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Activity: Density Bags

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

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Workshop: Density Bags

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Teacher Guide

M1U2-Density Bags Teacher Guide

Materials for Parts A-C

  • Small plastic bags
  • Scissors
  • Permanent marker
  • Labeling tape
  • Fresh water
  • Four beakers (e.g., 1,000 mL) or large containers, wide enough for plastic bags to float unobstructed
  • Small cup
  • Tray
  • Tape
  • Tables 2.1, 2.2, and 2.3
  • Towels
  • Food coloring (optional)

Additional Materials for Parts A and C

  • Salt water

 

Additional Materials for Parts B and C

  • Heat source
  • Ice bath (container in a tray of ice cubes and water, see Fig. 2.4)
  • Thermometer
  • Tongs

 

Procedure

Safety Note: When preparing hot water and handling hot containers, use tongs or heat-resistant gloves or gripping devices. Tongs can also be used to move small plastic bags into and out of containers.
 

A. Test the effect of salinity on the rising or sinking of bags of liquid.

  1. For each combination of fresh water and salt water, make a prediction as to whether the bag will
    1. sink (the bag and liquid are more dense than the liquid in the beaker)
    2. float (the bag and liquid are less dense than the liquid in the beaker)
    3. subsurface float (the bag and liquid are the same density as the liquid in the beaker).

Record your predictions in Table 2.1.

  1. Fill one 1,000 milliliter (mL) beaker with salt water and one 1,000 mL beaker with fresh water. Label your beakers.
     
  2. Observe as your teacher shows you how to fill the plastic bags for your experiment.
    1. Use a permanent marker to label an empty small plastic bag with the salinity of the water (fresh or salt) that will be poured into it.
    2. (Optional) Add a drop of food coloring to the empty plastic bag to make the liquid in the bag easier to observe.
    3. Fill the small cup with the same type of water that will be poured into the bag.
    4. Overfill the bag with water using the small cup. Do this step over a tray, spare beaker or towel.
    5. Seal the bag so it does not leak or have air bubbles inside. Do this step over a tray, if done correctly liquid will spill outof the overfilled bag.
    6. Shake the bag to distribute the food coloring.
    7. Pat the plastic bag dry.
    8. Cut off the excess plastic of the bag above the closure.
    9. If at any point in this procedure the bag starts leaking, seal the leak with tape.
       
  3. Make a bag of fresh water and a bag of salt water.
     
  4. Place one bag into one beaker and record your observations in Table 2.1. Repeat this step until you have tested and recorded your observations for each combination of bag and beaker. Make sure to pat the bags dry before placing them in the beakers or when transferring them between beakers.
     
  5. Compare your results with those of your classmates.

 

B. Test the effect of temperature on the rising or sinking of bags of liquid.

  1. For each combination of fresh hot water and fresh cold water, make a prediction as to whether the bag will sink, float, or subsurface float. Record your predictions in Table 2.2.
     
  2. Prepare hot water and cold water in 1000mL beakers. Make sure to use safety equipment and be careful with the hot water. Label your beakers.
    1. To prepare hot water, place a beaker of fresh water on a hot plate (Fig. 2.4 A), and heat to 50–70 degrees Celsius (˚C).
    2. To prepare cold water, place a beaker of fresh water in an ice bath until the water in the beaker reaches a temperature of -5˚C. Do not put ice cubes in the beaker (Fig. 2.4 B).
    3. The beakers can stay on the hot plate or in the ice bath during the experiment.

 

<p><strong>Fig. 2.4</strong>. (<strong>A</strong>) Place a beaker of water on a hot plate to create hot water.(<strong>B</strong>) Place water and ice cubes in a tray to create an ice bath. Place a beaker of water in the ice bath to create cold water.</p>

  1. Make a bag of hot water and a bag of cold water following the steps described in procedure 3 of Part A. Place each bag into each beaker and record your observations in Table 2.2. Repeat this step until you have tested and recorded your observations for each combination of bag and beaker.
    1. Pat the bags dry before placing them in the beakers or when transferring them between beakers.
    2. If the hot water is uncomfortably hot, use tongs to transfer the bags into and out of the water.
       
  2. Compare your results with those of your classmates.

 

C. Test the effects of both salinity and temperature on the rising or sinking of bags of liquid.

  1. Examine Table 2.3.
    1. Fill in the boxes that you already completed in Part B (Boxes 6, 8, 14 and 16)
    2. Based on your results in Part A (salinity), make predictions as to whether the bags in boxes 2, 5, 12, and 15 will sink, float, or subsurface float. Fill in your predictions in Table 2.3.
    3. Based on your results in Part B (temperature), make predictions as to whether the bags in boxes 3 and 9 will sink, float, or subsurface float. Fill in your predictions in Table 2.3.
    4. Based on your knowledge of how salinity and temperature affect density, predict whether the bags in boxes 1, 4, 7, 10, 11, and 13 will sink, float, or subsurface float. Fill in your predictions in Table 2.3.
       
  2. Examine your predictions in Table 2.3.
    1. Star (*) the predictions you are confident about.
    2. Circle the predictions you are unsure about. Write down why you are unsure.
    3. Discuss with your classmates which predictions they are confident about and which predictions they are unsure about.
    4. Decide which predictions you will test.
       
  3. Prepare the beakers and bags you need to carry out your experiment. Record your observations.
     
  4. Compare your results with those of your classmates.
Activity Questions: 
  1. How did your answers compare to your predictions?
     
  2. Explain why your classmate’s results were the same or why they were different for
    1. Part A (salinity)
    2. Part B (temperature)
    3. Part C (salinity and temperature).
       
  3. In Part C, which variable, temperature or salinity, tended to determine the behavior of the bag? Explain why you think this variable determined the results of your experiment.
     
  4. Using the term density, explain how a bag of salt water can sink in a beaker of salt water. Where might this occur in the ocean?
     
  5. If the temperature of the liquid in a bag and the liquid in a beaker were the same, under what conditions would the bag float? Where might this occur in nature?
     
  6. Would fresh water flowing into the ocean sink or float on top of seawater? Explain your reasoning.
     
  7. How might you use what you have learned in this activity to help explain the formation of layers in large bodies of water like lakes or the ocean?
     
  8. What do you think might happen to the liquid in the bags if the bags were not sealed? How could you test this?
     
  9. If a sealed bag of hot salty liquid is placed into a beaker of cold fresh water for 24 hours, what will happen to
    1. the temperature of the two liquids
    2. the salinity of the two liquids
    3. the density of the two liquids.
       
  10. If you used the food coloring, do you think it affected the density of the liquids in the bags? How could you verify your answer?

Table of Contents:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.