In this lesson, students will determine the pH of several liquids with litmus paper or a pH probe. Next, students will explore how pH affects the production of gas with Pop Rocks. Students will also investigate how Charles’ Law affects Pop Rocks. Finally, students will design their own experiment with Pop Rocks.
By the end of this lesson, students should be able to
- Explain what affects the rate of gas production when Pop Rocks are dropped in different liquids.
- Identify variables in the given experiment.
- Give an example of Charles’ law.
- Design an experiment using Pop Rocks with detailed procedure and variable identified.
This lesson supports students’ understanding of the following topics in chemistry:
- Acids and bases
- Chemical change
Teacher Preparation: 35 minutes, depending on the experience of the class, more prep time might be required if students are not used to getting lab supplies
Lesson: 2 45-minute class periods, if students already have some background on pH
For each group:
- One package of Pop Rocks
- Red and blue litmus paper or universal indicator paper or pH probes (or use several of these) or create your own indicator with red cabbage (shred, put in a blender, cover with distilled water and blend. Stain out the cabbage leaves. No boiling needed)
- Safety goggles
- 3 different liquids (one acidic, one alkaline, and one neutral) already in beakers labeled with the name of the liquid if time is an issue (acidic solutions = soda pop, vinegar, lemon juice; alkaline solutions = liquid soap, baking soda solution, ammonia; neutral solutions = table salt solution, distilled water, rubbing alcohol). Soda pop will produce the most gas production, but not because it is acidic. Could use carbonated water or other types of drinks.
- Balance and weighing pan or filter paper (or a few stationed around the room)
- Students should wear safety goggles at all time.
- Students should not mix any of the liquids together without supervision.
- Read and follow all safety warnings on labels.
- At the end of the lesson, students should pour their used solutions in a waste container. Dispose of this waste down the drain or according to local regulations.
- If using bleach in the design section….temperature rises over 100 oF ….Bleach contains sodium hypochlorite (NaOCl) which is quite reactive with organic compounds (it oxidizes them), including sugar. That's why it's a good antiseptic and cleanser.
- Students should wash their hands thoroughly before leaving the lab.
- Charles’ law
- Chemical reaction
candy, Pop Rocks, pH, liquids, Charles’ Law, experimental design, chemical reaction
- Acid is a substance that tastes sour, reacts with metals and carbonates, and turns blue litmus paper red. Acids react with certain metals to produce hydrogen gas.
- Litmus is an example of an indicator, a compound that changes color when in contact with an acid or a base. Sometimes chemists use other indicators to test for acids and bases, but litmus is one of the easiest to use.
- Bases are another group of compounds that can be identified by their common properties. A base is a substance that tastes bitter, feels slippery, and turns red litmus paper blue. Bases are often described as the “opposites” of acids. Unlike acids, bases don’t react with metals or carbonates. Bases do react with acids.
- Acids and bases are found almost anywhere. Acids are found in many fruits and other foods. Many acids have formulas that begin with hydrogen.
- A hydrogen ion (H+) is an atom of hydrogen that has lost its electron. An acid is any substance that produces hydrogen ions (H+) in water. H+ cause the properties of acids.
- Many bases are made of positive ions combined with hydroxide ions. The hydroxide ion (OH-) is made of oxygen and hydrogen with a negative charge. When bases dissolve in water, the positive ions and hydroxide ions separate. A base is any substance that produces OH- in water. Hydroxide ions are responsible for the bitter taste and slippery feel of bases. Hydroxide ions also turn red litmus paper blue.
- Chemists use a numeric scale called pH to describe the concentration of H+ in a solution. The pH scale is a range of values from 0 to 14. A low pH tells you that the concentration of H+ is high. In contrast, a high pH tells you that the concentration of H+ is low. When the pH is low, the concentration of OH‑ is high.
- A solution with a pH lower than 7 is acidic. A solution with a pH higher than 7 is basic. If the pH is exactly 7, the solution is neutral.
- A reaction between an acid and a base is called neutralization. After neutralization, an acid-base mixture is not as acidic or basic as the individual starting solutions were.
- A salt is any ionic compound made from the neutralization of an acid with a base. A salt is made of the positive ion of a base and the negative ion of an acid. In a neutralization reaction an acid reacts with a base to produce a salt and water.
Notes for the lesson
- Charles’ law: Temperature and volume of a gas are directly related when at constant pressure
- If students have not been introduced to pH yet use (add another class period )
- great simulation to review pH
- investigate the strength of acids and bases with this simulation
- simple interactive pH panel identifies if acids and bases by clicking on the name of the substance
- use cafeteria trays for students to conduct work and this also an easy way for them to gather their supplies from the front of the room
- have students work in pairs and take turns recording information. As students come up to collect three different liquids, supervise that a base, a neutral, and an acid solution are chosen (without identifying them). You could have everyone in the class use the same three liquids or you could have more options available.
- allow students to sit at their desks and face forward to be seen by the teacher at all times
- differentiation: some students might benefit from more detailed directions when designing their own experiment and creating a data table for observation in the lab.
- simulation on Charles’ Law
- Three-minute video on Pop Rocks: how they are made and myths
For the Student
Ask students if they have ever eaten Pop Rocks. What is their favorite flavor? Give each pair of students a bag of candy and instruct them to pour a small amount into their hand. Ask them to observe the candy and describe what they see. What would happen if you put it in your mouth? (But don’t allow them to eat them.)
Have students do the following:
- Hand out Student Activity Sheet: Chemistry of Pop Rocks
- Discuss independent and dependent variables. Have students identify what the variables were in the engage activity with their partner.
- Gather materials.
- Predict which liquids are acids, bases or neutral and then take turns writing or recording on a computer, each pair designs a table to collect information on the different liquids.
- Test the three liquids to determine the pH and record the information (color change, actual pH, acids, base, or neutral). Test the liquids by dipping a piece of litmus or universal paper into the liquid and remove once the paper is wet. Blue litmus paper turns pink in acids while red litmus turns blue in bases. Students can test all three same liquids or each pair can choose from a number of liquids mentioned in the material list). Liquids with a pH less than 7 are acidic, while above 7 are considered basic or alkaline, if using universal indicator paper.
- Take turns writing or recording on a computer, pairs collect information on the different liquids.
- Using the digital balance, students mass 3 grams of Pop Rocks. This sample is placed in first liquid. Students observe the gas production.
- Repeat the previous step with the other two liquids.
- Record your observations (could identify the most gas production as #1, least as #3)
Which liquids are acids? bases? Have students signal with their fingers from a list on the board (water =1, milk =2, pop = 3, soap =4 ) [acids = pop, vinegar, lemon juice; bases = soap, milk, tap water, ammonia; neutral = distilled water, salt solution, rubbing alcohol]
Which liquid caused the most gas production? [pop]
Students share their drawing of a pH scale labeled with the liquids they tested and gas production findings with the class. Could they predict what might happen if another acid or base was used? The class discusses why these patterns developed and how Charles’ law is demonstrated in the Engage thought activity. Can you think of other examples of this law? [a hot air balloon rises when the temperature of the gases inside the balloon are heated. Or a inflated balloon is dropped into liquid nitrogen and the balloon shrinks] What is the key word and hint that you can use to remember this law [temperature and the unit used to measure this in 0C, which is the first letter in Charles]. Students could use technology to create short video with an iPad, Flip camera, i-Movie, or a voice-thread project.
Students look at the animations to understand on a molecular level was is occurring. Students design their own experiment (and if they need some guidance) to investigate how Pop Rocks candy will dissolve
Students will be able to answer questions related to variables, why understanding pH is important, and why this knowledge is important. Students could use the simulation
- What were your variables in your experiment? Why is it important to do numerous trials? to keep some variables constant?
- How did you know if a liquid was an acid? base? Could one indicator give you that answer [litmus paper may not identify correctly an acid or base ]
- Why is it important to know about acids and bases? [answers will vary, but farmers need to know the soil pH to grow crops; in swimming pools or aquariums the pH is important]
- Are foods acidic or basic? [some foods are acidic while others are basic]
- What are Pop Rocks? [From howstuffworks.com: Hard candy (like a lollypop or a Jolly Rancher) is made from sugar, corn syrup, water, and flavoring. You heat the ingredients together and boil the mixture to drive off all of the water. Then you let the temperature rise. What you are left with is a pure sugar syrup at about 300 oF (150 oC). When it cools, you have hard candy. To make Pop Rocks, the hot sugar mixture is allowed to mix with carbon dioxide gas at about 600 pounds per square inch (psi). The carbon dioxide gas forms tiny, 600-psi bubbles in the candy. Once it cools, you release the pressure and the candy shatters, but the pieces still contain the high-pressure bubbles (look at a piece with a magnifying glass to see the bubbles). When you put the candy in your mouth, it melts (just like hard candy) and releases the bubbles with a loud POP! What you are hearing and feeling is the 600-psi carbon dioxide gas being released from each bubble]
Multiple Choice Items
- Which one of the listed liquids is an acid?
- If the pH of a liquid is 10, what type of substance is it?
- When the pH of a solution becomes more acidic, the number on the pH scale…
c. Stays the same
- Give an example of Charles’ Law. [a hot air balloon expand when filled with heated air. a regular balloon placed in liquid nitrogen shrinks. ]
- Why did the Pop Rocks produce more gas in certain liquids? [Liquids that are carbonated already contain carbon dioxide. The level increases when Pop Rocks are dropped into the liquid. The pH doesn’t affect the reaction but the presence of carbon dioxide will.]
- Eva wrote a summary report on eight different compounds that she tested in the science lab. Use the results below to fill in the chart for each compound. For example, would compound F be definitely an acid or a base? [ A=acid or base, B = acid, C =base, D =acid or base, E = base, F= acid, G = acid, H = base]
Compound A is a solution conducts electricity. Compound B reacts strongly with copper – seeming to “eat it” away. Compound C in solutions feels slippery. Compound D reacts with an indicator to produce a change in its color. Compound E has a bitter taste. Compound F has a sour taste.
Compound G has the chemical formula H2SO4.Compound H has the chemical formula NaOH.
|Definitely an acid||Definitely a base||Possibly an acid or a base|
Other [Project-based; Performance task]
- students could collect the gas using a balloon over a flask and calculate the % produced when Pop Rocks are dropped in different liquids.
- students could look at the pH of other foods like baked cakes or crushed candies (sour vs sweet)
- watch the video on making hard candy and then design an experiment on candy
Connect to Math
Students use a number line to identify pH, could get into powers of ten, logarithms, what pH really means mathematically; Students could practice calculations with Charles’ Law. Give each student a substance and they create the pH scale as a human pH scale.
Connect to Reading
Students could read about how Pop Rocks are made to bring in the idea of different careers such as food scientist or food engineering. Develop a SQ3R or QAR worksheet .
Connect to Writing
Students can write a 5 sentence paragraph about their lab results or a letter to the pop rock company explaining their findings.
Connect to Social Studies
Students can learn about the history of Pop Rocks or Charles’ Law
This lesson supports the following:
Practices of Science and Engineering
- Asking questions and defining problems
- Developing and Using models
- Planning and carrying out investigations
- Analyzing and interpreting data
- Constructing explanations and designing solutions
- Engaging in argument from evidence
- Using mathematical and computational thinking
- Obtaining, evaluating, and communicating information
- Scale, Proportion, and Quantity
- Systems and System Models
- Structure and Function
- Stability and Change
Disciplinary Core Ideas, Grades 6-8
- Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals). (MS-PS1-1)
- Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it. (MS-PS1-2),(MS-PS1-3)
- Gases and liquids are made of molecules or inert atoms that are moving about relative to each other. (MS-PS1-4)
- In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations. (MS-PS1-4)
- The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter. (MS-PS1-4)
- Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants. (MS-PS1-2),(MS-PS1-3),(MS-PS1-5)
- Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants. The total number of each type of atom is conserved, and thus the mass does not change. Some chemical reactions release energy, others store energy. (MS-PS1-2)
- The more precisely a design task’s criteria and constraints can be defined, the more likely it is that the designed solution will be successful. Specification of constraints includes consideration of scientific principles and other relevant knowledge that are likely to limit possible solutions. (MS-ETS1-1)
- A solution needs to be tested, and then modified on the basis of the test results, in order to improve it. (MS-ETS1-4)
- Models of all kinds are important for testing solutions. (MS-ETS1-4)
- Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process—that is, some of those characteristics may be incorporated into the new design. (MS-ETS1-3)