Countertop Chemistry Experiment 14
Rate of Solution Demonstration

Several factors can increase the rate of dissolution for a solid. In this demonstration, you will investigate some of these factors.

Materials Substitutions
3-600 mL beakers 3-1 quart jars
balance  
1 stirring rod 1 spoon
3 sugar cubes (sucrose) 3-sugar cubes
1 mortar and pestle 1 cup and spoon
1 large hot plate 1 warming tray with 2 burners
1-800 mL beaker 1 sauce pan
2-400 mL beakers 2 measuring cups (or Pyrex glassware)
club soda (small bottle)  
vacuum pump with bell jar attached  

Procedure

  1. Into the three 600mL beakers (labeled #1, #2, #3), add the following:

    #1 300 mL of hot H2O (about 80 oC)
    #2 300 mL of cold H2O (about 20 oC)
    #3 300 mL of cold H2O (about 20 oC)

  2. Drop one cube of sugar into beakers #1 and #2.
  3. Use a mortar and pestle or the back of a spoon to crush a sugar cube.
  4. Drop the crushed sugar into beaker #3.
  5. Using the stirring rod, stir the contents of beaker #2, leaving beakers #1 and #3 unstirred.
  6. Observe what happens. Which method increased the rate at which sugar dissolved most? Record your data below. Draw some conclusions based on your observations.

Data and Observations

Rate of dissolution for sugar cubes in water – first, second, and third

Beaker Contents Order of dissolving
#1 300 mL of hot H2O (about 80 oC)  
#2 300 mL of cold H2O (about 20 oC)  
#3 300 mL of cold H2O (about 20 oC)  

Questions

  1. How does crushing the solute (sugar) increase the rate of solution?
  2. Suppose you had a cube (6 sides) that measured 20 cm x 20 cm on each face. How much surface area would be exposed to the solvent?
  3. What would the surface area be if the same cube was crushed into 8 cubes with each face measuring 10 cm x 10 cm?
  4. How much area would be exposed if the cubes were crushed further into 8,000 cubes with each face measuring 1 cm x 1 cm?
  5. Why does stirring aid the solution process?
  6. What was the effect of increasing the temperature of the water? Why?

Extensions

  1. Carefully open the small bottle of club soda. Do not shake the bottle of club soda prior to opening it. Pour equal amounts into the two 400-mL beakers or measuring cups. Quickly, take the mass of each beaker and record these masses on the data table.
  2. Place one beaker of soda on the hot plate. As the soda heats, what do you observe? Take the second beaker of soda and place it under the bell jar of the vacuum pump. Turn the vacuum pump on. What happens? Is the solution boiling? When the bubbling stops, remove the beaker from the hot plate. Let it cool to room temperature for approximately 5-10 minutes and then reweigh it. Remove the beaker from the bell jar and weigh it. Record the masses below. What was the change in mass?

    a. Mass of soda in vacuum pump:
    before________g after___________g Difference___________g

    b. Mass of soda from hot plate:
    before________g after___________g Difference___________g

  3. What do these differences in mass tell you about the solubility of a gas in a liquid? Why should soft drinks be kept in the refrigerator?
    If you shake a bottle of soda before you open it, what will happen? Why?

Teacher's Notes

Prior to step 1, you may wish to divide the class into groups of 3 students, with each student responsible for one of the three beakers.

The greater the surface area of the solute, the faster the rate of dissolution of the solute. Crushing increases the amount of surface area exposed to the solvent.

1 face (20 cm x 20 cm) = 400 cm 2
1 face (10 cm x 10 cm) = 100 cm 2
1 face (1 cm x 1 cm)= 1 cm 2

1 cube (400 cm 2 x 6 faces) = 2400 cm 2
8 cubes 8(100 cm 2 x 6 faces)=4800 cm 2
1 cube (1 cm 2 x 6)=6 cm 2
8000 cubes=48,000 cm 2

Stirring distributes the solute throughout the solution and, therefore, increases the rate of dissolution. It may actually break the solute up into smaller units, thereby producing the same effect as "crushing" the solute. Most students will predict that heating the water is the single greatest way to increase solution formation. I ask that the students indicate the fastest technique for dissolution prior to the experiment. They are all surprised to find that, provided the water is not boiling, the sample in the heated water is the LAST to dissolve. The convection created by the hot solvent is not as effective as a larger surface area in accelerating the dissolution. I like to point out that both packets of sugar and sugar substitutes are granulated and, when stirred, dissolve very easily in cold beverages.

Henry's Law states that the mass of a gas dissolved in a given volume of liquid is directly proportional to the pressure of the gas applied. Soft drinks are bottled at 10-15 times atmospheric pressure to increase the concentration of CO2 in solution. Soft drinks are NOT BOILING as the gas is released! There are several grams of carbon dioxide in a small bottle of club soda. Your students might want to estimate the amount of gas they would consume with a 20-oz. soft drink—or the amount that is in a 3-liter cola! Certainly, some of the mass lost on the hot plate is due to evaporation. Some of the soda (water) will also be lost while the soda is under the vacuum, owing to the reduction of vapor pressure of the solvent. Remember the caution: “Do not shake the bottle of club soda”. Shaking causes carbon dioxide to escape from the solution, and the pressure will increase above the liquid, resulting in a premature release of the gas.

Safety Precautions

The hot plate should not be too hot! The water used with the sugar cubes must not be boiling (approximately 80¾ C is a reasonable temperature). Any glassware placed on the hotplate needs to be heat resistant.

Disposal

All solutions may be poured down the sink.

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