Match the Mystery Solutions
Materials Included In Kit
Aluminum potassium sulfate (alum) solution, AlK(SO4)2•12H2O, 1%, 100 mL
Citric acid, C6H8O7•H2O, 2 g
Sodium bicarbonate (baking soda) solution, NaHCO3, 0.5 M, 100 mL
Acetate sheets, 5½" x 8½", 2
Pipets, Beral-type, thin-stem, 90
Pipet holders, 15
Additional Materials Required
Water, distilled or deionized (DI), 100 mL†
Graduated cylinder, 100-mL†
Notebook paper, 1 sheet per student*
Paper towel, 1 per student*
*for each lab group
†for Prelab Preparation
Part 1. Solution Preparation
- Use 100 mL of DI water to prepare the 2% citric acid solution. Add 100 mL of DI water to the 2 g of citric acid powder in the bottle and mix.
Part 2. Preparing the Pipets
- Cut off all but 1 cm of the stem from each thin-stem pipet, enough for three shortened pipets for each student (see Figure 1).
- Using a permanent marker, label 15 of the thin-stem pipet bulbs “A,” 15 of them “B” and 15 of them “C.” Note: To avoid students copying from other groups, you may letter each set of three pipets differently (e.g., D, E, F).
- Using a permanent marker, label 15 of the thin-stem pipet bulbs “1,” 15 of them “2” and 15 of them “3.” Note: To avoid students copying from other groups, you may letter each set of three pipets differently (e.g., 3, 4, 5).
- Take one pipet labeled “A” (or its equivalent), draw up half a bulb-full of the alum solution and place the pipet in the pipet holder. Repeat with the other “A” pipets. Note: By pouring each solution into a labeled wide-mouth cup, this step may be easily done with two pipets at a time.
- Repeat step 4 for the pipets labeled “2” or its equivalent. Place these pipets in the pipet holder (see Figure 2).
- Take one pipet labeled “B” (or its equivalent), draw up half a bulb-full of the baking soda solution and place the pipet in the pipet holder. Repeat with the other “B” pipets.
- Repeat step 6 for the pipets labeled “3” or its equivalent. Place these pipets in the pipet holder (see Figure 2).
- Take one pipet labeled “C” (or its equivalent), draw up half a bulb-full of the citric acid solution and place the pipet in the pipet holder. Repeat with the other “C” pipets.
- Repeat step 8 for the pipets labeled “1” or its equivalent. Place these pipets in the pipet holder (see Figure 2).
Part 3. Preparing the Acetate “Spot Plates”
- Cut the acetate sheet into 3 cm x 5 cm pieces.
- Using a permanent marker, place a dark dot in opposite corners of each small acetate sheet. Note: The dark dots make it easy to find the acetate sheets should they fall onto the floor as well as help the instructor confirm the sheets were returned to the pipet holders at the end of the lab.
- Slide two acetate spot plates into each of the pipet holders, behind the pipets (see Figure 2).
- Close each pipet holder and store with pipet tips up.
Citric acid is a severe eye irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Do not taste, touch or smell any solutions or chemicals used in the lab. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
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. Solutions on the acetate sheets may be wiped up with a paper towel and thrown away in the regular trash. All leftover solutions may be rinsed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.
- Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the questions may be completed the day after the lab.
- Students may want to taste, smell or feel the solutions for identification. Remind students that such practice is not allowed in the laboratory.
- While discussing their ideas for solving the mystery, invariably a student will offer the approach of mixing the solutions. Discuss the merits of this approach, and show the students how to use the cut-off pipets to squeeze out drops of solutions and mix them on the acetate “spot plate,” and how to avoid contamination. Stirring the drops together will not be necessary— the reactions take place immediately and the results are obvious.
- Additional pipet holders, Catalog No. AP1519, are available from Flinn Scientific.
- Extra-large bulb pipets (available form Flinn Scientific, Catalog No. AP1445) are great for holding thin-stem pipets in place in the cassette boxes. Cut the extra-large bulbs in half, discarding the part attached to the stem. Place six of the halfbulb “cups” in the cassette box. Insert one filled pipet into each cup.
- This is a great lab for the first day of class. The purpose of the “no visual comparison” restriction is to prevent one partner from monopolizing the lab. When conducted as written, 100% participation is guaranteed, and good inquiry and communication skills are emphasized.
- If sitting back-to-back is awkward for the students, a barrier such as a notebook or pocket folder may be placed between their work areas.
- Even though understanding the chemistry involved is not the focus of this activity, students may be curious as to what took place. The reaction of citric acid and sodium bicarbonate produces sodium citrate and carbonic acid (Equation 1).
The carbonic acid then decomposes into water and carbon dioxide (Equation 2). The carbon dioxide forms the bubbles that students observe.
Sodium bicarbonate is the salt of the weak acid, carbonic acid, H2CO3. When dissolved in water, sodium bicarbonate, NaHCO3, forms a slightly basic solution (Equation 3).
Alum, AlK(SO4)2, when dissolved in water, forms the cations Al3+(aq) and K+(aq) in solution, along with the sulfate anion, SO42–(aq). When the alum and baking soda solutions are combined, a white precipitate of aluminum hydroxide is formed (Equation 4).
None of the products of the citric acid and alum reaction are insoluble in water; therefore no change is observed.
- A video of this lab activity, Mystery Solutions, presented by Bob Becker, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. Please visit the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Scientific Methods Inquiry Labs video package.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Planning and carrying out investigations
Analyzing and interpreting data
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Answers to Prelab Questions
- Which of the following is NOT a characteristic of the scientific method: (a) logic, (b) creativity, (c) bias, (d) evidence? Explain your answer.
The correct answer is (c) bias. Bias can enter scientific reasoning in subtle ways. A scientist may ignore an observation or measurement that seems “out of line.” Another scientist may propose a “cause and effect” relationship without doing proper control experiments to see if the same effect is observed in the absence of the proposed cause. Note: Some students may challenge the importance of creativity in the scientific process. The best science is usually the most creative as well. It takes creativity to propose hypotheses to explain how or why something works and then to design experiments that truly test these explanations.
- What safety precautions should be taken during this laboratory activity? Explain why.
Since all the solutions in this activity are unknown and one of the solutions is a severe eye irritant, it is important to wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. The sense of taste, touch or smell should not be used to identify any solutions or chemicals in the lab. Hands must be washed thoroughly with soap and water before leaving the laboratory. All laboratory safety guidelines must be followed.
Mixtures of solutions were systematically tested pairwise within each set of three solutions. Results of each mixture were recorded and compared.
A + B = cloudy white
A + C = no visible reaction
B + C = bubbles
1 + 2 = no visible reaction
1 + 3 = bubbles
2 + 3 = cloudy white
Answers to Questions
- Write your matches in the box below and describe the reasoning that led to your conclusion.
Solution B was the only solution involved in both the cloudy white and the bubbling reactions. Likewise for solution 3, therefore solution B must be the same as solution 3. C was the other solution involved in making bubbles, as was solution 1. Therefore solution C must be the same as solution 1. By process of elimination, solution A must be the same as solution 2 (both solutions A and 2 were involved in the cloudy white precipitate reaction and the combination that produced no reaction).
- What steps of the scientific method listed in the Background section were used in solving the mystery of the unknown solutions?
Define a problem, make observations, design and implement an experiment, record and analyze data, and draw a conclusion were steps included in this investigation.
- Forming a hypothesis is often considered an essential step of the scientific method. Why was it not appropriate to develop a hypothesis in this experiment?
No hypothesis was developed since the solutions were unknown. Any possible answer prior to testing would be simply a guess, not based on facts or observations.
- “Don’t mix chemicals unless instructed to do so” is a good general safety rule—unpredictable reactions may take place. The following are some common “chemicals” found in most homes. Identify 1–5 based on the following data. Note: See student worksheet for data tables.
- Vitamin C 3 = A. Vitamin C
- Baking soda 5 = B. Baking soda
- Washing soda 1 = C. Washing soda
- Epsom salts 2 = D. Epsom salts
- De-icing salt 4 = E. De-icing salt
Special thanks to Bob Becker, Kirkwood High School, Kirkwood, MO, for providing the idea and the instructions for this activity to Flinn Scientific.