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Determining the Composition of Bridge Straw Stalactites (0 Favorites)

LAB in Observations, Polyatomic Ions, Physical Properties, Solubility, Chemical Change, Covalent Bonding, Ionic Bonding, Identifying an Unknown, Chemical Properties, Experimental Design. Last updated October 11, 2021.


Summary

In this lab, students will investigate “straws” that hang from a local bridge, and then determine various tests that can help to determine their chemical composition. Evaluating both the test results, as well as given information students will then make a claim about the composition, while providing evidence and supporting it with reasoning.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
  • HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
  • Scientific and Engineering Practices:
    • Asking Questions and Defining Problems
    • Analyzing and Interpreting Data
    • Planning and Carrying Out Investigations
    • Engaging in Argument from Evidence
    • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lab, students should be able to:

  • Make observations and design experiments to determine physical and chemical properties of a substance.
  • Use a solubility table to determine to what extent any given substance will dissolve in water.
  • Use properties of a substance to determine its identity.
  • Analyze and interpret data to make a claim using CER (claim-evidence-reasoning) format.

Chemistry Topics

This lab supports students’ understanding of:

  • Identifying an Unknown
  • Chemical Properties
  • Physical Properties
  • Chemical Reactions
  • Chemical Change
  • Ionic Bonding
  • Covalent Bonding
  • Polyatomic Ions
  • Polarity
  • Solubility

Time

Teacher Preparation: 30-45 minutes
Lesson: Two 60-minute class periods and one 60-80-minute lab period (several modifications offered below)

Materials

  • Bridge Straw samples (crushed/powdered calcium carbonate can be substituted, but don’t tell the students)
  • Conductivity Meters
  • Variety of polar and nonpolar solvents (ex: mineral oil, vegetable oil, ethanol, water)
  • Variety of acids and bases (ex: vinegar, milk, 0.1M HCl, 0.1M NaOH)
  • Thermometer
  • Hotplate
  • Foil/metal boats (for melting substances on hot plate)
  • Flame (ex: candles or Bunsen burners)
  • Magnets
  • Beakers (variety of sizes)
  • Glass Stirring Rod
  • Test Tubes
  • Limewater (for gas testing)
  • Wooden Splints (for gas testing)
  • Magnifying Glass (optional)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Depending on the experiment selected/determined by students decide to do, the following may be relevant:
    • Always use caution around open flames. Keep flames away from flammable substances.
    • Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
    • Open flames can cause burns. Liquid wax is hot and can burn the skin.
    • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
    • When lighting a match and wooden splint, be cautious with the flame.
    • An operational fire extinguisher should be in the classroom.
    • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • Refer to the Safety Data Sheets (listed in materials section) for any substances used.

Teacher Notes

Overview:

  • If you are able to complete this lab over multiple class periods, I suggest the following:
    • Day One: Introduce the phenomenon, ask questions, plan experiments.
    • Day Two: Student experiments (or teacher-led demos).
    • Day Three: Analysis of class data.
  • You can find some helpful background information about straw stalactites growing from concrete structures in this 2017 article by Garry K. Smith, as well as on Wikipedia.
  • Based on the ability of your students, this lab can be completed differently, or even as teacher-led demonstrations. Some suggestions are included below.
  • I suggest that students work in small groups for this lab.
  • An Answer Key document is available for teacher reference.

Day One:

  • First I show photos of the bridge straw stalactites phenomenon. Both images below are found in the Public Domain via Wikimedia Commons.
Images from: Chemicalinterest, Public domain, via Wikimedia Commons
  • I also show this short video I made while kayaking under a local bridge (~1 minute).
  • I then wonder aloud about what these substances are and how they are made. I engage students in a conversation about how to figure out their composition in the laboratory. Once they identify that determining properties is important, I start compiling a list of questions based on their feedback that may lead them to answer the question. (A shared Google Doc works well for this especially if students use electronic lab books.) I encourage both physical and chemical observations as well as questions such as, “what time of the year do you see them?” (typically, spring, summer and fall based on geographical location).
  • This could be done as a class discussion, in partners or in lab groups and then shared out as a class. There is space provided for this task on the Student Handout document.
  • When the list of questions is done, have lab groups develop procedures to answer their questions. Alternatively, you could provide procedures for the particular tests that students identify as being useful to identify the composition of the bridge straws. You may need to divvy up the questions depending on how much time you want students to spend in the lab.

Day Two (Lab):

  • Be sure to review all student-generated experiments and then review safety precautions with lab groups and the class.
  • Students should perform their respective lab experiments. It is up to the teacher to decide which tests to allow students to do. This can also be accomplished with a combination of student experimentation and teacher-led demonstrations. For example, maybe students test solubility and the teacher does the melting point as a demonstration. Alternatively, different lab groups could do different tests such as one group does solubility and another group does physical observations while a third group does melting on a hot plate, and the results are shared.
    • Warning: Remember not to have open flames near flammable substances.
  • It helps if students put their lab results on a shared class document or they could present them to the class the next day.
  • Tests that are encouraged/results that are beneficial to determining the composition of the bridge straws are listed in the data table below and shown in the video below.
  • In the lab students may find it difficult to determine if the substance dissolves. When they stir, it seems to disappear, but that’s not accurate. I recommend doing a solubility test in a test tube, shaking, and then waiting for it to settle out.
  • For a melting point test, you may need to use an infrared thermometer to determine the temperature of a hot plate. A Bunsen burner flame can reach anywhere from 1000°-1560°C. This should be high enough to melt the calcium carbonate (MP = 825°C), however this was not attempted in the classroom.
  • This lab was originally designed during the pandemic and so based on the investigative questions developed by the students, the teacher conducted a series of experiments at home, and created a video (~4 minutes, 30 seconds) for students to watch and record their data in a provided data table (see below). This option could be completed if there is a need for virtual learning, or if materials are unavailable. In this can the data table below can be copied to the student handout and completed while watching the video:
Property
Observation
Color
Texture
Brittle vs. malleable
Electrical conductivity
Reaction to freezing temperature
Solubility/reaction in water
Solubility/reaction in oil
Solubility/reaction in bleach
Temperature
Melting point (you can’t give me a specific melting point, but what do we know based on the video?)
Flammability (does it burn?)
Magnetic or not
Odor
Solubility/reaction in vinegar

Day Three:

  • It’s helpful if the results of each test are shared with the whole class (by lab groups or on a shared document).
  • Students can work independently or in their lab groups to complete part 2 of the lab. Using the experimental results and the series of guiding questions, students should be able to determine the composition of the bridge straws.
  • The chemistry of the bridge straws is more complex than I cover in my chemistry class. Teachers can reference some helpful background information about straw stalactites growing from concrete structures in this 2017 article by Garry K. Smith, as well as on Wikipedia.

For the Student

Lesson

Part 1: Lab Testing

Brainstorming

List any questions that you have about the bridge straw phenomenon below. These questions will help to guide you in designing lab tests.

Procedures

In the space below, design specific lab procedures that would help you to determine what the bridge straws are made of. Some could be as simple as observations, but others may involve several steps.

*Your teacher must approve all procedures before you can conduct them in the lab.

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Always use caution around open flames. Keep flames away from flammable substances.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
  • Open flames can cause burns. Liquid wax is hot and can burn the skin.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
  • When lighting the match and wooden splint, be cautious with the flame.

Lab Data

Create a data table to organize the results of the procedures you have designed.

Part 2: Analysis

  1. What is a bridge made of?! The answer to that will give a hint about what might be dissolved in the water that is dripping from the bridge and making the straws.

    Use the resource https://www.cement.org/cement-concrete-applications/cement-and-concrete-basics-faqs to answer the following:

    Explain the difference between concrete and cement:
  2. What metal ions can be found in cement? Look at the following compounds that make up cement and then list the metal ions you see in these formulas. You should find three.

3CaO·SiO₂
Ca₃Al₂O₆
Ca2(Al,Fe)2O5

  1. Recall the difference between ionic and covalent substances. You may have done a lab where you dissolved substances in water and oil. What are the two main elements that are common in nonpolar substances that dissolve in oil?
  2. Based on the solubility test results and question #2, is the straw made of an ionic or a covalent substance? Explain your reasoning.
  3. Why doesn’t it dissolve in water? (Think about a solubility chart.)
  4. The next informative test was that the straw produced bubbles when reacted with vinegar. Substances that bubble in vinegar are usually producing carbon dioxide. What polyatomic ion could break down to produce carbon dioxide?
  5. Now you have three possible cations (from #2) and the one, most probable anion (from #5). Combine the possible the cations with the polyatomic ion to make 3 different compounds.
  6. Using your solubility chart look up the solubility of the three compounds formed in #7.  Only two of the possible compounds have solubilities on the chart (the other doesn’t normally form). What are they?
  7. Only one of those is the same color as our straw. Which one is it?
  8. Finally, put everything together and determine the composition of the straws using CER (claim-evidence-reasoning) format.