Countertop Chemistry Experiment 5
Ziptop Bag Chemistry

Three reactions are performed in a sealed Ziploc™ bag so that they can be easily observed.

Materials per lab group	Substitutions per lab group
4 ziptop bags
1 Tbsp. calcium chloride
2 Tbsp. sodium hydrogencarbonate	2 Tbsp. baking soda
30 mL water
35 mm film canister (top optional)	1 small paper cup of water
30 mL indicator solution (phenol red)	30 mL red cabbage juice

Procedure

1. Add 2 Tbsp. sodium hydrogen carbonate to a Ziploc™ bag. Gently place a film canister (approximately1/3 full of water) inside the bag in the upright position. Squeeze out any excess air and seal the bag. Spill the water into the bag by shaking. Look, listen, and feel. Record your observations.
2. Add 1 Tbsp. of calcium chloride to a second Ziploc™ bag. Repeat the remaining steps in procedure 1 for the calcium chloride, and record your observations for this material.
3. Mix 2 Tbsp. of sodium bicarbonate and 1 Tbsp. of calcium chloride in a third Ziploc™ bag and mix thoroughly. Repeat the remaining steps.
4. Repeat procedure 3, replacing the water in the film canister with 30mL of indicator solution.

Data and Observations

1. sodium hydrogencarbonate in water_________________________________
   ________________________________________________________
2. calcium chloride in water____________________________________
   ________________________________________________________
3. NaHCO₃ + CaCl₂ in water__________________________________
   ________________________________________________________
4. NaHCO₃ + CaCl₂ with indicator solution________________________________
   ________________________________________________________

Questions

1. Classify each of these changes as chemical or physical. Use your observations to help you make your decisions.
2. In the fourth bag, what did the indicator tell you about the observed reaction?
3. What gas is being produced? How could you test this?
4. Write an equation for any chemical changes that have taken place.
5. Define heat of solution. Is it a physical change or a chemical change?

Teacher's Notes

1. a) There is a physical change in the first bag. See note 5 below for explanation
   b) A physical change occurs in the second bag.
   c) In the third and fourth bags, a chemical change occurs. See note 4 for the equation.
2. The indicator should show that the reaction occurring in the third bag is acidic. Cabbage juice will turn from red to blue in color, while phenol red will turn from red to yellow.
3. The gas that is produced is carbon dioxide, CO₂. It is formed from the carbonate ion, HCO₃ -. A burning splint would show that the gas extinguishes the flame. Some fire extinguishers use carbon dioxide for this reason.
4. The chemical changes that occur in bags 3 and 4 can be represented by the following equation:
   2NaHCO₃ (aq) + CaCl₂ (aq)-> CaCO₃ (aq) + 2NaCl (aq) + H₂O (l) + CO₂ (g)
5. A physical or chemical change may be accompanied by a change of energy. If the change requires heat from the environment, it is said to be endothermic.
   Solute + Solvent + HEAT ---> Solution
   If it releases energy to the environment, it is said to be exothermic.
   Solute + Solvent ----> Solution + HEAT
6. Phenol red can be used to show the presence of an acidic solution. It can be purchased at a swimming-pool supply store. Many foods also contain indicators. One of these is red cabbage juice.

Safety Precautions

As the Ziploc bags expand, care should be used to prevent excessive pressure build-up. The bags may burst.

Disposal

Solid wastes may be placed in a solid-waste container. All solutions may be poured down the drain, followed by water. When calcium chloride is dissolved in water, heat is given off. Handle these solutions with care.

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