Kitchen Chemistry Experiment 14
Rate of Solution Demonstration

How does heating a solvent, crushing a solute, or stirring a mixture aid the solution process? Which factor would have the greatest effect on solubility of a solid in a liquid?

Materials	Substitutions
3-600 mL beakers	3-1 quart jars
balance
1-stirring rod	1-spoon
3-cubes of 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)

Procedure

 

1. Have 3 students set up 3 jars for a contest to test solubility of a sugar cube

#1 300 mL of hot H2O with a sugar cube

#2 300 mL of cold H2O with a sugar cube to be stirred

#3 300 mL of cold H2O with a crushed sugar cube

Use a mortar and pestle or the back of a spoon to crush the sugar cube.

2. When everyone is ready, drop the sugar cubes into the water and observe what happens. The student who is stirring should do so with medium intensity. The other two students cannot stir. Which method increased the solubility of the sugar cube the most? Record you data below. Draw some conclusions based on your observations.

 

Data and Observations

 

Rank Order Rate of Solution for sugar cubes in water - #1, #2, #3

___ 300 mL of hot H2O with a sugar cube

___ 300 mL of cold H2O with a sugar cube to be stirred

___ 300 mL of cold H2O with a crushed sugar cube

Extensions

1. How does crushing the solute 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 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 did you predict would happen with the heated water and the sugar cube? Why?

 

Additional Extension

 

1. Carefully open the small bottle of club soda. Pour equal amounts into the 400 mL beakers or measuring cups. Quickly , take the mass of each and record this on the data table.
2. Place one beaker of soda on the hot plate. Take the second beaker of soda and place it under the dome of the vacuum pump. Turn the pump on. What happens? Is the solution boiling? When the activity slows or stops, remove the beaker and reweigh it. Record the mass below. What was the change in mass?
3. Now, remove the beaker from the hot plate. Allow it to cool before placing it on the balance. Reweigh it and record its mass below.
4. What is Henry's Law ? Why should soft drinks be kept in the refrigerator? If you shake a bottle of soda pop before you open it what will happen? Why? What is effervescence ?
5. 1. Mass of soda in vacuum pump before________g after__________g

6. Difference___________g

7. 2. Mass of soda from hot plate before________g after__________g

8. Difference___________g

 

Teacher's Notes

The answers to the questions about crushing the solute can be complex for some students. Crushing increases the amount of surface area exposed to the solvent.

1 face (20 cm x 20 cm) = 400 cm ² 1 cube (400 cm ² x 6 faces) = 2400 cm ²

1 face (10 cm x 10 cm) = 100 cm ² 8 cubes 8(100 cm ² x 6 faces)=4800 cm ²

1 face (1 cm x 1 cm)= 1 cm ² 1 cube (1 cm ² x 6)=6 cm ²

8000 cubes=48,000 cm ²

Stirring allows more of the solvent to pass over the solute surface area. It may actually break the solute up into smaller units therefore mirroring the same effect "crushing" has on the surface area. Most students will predict that heating the water is the single greatest way to increase solution formation. I make the students raise their hands and commit to a hypothesis before we test it. They are all surprised to find that, provided the water is not boiling, the trial with the heated water is the LAST to dissolve. Apparently the convection created by the hot solvent is not enough to overcome the advantage of more surface area exposed to the solvent. I like to point out that packets of sugar and sugar substitutes are granulated and, when stirred, dissolve very easily in cold beverages, like ice tea, very easily.

 

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. Soft drinks are bottled at 10-15 times atmospheric pressure to keep the CO2 in solution. Stress to the class that the soft drinks are NOT BOILING !!! So many of them believe this is what they are seeing and are not impressed. 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's in a 3-Liter cola!! Certainly, some of the mass lost on the hot plate is due to evaporation. That is why this trial will register a greater difference than the one that is put in the vacuum pump. Let your students hypothesize as to why the hot plate provides a greater difference. A very bright youngster will mention the evaporation. If a bottle of soft drink has been shaken before you try to open it, you will not be able to tell. Shaking will cause many carbon dioxide molecules to build up in the neck of the bottle above the liquid. It looks the same but the gas is waiting to rush out in a show of effervescence.

 

Sugar cubes and club soda can be purchased at the grocery store.

Safety Precautions

The hot plate should not be too hot! (setting 4 out of 6) The water used with the sugar cubes must not be boiling. Any glassware placed on the hotplate needs to be heat resistant.