Reaction in a Bag

Description

Equal volumes of anhydrous calcium chloride and sodium bicarbonate are placed in a sealable plastic bag. Aqueous phenol red is added, the bag is sealed, and the ensuing reaction is observed.

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Phenol red causes stains.

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Wear old clothing. Wash spills immediately with large amounts of water.

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1. Open a sealable plastic bag.
2. Hold the bag so that the center of the bottom of the bag is tucked up and the two bottom corners are readily accessible.
3. Place a teaspoon of NaHCO₃ in one bottom corner. Place a teaspoon of anhydrous CaCl₂ in the other corner (right of movie).
4. Place a vial of phenol red solution in the bag. Seal the bag without mixing the solids or spilling the solution.
5. Tilt the liquid onto one of the two solids. Mix by squeezing gently to form a solution.
6. Tilt the bag so that the solution comes in contact with the second solid. Mix again.
7. Note evidence for reaction.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 097 Reaction in a Bag

Answer the following questions as you watch the movie.

1. Describe the experiment.
2. Record the color changes observed.
3. Note any other sign of reaction.

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Purpose

To explore variables in a chemical reaction.

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(for 10 students working in pairs)

• 25 g CaCl₂ in sealable jar with dispenser (plastic spoon)
• 25 g NaHCO₃ with dispenser (plastic spoon)
• 5 sealable plastic bag (e.g., Ziploc^® bag)
• 50 mL 0.1 % aqueous phenol red
• 5 vial (tube, small bottle, or top of Beral pipet)

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• Keep the calcium chloride dry. This is an excellent experiment for the first week or even the first day of school.
• It is a good discrepant event.
• To prepare phenol red from the solid, add a tiny amount (the volume of a pin head) to 100 mL of water. Stir to dissolve. If necessary, dilute until the color is red rather than magenta.

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Teacher preparation: 10 minutes

Class time: 20-25 minutes

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All of the materials used in this experiment may be disposed of safely at the sink or with ordinary solid trash.

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• Dissolving calcium chloride is exothermic. If the water solution first contacts the calcium chloride, it heats. Reactions from the warm solution proceed quickly. Dissolving sodium bicarbonate is endothermic. If the water solution first contacts the sodium bicarbonate, it cools slightly. Cooled reactions proceed slowly.
• Regardless of the temperature, the two solutions react to form carbon dioxide and calcium carbonate:
  • Ca²⁺ + 2 HCO₃^- --> CaCO₃ + H₂O + CO₂
• The sealed bag fills with gas. The carbon dioxide is acidic and changes the color of the phenol red from red to yellow.
• Students can vary the relative amounts of the chemicals, and select different chemicals on which to place the added liquid inside the sealed sack.

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The experiment was developed through the:

GEMS Project,

Lawrence Hall of Science,

University of California-Berkley.

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1. Two reactants are mixed inside a plastic bag. Phenol red is added as an indicator. The color changes and gas bubbles are observed with all three components are mixed.
2. The indicator is first added to the CaCl₂ resulting in a red solution. The red solution is then added to the NaHCO₃ resulting in a change to yellow.
3. Bubbles are also evolved. Gas is produced in the bag.

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• discrepant event
• exothermic
• endothermic solution
• reaction rates

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