Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium hydroxide solution is irritating to eyes and skin. Always wear goggles or safety glasses when working with chemicals in the laboratory. Once food items are brought into the lab they are considered chemicals and should not be consumed. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

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. The leftover Pop Rocks^® and indicator solution may be disposed of by pouring down the sink with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• In step 10, if students do not fill the centrifuge tube all the way to the top, they will begin with air in the tube which is not carbon dioxide. Thus, it is important if there is an air bubble at the tip immediately after the tube is inverted, that the volume be recorded to obtain accurate results.

Answers to Prelab Questions

1. Why do Pop Rocks^® make their signature popping sound when placed in your mouth?

   Saliva causes the hard candy to dissolve which releases the carbon dioxide with its characteristic popping sound.

2. The CO₂ is added to the melted candy mixture once it has cooled. Why would it not be beneficial to add the CO₂ after it is first melted?

   The solubility of CO₂ increases as the temperature decreases. As the temperature increases, a greater volume of CO₂ would be required to stay in solution as the majority of it would bubble out and be released as a gas.

Sample Data

Answers to Questions

1. What was the total volume of CO₂ collected in the centrifuge tube?

   The total volume collected from 2.0 g of Pop Rocks was 0.7 mL (final volume – initial volume)

2. Explain the observed color change of the bromthymol blue indicator solution.

   Bromthymol blue indicator was used to confirm a decrease in pH as the Pop Rocks dissolved. A small portion of the CO₂ reacts with the water to produce carbonic acid which dissociates to H⁺, which results in a lower pH.

3. Why was it necessary to add 0.01 M sodium hydroxide to the indicator solution before starting the experiment?

   Bromthymol blue is a pH indicator which changes over a pH range of 6.0–7.6. Once the carbon dioxide dissolved in water it would produce carbonic acid which will lower the pH below 6. To see a change, the pH had to be higher than 7.6 to start the experiment.

4. How much CO₂ is released (in mL) per gram of Pop Rocks?
   {12830_Answers_Equation_2}
5. Based on the volume of CO₂, calculate the volume in liters of CO₂ that would be required to make a 10-kilogram batch of Pop Rocks?

   0.35 CO₂/g x 10,000 g = 3500 mL = 3.5L CO₂

6. Based on the answers to Questions 2–5, would you predict the actual volume of CO₂ originally added during manufacturing of 2.0 g of Pop Rocks more than, less than, or the same as the value calculated in Question 1?

   The actual volume of CO₂ added to 2.0 g of Pop Rocks would most likely be greater than 0.7 mL because the volume of CO₂ that reacted with water forming carbonic acid was not quantified.

References

Cesa, I. Flinn ChemTopic™ Labs: The Gas Laws, Volume 9, Flinn Scientific Inc. 2003.

Introduction

Everyone remembers eating Pop Rocks^® and listening to them crackle as they dissolve in your mouth. Dissolve a package of Pop Rocks in water and collect the gas to determine the volume of carbon dioxide in each package of Pop Rocks.

Concepts

• Consumer chemistry
• Volume of a gas

Background

Matter exists in three physical states—solid, liquid or gas. We are constantly surrounded by gases in the atmosphere. Although few elements exist in the gaseous state at room temperature they play a very important role.

Carbon dioxide, CO₂, has many important commercial uses. About 25% of all CO₂ produced is used in the soft drink industry. Carbon dioxide, has a relatively high solubility compared to other gases. Like most gases, it is also more soluble in lower temperature solutions. Solutions retain more CO₂ at lower temperatures. As the solution of CO₂(aq) is warmed, CO₂(g) is released as bubbles. This is noticeable when a carbonated beverage is warmed.

Not all the CO₂ in soft drinks is released as a gas. A very small portion of CO₂ remains in solution. The carbon dioxide reacts with water to produce carbonic acid, a weak acid (see Equation 1). This explains why soft drinks are acidic (have a low pH).

Pop Rocks were first developed by General Foods research scientist William A. Mitchell (1911–2004) in 1956. Pop Rocks are made in a similar fashion to carbonated beverages. First, hard candy is melted down. As it cools, carbon dioxide is added at a pressure of 600 pounds per square inch (psi). When Pop Rocks are eaten, the saliva inside the mouth dissolves the hard candy, releasing the CO₂ gas and resulting in the characteristic popping sound of Pop Rocks.

A pH indicator such as bromthymol blue can be used to determine the relative amount of CO₂ dissolved in water. Bromthymol blue is blue in a solution of a pH greater than 7.6, green between 7.6 and 6.0, and yellow at a pH below 6.0. The CO₂ that remains in the water to form carbonic acid is displaced from the centrifuge tube as the CO₂ gas is released. Thus the weakly acidic carbonic acid enters the bromthymol blue solution and changes the pH of the solution.

Experiment Overview

In this activity, Pop Rocks will be dissolved in a centrifuge tube filled with water. The volume of CO₂ released as a gas will be collected and measured in the centrifuge tube.

Materials

Prelab Questions

1. Why do Pop Rocks make their signature popping sound when placed in your mouth?
2. The CO₂ is added to the melted candy mixture once it has cooled. Why would it not be beneficial to add the CO₂ after it is first melted?

Safety Precautions

Sodium hydroxide solution is irritating to eyes and skin. Always wear goggles or safety glasses when working with chemicals in the laboratory. Once food items are brought into the lab they are considered chemicals and should not be consumed. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Pour 50 mL of distilled or deionized water to a 100-mL glass beaker.
2. Add 20 drops of 0.04% bromthymol blue to the DI water. Stir the solution using a stirring rod.
3. If the solution is not blue in color, add 0.01 M NaOH dropwise, stirring after each drop, until the solution is blue in color. Note: It is important to stir/test the solution after each drop of NaOH is added so it does not become too basic.
4. Using a pushpin, poke four holes into the cap of the centrifuge tube (see Figure 1).
   {12830_Procedure_Figure_1}
5. Place a weighing dish on an electronic balance and measure 2.0 g of Pop Rocks^® candy.
6. Add 2–3 mL of distilled or deionized water to the centrifuge tube.
7. Read through all of the steps 8–10 before starting step 8. It is extremely important that the steps be done quickly to obtain accurate results.
8. Fold the weighing dish in half to create a spout. Carefully pour the Pop Rocks into the centrifuge tube.
9. Quickly fill the centrifuge tube with distilled or deionized water to the very top and then cap the tube.
10. Invert the centrifuge tube into the beaker filled with bromthymol blue indicator. If there is any air space at the tip of the centrifuge tube immediately after it is inverted, record the volume on the worksheet. If the tube is completely filled with water, record zero.
11. Allow the Pop Rocks to dissolve. Once the Pop Rocks have dissolved, record the final volume of gas in the centrifuge tube on the Pop Rocks Science Worksheet.

Student Worksheet PDF

12830_Student1.pdf