Chemistry in a Ziploc Bag: Mini-lesson Vanderbilt Student Volunteers for Science Fall 2018

Why is the science in this lesson important? Observation is an important skill in many different careers, including medicine and forensics. Doctors have to take detailed observations on symptoms and illnesses in order to properly select medicine and treatments for patients. In forensics, scientists have to make detailed observations on evidence found at crime scenes in order to draw conclusions on cases. Scientists and researchers must design careful experiments to figure out why something that they observe happens

I. Introduction Learning Goals: To introduce students to the nature of scientific observations and experimentation Explain to students the importance of making careful observations and recording them. Write the following chemicals on the board: sodium bicarbonate NaHCO3, calcium chloride CaCl2, water H2O, phenol red Tell students this lesson involves a chemical reaction that produces a gas. Warn them that the ziploc bag is going to fill up with air. Assure students that the chemicals are safe, but they should not do anything with them that they are not instructed to do. For this experiment, students work in groups of 3 or 4

I. Introduction - Preparation Count students. Prepare enough cups for students to do the experiment in pairs. Pour 15 mL phenol red into 1oz cups (to the marked line). Explain to students that scientists are always very careful how they handle chemicals. Some liquids might look like juices, and some white powders might have the same formula as those used in the kitchen. But if they are being used in a science experiment, they are ALL treated as CHEMICALS, and are kept well away from mouths and eyes.

II. Making Observations When Calcium Chloride Is Added To Baking Soda/Phenol Red Solution Learning Goals: To give students the opportunity to record color and temperature-based observations during chemical reactions Tell students write down their observations. Give each pair one ziploc bag containing baking soda, one 1oz cup containing 15 mL of phenol red solution, and one plate. Ask students: How can you tell when a chemical change has occurred?

IIA. Experiment Tell students to: Hold the bag upright over the plate. Open the bag and add a teaspoon of calcium chloride. Add the phenol red solution (includes water) to the bag and seal the bag. Gently shake the contents of the bag while holding the bag over the plate. Ask students what they observe. Record students observations about color changes, temperature changes (is the bag cold or warm or both since there can be localized heating), changes in bag size, and gas given off or foaming.

II. Possible Student Observations Learning Goals: To provide students with information that can help them identify key indicators of the occurrence of chemical reactions. (1) When phenol red solution is added to the baking soda bag, it remains red. (2) The bag feels cold (due to the baking soda dissolving). (3) When the calcium chloride is added, the phenol red turns yellow. (Phenol red changes to yellow around pH 6.8.) Students may feel a short-lasting warming while the anhydrous calcium chloride dissolves in water. (4) The bag fills up with gas and continues to feel cold. Ask students: What evidence for chemical changes did they observe in today’s experiment? – see lesson

II. Experiment B – Brainstorming To Determine the Roles of the Different Reagents in the Reaction . Learning Goals: To have students use the scientific method to design an experiment. Write the overall equation on the board: 2NaHCO3 (aq) + CaCl2(aq) -----> CaCO3 (s) + 2NaCl (s) + H2O(l) + CO2 (g) For this section, students will brainstorm how to determine the role of each of the chemicals Tell students that they will determine which combination of chemicals produced the following observations: The bag feels cold(er). The bag feels warm(er). The mixture fizzes, and the bag fills up with gas. The mixture turns from red to yellow.

II. Experiment B – Brainstorming To Determine the Roles of the Different Reagents in the Reaction . Take the students through the steps of a systematically designed procedure to test each observation by taking one reagent (called the CONSTANT) and adding another reagent (called a VARIABLE), one at a time. Tell students that scientists design experiments that change only one variable at a time.  Make a list on the board (see lesson).

Experiments 1 & 2 NaHCO₃ H₂O Phenol red Experiment 1 Experiment 2

Experiments 3 & 4 CaCl₂ H₂O Phenol red Experiment 3 Experiment 4

Experiments 5, 6, & 7 NaHCO₃ CaCl₂ H₂O Phenol red Experiment 5

IIC. Experiment: Students Make Observations When Controlling the Number of Reagents. Tell students to follow instructions and to write their observations for each experiment. At the end of each experiment, ask students what they observed. Write the results on the board. Tell the class the reasons for the observations (see chart below) and write reasons on the board for students to copy. Repeat for Experiment 2 in the same test tube. Repeat for Experiment 3 and 4 in the second test tube Repeat for Experiment 5, 6 and 7 in the third test tube

IIC. Experiment: Students Make Observations When Controlling the Number of Reagents. Ask students: What chemicals are responsible for the following – see lesson The bag feels cold(er)? The bag feels warm(er)? The mixture fizzes, and the bag fills up with gas? The mixture turns from red to yellow?

IV. VSVS BACKGROUND INFORMATION ON CHEMICAL EQUATIONS This information is beyond the scope of 7th graders, and is for VSVSer only. The equations for the reactions that occur when anhydrous calcium chloride is added to the sodium bicarbonate solution are given in the manual.

Clean Up In the event one bag leaks or explodes, use paper towels to clean up any mess. The VSVS team should collect all Ziploc bags and used cups, and put them in the trash bag. Make sure the Ziploc bags with the reaction mixture is sealed before you put them in the trash bag. Return trash bag to the VSVS lab. Put everything else in the kit box along with the trash bag and return it to the VSVS lab.