Adapted from April Aldott’s: Reaction in a Plastic Baggie
http://www.elmhurst.edu/~chm/demos/reactioninbag2.htm
Reaction in a Plastic Baggie
Science Concept: This chemical reaction between two solids creates a new substance in
the form of a gas that is observable by dramatically inflating the plastic baggie.
Because 3rd grade students often miss observing the heat, they will do this reaction in two
steps: First Ss mix the CaCl2 with water in order to observe the heat, then they add the
baking soda and phenol red to make further observations.
Safety Precautions: When handling the phenol red, use caution. If it contacts skin or
clothing, it may stain. Additionally, be careful in letting the bag fill up with gas. If it gets
too full, the bag might pop.
Waste Disposal: Dispose of the plastic bag and its contents in the trash.
Introduction & Commentary: How many of you like balloons? Do you like blowing
them up? Sometimes that can make you kind of tired, though. I'm going to show you a
way in which this plastic bag will fill up like a balloon, without having to blow air into it.
Materials & Pre-Demo Preparation:Note to Teacher: It is highly recommended that
teacher try out the chemical reaction before doing it in the classroom.
Preparation:
Prepare a tray with the following supplies for each group-Be sure everything is labeled!
One quart-size ziplock baggie, goggles, plastic gloves, paper towels for clean up, one
plastic medicine or small salsa cup, container for waste, plastic tablespoon labeled
Calcium chloride and 3 plastic teaspoons (labeled “water”, “baking soda”, “ phenol red
solution”
In addition, the following supplies will be needed for each group for each investigation:
Investigation 1:
Calcium chloride -1 tablespoon
Water bottle-students will need about 2 teaspoons
Investigation 2:
Baking soda- 2 teaspoons
Phenol red solution-1 teaspoon or 5 ml
(If only phenol red is available, a phenol red solution is made by adding four drops of
phenol red to 10 ml of distilled water.)
Directions for Investigation 1 (Produces Heat)
1. In the ziplock bag, place 1 tablespoon of calcium chloride into the baggie. Add 2
teaspoons of water into the baggie and seal.
2. Record your all your observations. Utilize all your senses to make your
observations. (Teacher: Guide Ss to feel the bottom of the baggie if they are not
doing so.)
Directions for Investigation 2 (Produces Carbon Dioxide gas and a Color Change)
1. Measure 1 tsp of phenol red solution and pour into medicine cup. Carefully stand
the medicine cup inside the baggie. Do not let the cup tip over.
2. Add 2 teaspoons of baking soda to the baggie and quickly seal the baggie (to
prevent gas from escaping).
3. Now tip over the cup so that all substances mix.
4. Record your all your observations. Utilize all your senses to make your
observations. (Teacher: Again, guide Ss to feel the bottom of the baggie! If baggie
gets very big, teacher may open it or face it away from students.)
Explanation and Background for Teacher
Calcium chloride and baking soda (bicarbonate) react to form calcium carbonate, sodium
chloride (salt), water, and carbon dioxide. To produce this reaction, calcium chloride and
baking soda, which are both solid, are mixed together in water, which provides a solvent
for the chemical reaction to occur. The two solids switch partners and form a new
substance--the precipitate calcium carbonate. The calcium carbonate is a white solid
precipitate that sinks to the bottom of the water. As the precipitate is formed, the
bicarbonate breaks down first to make hydrogen ions, an acid. This acid then converts
some of the bicarbonate to carbon dioxide gas which begins to blow up the plastic bag. It
can also be observed as a fizzing or bubbling. The phenol red turns yellow when the CO2
produced dissolves in water to produce carbonic acid.
The balanced chemical reaction is:
2 NaHCO3 + CaCl2 => CaCO3 + CO2 + 2 NaCl + H2O.
Also, as part of the chemical reaction between these two solids, changes in temperature
can be felt--an exothermic reaction causes the calcium chloride to get warm, while the
endothermic reaction makes the baking soda get cold.
For a relatively simple explanation, see: What Do Calcium Chloride & Baking Soda
Make? | eHow.com http://www.ehow.com/how-does_5498056_do-amp-baking-sodamake.html#ixzz1uEDHM6n4
Additional Teacher Information
Calcium chloride + baking soda (sodium bicarbonate) --- calcium carbonate + sodium
chloride + hydrogen ions
CaCl2 + 2 NaHCO3 ---> CaCO3 + 2 NaCl + H+ ions
Hydrogen ions + sodium bicarbonate ---> carbon dioxide + water + sodium ions
H+ ions + NaHCO3 ---> CO2 + H2O + Na+
A chemical reaction involves the breaking of bonds in the reactants and the making of
bonds in the products. It takes energy to break bonds (endothermic; feels cold) and that
energy is released when bonds are formed( exothermic; feels warm/hot).
In an endothermic reaction, it takes more energy to break the bonds of the reactants than
is released when the bonds in the products are formed. In an endothermic reaction, the
temperature goes down.
Additional Explanation:
From: http://agpa.uakron.edu/p16/lesson.php?id=ziplock_chem&pg=procedures
The solution gets warm because heat is given off when calcium chloride dissolves in water
(not a chemical reaction). This process is exothermic. When sodium bicarbonate dissolves
in water, heat is absorbed and the solution gets colder. This process is endothermic. The
phenol red turns yellow when CO 2 produced dissolves in water to produce carbonic acid.
Note: the addition of some solutes to a solvent will raise the temperature of a solution, while
others will have no noticeable effect. This behavior depends on the heat of solution of the
solute in a given solvent. The heat of solution, i.e., the amount of heat given off or absorbed
during the process of solution, is equal to the difference between the energy that must be
supplied to break up the crystals of the solute and the energy that is released when the
solute particles are taken into solution by the solvent. If more energy is required to break up
the crystals than is released in forming the solution, the temperature will decrease. If more
energy is released in forming the solution than in breaking up the crystals, the temperature
will increase.
The following is for your information and may be too detailed for your students:
The gas evolved is carbon dioxide. The bicarbonate ion (HCO 3-) is a weak acid and partially
ionizes in solution:
HCO3- ↔ H+ + CO32The calcium ion (Ca2+) from the calcium chloride reacts with the carbonate ion (CO 32-) to
form insoluble calcium carbonate:
Ca2+ + CO32- ↔ CaCO3
The hydrogen ions react with the bicarbonate ion to yield carbon dioxide:
H+ + HCO3- ↔ H2O + CO2
The indicator changes colors because the carbon dioxide dissolves in water to produce an
acidic solution. Phenol red is red in basic solutions and yellow in acidic solutions.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-