Layering Liquids Teacher s Guide A Hudson River Plume Activity

Transcription

1 Layering Liquids Teacher s Guide A Hudson River Plume Activity This activity challenges students to layer 4 different colored liquids (of different densities) in a clear straw. Through trial and error, students keep track of their successes and make predictions as to the correct order. Most teams come up with the correct layering after about 15 minutes. Then, through a teacher-facilitated class discussion of what they have observed and the Density Jar demonstration, students develop a better understanding of density. Objective: The objective of this activity is for the students to learn that liquids have a certain property (density) that causes some liquids to float on top of others, and to express their understanding of this property in their own words. NJ State Standard: 8 Structure and Behavior of Matter. Time Required Preparation: approximately 25 minutes (10 to cut potatoes, 15 to make solutions) Activity: 20 min for activity, 5 min cleanup, min discussion/demo This activity was adapted from a GEMS Discovering Density, by Lawrence Hall of Science, University of California Berkeley. Classroom Setup Each student will have their own potato base and about 6 clear straws. In groups of 4, students will share access to the liquids in the middle of the table. Each table will have one small beaker (or cup) of each liquid with 4 pipettes in each. Materials Sometimes it is difficult to find clear plastic straws. It is important to make sure the straws are NOT the super-thin straws with a small diameter ~ the glycerin cannot flow down them. Often fast-food restaurants use clear straws that are wide enough. Or you can try restaurant supply stores or party suppliers. Costco sells a large box of clear straws that are the correct diameter (5-6 mm diameter). Glycerin and isopropyl alcohol are available in drug stores. Kosher or pickling salt is available in supermarkets. These are pure salts with no additives, so they dissolve easily and make clear salt water. Table salt can be substituted, but it makes a cloudy mixture. For the Class: 1 sharp knife 4 food colors (red, green, yellow, blue) One potato for every 2-3 students 5 tablespoons (about 50 g) of kosher or pickling salt 32 oz. of tap water 16 oz. glycerin 16 oz. isopropyl or denatured alcohol 1 tablespoon measurer 1 measuring cup

2 For Density Jar Demo during class discussion: one large beaker (1000 ml) enough marbles to fill the beaker enough salt to fill the spaces between the marbles (400 ml) a medium beaker of water (300 ml) For each group of 4 students: 4 small beakers or wide-mouthed, clear plastic cups (9 oz.) 16 pipettes (4 per beaker) or only 8 pipettes (2 per beaker) For each student: 3 full length clear plastic straws (which will be cut in half making 6 per student) Liquid Layer Data Sheet Liquid Layering Student Sheet (has a place for them to take notes during the class discussion at the end of the activity) Preparation In Advance: Cut the clear straws in half with scissors or a paper cutter. You can cut straws at a time if you bundle them with a rubber band. (Leave on the paper wrappers to prevent slipping. Remove the wrappers before giving them to students). Before Class: Slice the potatoes and prepare the layering liquids. Use a sharp knife to slice the potatoes. Most potatoes will give you 2 to 3 bases. Place them in a large container, covered with water, to keep them from turning brown. Prepare the 4 stock colored mystery liquids, and then fill 4 plastic cups or small beakers about ¼ full with a different liquid in each cup for each group. Glycerin add about 25 drops of blue food coloring to 16 oz. of glycerin and shake well in a container. (If you purchase the 16 oz. size container, you can add the color directly). Test the consistency to make sure it flows to the bottom of the straw. If it is too thick, add a little water to thin it a bit. It should be similar to syrup. Salt water add 5 level tablespoons (about 50 g) of kosher salt to 16 oz. of tap water add drops of green food coloring and shake until all the salt dissolves. Water add drops of yellow food coloring to 16 oz. (about ½ liter) of tap water and shake. Alcohol add drops of red food coloring to 16 oz. of isopropyl or denatured alcohol in a bottle and shake well. Set up a central location along with one set of materials for you to demonstrate the layering technique to the class. Teacher Notes Introducing the Challenge:(It is recommended to hand out materials to the students only after you have completed demonstrating the liquid layering technique.) The Layering Liquids PowerPoint can be used during the demo and discussion portions of this activity. plume/activity_layeringliquids.ppt 1. Tell the students that today they will attempt to layer 4 unknown liquids in the same straw so that none of the colored liquids mix or blend (run through another).

3 Caution them to NOT taste any of the unknown liquids. 2. Demo as you explain: Insert the straw into the potato at a 45-degree angle. Explain that the reason the straw is placed at this angle is so the liquid can run down the inside of the straw. If the straw is not pushed into the potato far enough or all the way through, the liquids will leak out. Food coloring stains, so it is important to set the straw correctly. 3. Demo as you explain: Remind students of proper pipette technique: Squeeze the bulb and insert the tip into the liquid Release the bulb so the liquid is drawn up into the pipette Carefully squeeze the bulb so that liquid slowly flows into the straw one drop at a time (shooting the liquid forcefully will ruin the results). Remind students to replace pipettes in the proper liquid container so as to NOT contaminate the liquids. Students will use the Liquid Layering Data Sheet to write their predictions of how to layer and record their successes. Students will insert a new straw into their potato for each new layering experiment. Remind students not yell out the correct sequence. Experimenting: Students attempt to layer the liquids. As you circulate, students may complain that the blue liquid (glycerin) is too thick and won t move down the straw. Tell them it will move down the straw, just slowly. It helps to add only one drop at a time and not force the blue liquid into the straw. As students finish, have them repeat their experiment, carefully layering the liquids in the expected order to see if they can get very clear separations between the layers. Clean Up: Come around with a large container (or garbage can) and have students carefully place their entire potato base with straws into the container. Or you can have a container at each table for students to place their potato bases into and have one student return it to the lab table or dump into the trash. Oftentimes, students don t realize that dropping the potato base into the garbage can creates backsplash forcing the food coloring to spray out of the straws and stain their clothing. Discussing the Results: Draw several large columns on the board like those on the data sheets. Ask the students to report how they ordered the layers, starting with the bottom layer. Sometimes, one or two students will find another order that worked. If so, list those results as well. Ask the students if they can guess what some of the liquids are. Reveal the identity of the liquids to the students. Write the names of the liquids alongside one of the columns on the board. As the students why they think the liquids layered the way they did. Be careful of the words heavy or light use their words for now. Some students may mention that some of the liquids were thicker. Others might mention differences in density. If they do use density ask them to explain what

4 they mean by it. Encourage alternative explanations and argumentation about the differences between the liquids are and why they layer that way. Remember to refrain from defining density yourself at this time ~ probably the most difficult part in all of this! If teams have different results, ask the students why they think different teams came up with different ways to layer the liquids. Demonstration and Discussion To help present the content in this demonstration, you can use the Jar Full of Marbles PowerPoint presentation. s/plume/docs/demo_densityjar.ppt Note: This works best if you have your supplies out of their direct view. Hold up a large beaker filled to the top with marbles. Say: This beaker is filled with marbles. Can I fit anymore into this space? Take responses from students. The way you have worded the question will lead them to think of just more marbles ~ see if anyone thinks about the spaces between the marbles. Now hold up a beaker about ¾ full of salt. Say: Will this fit in the beaker? Pour the salt into the marbles. There is now more stuff in the same space. Say: Now there is more stuff in here than before. Can I add anything more to this space? If no one comes up with it, pull out the beaker about ½ filled with water and add it to the beaker of marbles and salt. Say: This is the same amount of space as when I started with just the marbles, but now there is a LOT more stuff in here. Scientists have a word to describe how much stuff is in a certain amount of space ~ density. Write on the board to allow students to copy into their notes. density = stuff space Say: These terms don t seem scientific. In math, what is space called? See if any of the students can answer, Volume. Extend the equation out and write volume across the bottom. So now it looks like this: density = stuff = space volume Say: What do we call stuff in science? Hint: it is what we measure with the triple beam balance. See if any of the students can answer, Matter which has mass Now add mass across the top, so that you now have: density = stuff = mass space volume Encourage students to write this in their notes.

5 Glycerin was the most dense and always ran to the bottom of the straw. Salt water is more dense that fresh water, because in the same volume (amount) there are dissolved salts and other substances along with the water molecules. This means there is more mass in the same volume ~ making salt water more dense. We see that in this activity because the fresh water floated on top of the more dense salt water. Isopropyl alcohol is the least dense and floats on top of all the other liquids in the straw.

6 Name: Class: Layering Liquids Student Worksheet In this activity you are being asked to layer 4 unknown liquids, each a different color, in one clear straw without any liquid mixing (running through) another. Be careful to replace the pipettes back into the correct container. If you don t, you may contaminate the liquids and ruin the outcome of the activity. Observations What do you notice about each liquid, besides the different color? Red: Yellow: Blue: Green: Layering the Liquids Use the Liquid Layers Data Sheet to make predictions as to the correct order and keep track of your successes. Remember to push the straw into the potato base at a 45 angle far enough to not leak, but not all the way through. Class Discussion Notes:

7 Name: Class: First predict which color will float on top of another color. Try your idea. Record which colors are successful (that is, which float on top of the other). Continue until you have placed all 4 colors in the straw without any color running through another. Remember to keep the answer quiet Try the correct layering a second time see how clearly you can get the separation between the layers.