solid solute dissolving in a solvent is a surface phenomenon. Dissolving is a surface phenomenon because it is those molecules or ions at the surface of the solid, not those in the interior, or bulk, of the solid, that interact and dissolve in the surrounding solvent.

In aqueous solutions, the solvent is water. Some of the water molecules dissociate to form hydrogen ions, H⁺, and hydroxide ions, OH^–. If an ionic crystal is added to water, the H⁺ and OH^– ions interact with the ions at the surface of a crystal, pulling them from the crystal lattice into the solution. For example, if a sodium chloride crystal is added to water, the sodium ions, Na⁺, and chloride ions, Cl^–, on the surface of the crystal dissociate (move from the crystal lattice to solution) to form aqueous Na⁺ and Cl^– ions. The positively charged Na⁺ ions will be surrounded by the negatively charged OH^– ions, while the negatively charged Cl^– ions are surrounded by the positively charged H⁺ ions. These ionic attractions between the crystal’s ions and water’s ions make it favorable for the sodium chloride crystal to dissolve in water.

As the surface ions dissolve, the next layer of ions now becomes the surface layer. This new surface layer interacts with the ions already in solution as described above. This interaction at the surface of a crystal continues until the crystal is completely dissolved, or until the solution can accept no more solute.

Factors Affecting the Rate of Dissolving
The rate at which a substance dissolves in water depends upon four factors: stirring, temperature, surface area, and the amount of solute already dissolved in solution. Each of these is addressed in detail.

Stirring. Stirring helps to dissolve a solute by bringing fresh solvent in contact with the solute crystal at a faster rate. As the surface layer of a solute crystal dissolves, the solution surrounding the crystal tends to have a high concentration of dissolved solute, which tends to slow down the rate of dissolving. By stirring the solution, the dissolved solute is transferred to other regions of the solution more quickly and fresh solvent is made available so that the next surface layer on the undissolved solute crystal can now dissolve.

Temperature. When the temperature of a solution is increased, the average kinetic energy of the molecules and ions in solution is also increased. This means that both the solvent and dissolved solute particles are traveling from one region of the solution to another region more quickly as the temperature is raised. This increased movement increases the rate at which fresh solvent is brought into contact with the undissolved solute crystal. Just as with stirring, bringing fresh solvent in contact with the undissolved solute crystal at a faster rate increases the rate at which the solute crystal will dissolve. In addition, the solvent particles have more energy to remove particles from the surface layer of the solute crystal.

Surface Area. Because dissolving occurs at the surface of a crystal, the more solute surfaces that are exposed to solvent, the faster the solute will dissolve. Therefore, increasing the surface area of the solute to be dissolved increases the rate at which it will dissolve. The surface area is increased, for example, by grinding up one large crystal into many small crystals or granules. Each of the small pieces is surrounded by a surface layer which can dissolve in solution as soon as the solute is added to solution.

Amount of Solute Already Dissolved in Solution. As a solute dissolves, the concentration of dissolved solute is greatest in the region directly adjacent to the solute crystal. This slows the rate at which additional solute can be dissolved. Stirring or raising the temperature helps to distribute these dissolved solute particles into other regions of solution more quickly, which increases the rate of dissolving. Why does the concentration of solute already dissolved affect the rate at which additional solute can dissolve? Each substance has a given solubility in water. Its solubility determines how much of that solute can dissolve in a given volume of water at a given temperature. Solubility varies from solute to solute. Some solutes are extremely soluble, while others are only slightly soluble. This means that the water surrounding the solute can only accept a certain number of grams of solute in a given volume. Imagine that the water has a specific number of seats in which dissolved solute particles can sit. As these seats become filled, it is harder for an undissolved solute particle to find an empty seat, and it takes that particle longer to find an empty seat before it can dissolve.

Different solutes dissolve at different rates depending on the four factors mentioned above. However, keep in mind that the rate at which a solute dissolves does not change how much of that substance can dissolve or whether it even can dissolve in a particular solvent. These ideas encompass higher level solubility concepts which are beyond the scope of this laboratory activity.

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part A. Preparation

1. Fill a small beaker half-full with water. Warm on a hot plate to about 80 °C.
2. Fill a second small beaker half-full with water. Add several ice cubes and allow the ice-water to cool to about 5 °C.
3. Fill a third small beaker half-full with water. Let this water sit on the lab bench and come to room temperature.
4. These beakers of water will be used in Part C. Proceed with Part B while they are coming to the desired temperatures.

5. Place two test tubes in a test tube rack. Label the tubes 1 and 2. Place about 0.2 g of salt crystals in each test tube. The amount added is not as important as adding equal amounts to each tube.
6. Add enough water to fill each test tube about two-thirds full. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added.
7. Stopper test tube 2 and invert it. If all of the salt does not dissolve, invert it again. Continue inverting until all of the salt is dissolved. Count the number of inversions required. One inversion consists of turning the stoppered test tube upside down, then bringing it back right-side-up.
8. Compare the rate of dissolving between the two tubes. Record your observations in Data Table 1.
9. Rinse the contents of both test tubes down the drain. Rinse and dry each test tube.

10. Place three test tubes in a test tube rack. Label the tubes 1, 2 and 3. Add about 0.2 g of salt crystals to each tube. The amount added is not as important as adding equal amounts to each tube.
11. Measure the temperature of the water in the beakers from steps 1, 2 and 3 with a thermometer. Once they have reached the desired temperatures, record the temperatures in Data Table 2 and proceed with step 12.
12. Fill the first test tube about two-thirds full with the cold ice-water. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. Count the number of inversions required. Record your observations and the number of inversions required in Data Table 2.
13. Fill the second test tube about two-thirds full with room temperature water. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. Count the number of inversions required. Record your observations and the number of inversions required in Data Table 2.
14. Fill the third test tube about two-thirds full with the hot water sample. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. Count the number of inversions required. Record your observations and the number of inversions required in Data Table 2.
15. Rinse the contents of each test tube down the drain. Rinse and dry each test tube.

16. Place three test tubes in a test tube rack. Label the tubes 1, 2 and 3.
17. Obtain two rock salt crystals that are approximately the same mass. Weigh them on a balance. Place one of the rock salt crystals into the first test tube.
18. Grind the second rock salt crystal with a mortar and pestle until it is a fine powder. Transfer the powdered salt to the third test tube.
19. Add the same mass of salt crystals to the second test tube.
20. Fill the first test tube containing the rock salt crystal about two-thirds full with water. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. If the number of inversions required to dissolve the crystal is greater than 25, stop and record “>25” for the number of inversions in Data Table 3.
21. Fill the second test tube containing the salt crystals about two-thirds full with water. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. Count the number of inversions required. Record your observations and the number of inversions required in Data Table 3.
22. Fill the third test tube containing the powdered salt about two-thirds full with water. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. Count the number of inversions required. Record your observations and the number of inversions required in Data Table 3.
23. Rinse the contents of each test tube down the drain. Rinse and dry each test tube.

24. Place two test tubes in a test tube rack. Label the tubes 1 and 2.
25. Place about 0.2 g of salt crystals in test tube 1. Fill this tube about two-thirds full with water. Pour the water carefully down the side of the tube so that little mixing occurs as the water is added. Stopper the tube and invert it to dissolve all of the salt. Count the number of inversions required. Record your observations and the number of inversions required in Data Table 4.
26. Add 0.4 g of salt crystals to test tube 2. Fill this tube about two-thirds full with water and invert (without counting) until all of the salt is dissolved.
27. Now add about 0.2 g of additional salt crystals to test tube 2. Stopper test tube 2 and invert until all of the salt is dissolved. Count the number of inversions required. Records your observations and the number of inversions required in Data Table 4.
28. Rinse the contents of each test tube down the drain. Rinse and dry each tube.

Student Worksheet PDF

11829_Student1.pdf