Module_2_Polarity_2023

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George Washington University *

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1111

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Chemistry

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Feb 20, 2024

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BISC 1111 Fall 2023 – Module 2: Exploring chemical bonds with slime and Silly Putty Objectives & Purpose: Students will practice basic wet-lab techniques such as measuring set amounts of reagent with a scale or graduated cylinder, following basic written procedures, and observing lab safety measures while making a classic child’s toy (slime). Using the slime they produce, and commercial Silly Putty, students will conduct a basic experiment comparing and contrasting their chemical bonding properties. In the last part of this week’s protocol, students will be exposed to and gain experience using one of the most important pieces of equipment for their future as a scientist: the micropipette. This module introduces students to basic chemistry concepts relevant to their understanding of molecular biology, while simultaneously preparing them for more advanced lab work by starting off with a relatively “low stakes” set of activities. Students are encouraged to watch the following video on proper micropipetting technique prior to coming to class ( https://www.youtube.com/watch? v=FJuceccl9Ns ). Introduction: Have you ever played with Silly Putty and newspapers? If not, know that the ink in the newsprint can be transferred to Silly Putty. This phenomenon is due to the chemical characteristics of polarity. Polarity is based on two primary factors: the shape and electronegativity of a given molecule. Bond polarity exists when two atoms bonded covalently share electrons unequally. Remember, covalent bonds form when two atoms share one or more pairs of valence electrons. Polarity of substances are important considerations not just in the lab, but also in your everyday life. Loads of common household goods are great examples of polar (water, ethanol, ammonia, etc...) and non-polar (gasoline, turpentine, benzene, etc...) molecules. In many cases, the usefulness of these household items for their particular task is largely based on types of atomic bonds in the molecules of the solvent. In this lab module, we will be exploring polarity by playing with two favorite (if old-fashioned) toys; slime and Silly Putty. The etymological origins of the word “polarity” stretch back to the 1640s, when the root Latin word “polus” (an end of an axis) was used to describe opposites. There are strong attractions in a molecule that induce polarity, like the North and South poles of the globe or the poles of a magnet. That is to say, molecules can be polar, or have poles, too. Electronegativity is a term that describes the attraction an atom has for electrons. Fluorine has the strongest attraction for electrons of all elements, and therefore has the greatest electronegativity. It is measured on various scales like the Pauling scale , and Mullikan scale. Pauling scale is the most widely used to measure electronegativity of any element (and will be used in the remainder of this lab module). Electronegativity is a relative property of elements, it has no measurable numerical value, and therefore it has no units. Therefore it is important to indicate in your writing what scale you are referring to when discussing the electronegativity of an atom. Elements on the right side of the periodic table close to where fluorine is located tend to have greater electronegativities than elements located on the left side of the periodic table (the hydrogen side of the table). If all the atoms in a molecule have similar electronegativities, the molecule is nonpolar. Hexane is an example of a nonpolar molecule: it has only carbon and hydrogen atoms. Carbon has a Pauling Scale electronegativity value of 2.5 and hydrogen has a value of 2.1. This difference of only 0.4 in electronegativity results in a nonpolar molecule. If there is a large difference between the electronegativities of two covalently bonded atoms, the bond is considered polar. For a covalent bond to be polar, the atoms bonded must have an electronegativity difference of between 0.5 and 1.9 (using the aforementioned Pauling Scale). However, electronegativity is not the whole story when assessing the overall polarity of a molecule. Even if a molecule has a bond between two atoms with a large difference in electronegativity, it does not automatically mean that the molecule is polar. The shape of a molecule also must be considered. To understand how the shape of the molecule affects the polarity, imagine the atoms within a molecule playing tug-of-war. 1

If there is an equal pull on each side (the “teams” are tied in strength during the tug-of-war), then the rope does not move. Similarly, if there are two polar bonds that have equal electronegativity differences, they each pull with the same force, and the overall pull is zero. Figure 1. Four different ways to depict a carbon dioxide molecule. Clockwise from the top left: molecular formula, space-filling model, structural formula, and ball-and-stick model. An example of this shape phenomenon affecting polarity is the compound called carbon dioxide (Figure 1). In this case, there are two polar bonds (between Carbon and Oxygen which read 2.5 and 3.4 on the Pauling scale respectively), but the molecule is linear and non-polar because the overall pull is zero. Figure 2. Four different ways to depict a water molecule. Clockwise from the top left: molecular formula, space-filling model, structural formula, and ball-and-stick model. If there is an unequal net pull, however, the molecule is bent. Water (Figure 2) is an example of this: it has a bent shape and is polar because the pull is unequal. Many of the common household molecules mentioned earlier in the introduction (water, ethanol, turpentine, benzene, etc...) are used in cleaning solutions because of their usefulness as solvents. A solvent is a substance, usually a liquid, that is capable of dissolving other compounds. Polar solvents such as water, vinegar, or ethanol dissolve other polar molecules. Non-polar solvents, such as oil or gasoline, dissolve other nonpolar molecules. This trend is colloquially known as "like-dissolves-like." Consider this: since sugar dissolves in water, is sugar polar or nonpolar? How do you know? Although you may not realize it, the inks that are used in writing utensils are chemical solutions that include different molecules. Some inks are polar, while others are non-polar. A polar solvent will attract (or “dissolve”) polar inks, while a nonpolar solvent will attract nonpolar inks. In this lab, you will use various inks to determine whether slime and Silly Putty are polar or nonpolar. Generally speaking, water soluble inks are polar and include things like highlighter and Uni-ball pen. Contrastingly, water insoluble inks are non-polar and include Sharpie pens/markers, newsprint, and most dry-erase markers. 2

Students are required to wear all PPE for the following protocols. That means, closed toed shoes, lab coats, safety glasses/goggles, and gloves. While none of the reagents in this lab are dangerous in the listed concentrations, students must adhere to these standards to practice the culture of safety that all more advanced labs hew to. Protocol Part 1: Making Slime 1. Weigh out 0.5g of guar gum into a 250mL beaker. Make sure to tare the scale with the weigh boat first. To tare a scale means to “zero out” the weight of whatever is already on the scale, in this case the weigh boat prior to adding guar gum. 2. Measure 50.0mL of distilled water into a 100mL graduated cylinder and pour it into the 250mL beaker containing the guar gum. When using a graduated cylinder, make sure you measure from the bottom of the meniscus. 3. Rapidly stir the mixture with a stirring rod for at least 3 minutes and until the guar gum is dissolved. 4. Measure 4.00 mL of a 4% Borax solution into a 10 mL graduated cylinder and add it to the guar gum and water. 5. Stir the solution until it becomes slime. This will take a few minutes. If the slime remains too runny, add an additional 1.0 mL of the 4.0% Borax solution and continue to stir until the slime is the right consistency. 6. Once you are satisfied with the slime, pour it into your hands. Be sure not to drop any of it onto the floor. 7. Manipulate the slime in your hands. Write down observaftions made about how slime pours, stretches, breaks, etc… in your lab notebook. CAUTION: Slime is slippery and if dropped it can make the work area slick. 8. Place the slime back into the beaker, and be sure WASH YOUR HANDS. Remember, the central question driving today’s protocol is “Are the chemical bonds making up slime and Silly Putty polar or nonpolar?” Before attempting part 2 of the protocol, discuss reasonable hypotheses with your group mates. Once you reach a consensus, write down your hypotheses (one for slime and one for silly putty) in your lab notebook , making sure that they are clearly labeled. If you are having trouble elegantly wording your hypothesis, just fall back on the traditional “If…then…because” paradigm. For example, a gardener might hypothesize: if the amount of fertilizer used on the tomato plant is changed, then the tomato plant will experience a different growth rate, because fertilizer provides nutrients needed for plant growth. Make sure your hypotheses address the polarity of both slime and silly putty. Protocol Part 2: Slime and Silly Putty Ink Assays 1. On a piece of provided paper on the lab bench, make one 20-25mm long mark of each of the inks you are testing. Space the marks at least one inch apart. Use a pencil to label each mark with its description. 2. Water soluble (polar) inks include those in highlighters and certain pens like Uni-ball pens. 3. Water insoluble (non-polar) inks include those in permanent pens/markers, newsprint, and a dry-erase marker. 4. While the inks are drying, select a passage or a picture in the newspaper to test with the slime. 5. Before performing ink tests on slime, hypothesize which inks the slime will pick up. Make sure to note your hypotheses in your lab notebook! 6. Break off a small piece of slime that is 3 - 5cm in diameter. Gently place this piece on top of the newspaper print, and then carefully pick it up again. 7. Observe and record in Table 1 (in your lab notebook) whether or not the ink was picked up onto the slime. 8. Once the inks from Step 1 have dried, break off another small piece of slime and gently place the slime on top of the first ink on the prepared paper, and then carefully pick it up. Repeat this for each of the inks. Observe and record which inks were picked up (dissolved) by the slime in Table 1. 9. Repeat this ink testing two more times for accuracy (three total trials per ink type assayed). 10. Before performing ink tests on Silly Putty, hypothesize which inks the Silly Putty will pick up in your lab notebook. 3

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11. Perform ink tests on Silly Putty in the same manner as above. Record your results in Table 2 in your lab notebook. Table 1. Results of Slime ink assays Picked up (dissolved) Yes or No? Ink Type Trial 1 Trial 2 Trial 3 Dry-erase marker Highlighter Newsprint Sharpie Uni-ball point pen Do your results support your hypothesis? Why or why not? Answer these questions with 2 or more well- reasoned sentences in the “Results & Analyses” section of your lab notebook. Table 2. Results of Silly Putty assays Picked up (dissolved) Yes or No? Ink Type Trial 1 Trial 2 Trial 3 Dry-erase marker Highlighter Newsprint Sharpie Uni-ball point pen Do your results support your hypothesis? Why or why not? Answer these questions with 2 or more well- reasoned sentences in the “Results & Analyses” section of your lab notebook. Prior to starting the par 3 protocol below , make the following table in the “Data tables & observations” section of your lab notebook: Type of Micropipette ID Number of Micropipette used on [Date] P-10 P-20 P-200 P-1000 * For EVERY lab module that calls for the use of micropipettes should have this style of table made in your “Data & observations” section of your lab notebook as part of your pre-lab prep. During the lab, you are to record the identification number of each micropipette of each type that you use. If you do not use a micropipette 4

of a certain type, write “none” or “N/A” in the appropriate cell of the table. We do this for two reasons: 1. to help you keep track of which micropipettes you have used. This could be informative for any potential equipment errors or malfunctions affecting experimental outcomes or results. 2. to help us (the instructions staff) identify students who may be causing damage to equipment, and offer them additional training and assistance. Repeated breakage of equipment after a training intervention may result in loss of points of the student’s overall grade. * Part 3: A quick micropipette activity *This section’s protocol DOES NOT have to be transcribed into your lab notebook, but you do need to read it before taking the pre-quiz* The following information and quick activity is merely to introduce you to one of the most important, and finicky, pieces of equipment you will be using in every biology laboratory here at GWU. Figure 3. What do the numbers on the plunger of the micropipette mean? What do the numbers in the vertically oriented windows of the micropipette represent? Pipettes are instruments used by researchers to accurately measure incredibly small volumes of liquids. Pipettes you will encounter in this class have a range of volumes they are designed to hold. Note the label on the top of the pipette plunger to determine the maximum volume. You can set the volume the pipette can hold by turning the dial on the pipette so that the numbers in the display winders change. Remember, you cannot use volumes higher than the pipette can possibly hold (see Figure 3 above). 5

Figure 4. Proper micropipetting technique diagram. Tips for using a micropipette: 1. The numbers displayed in the micropipette window represent different volumes for different sized pipettors. 2. Keep the micropipette vertical at all times to keep solution in the tip and not in the body of the pipettor. 3. Each micropipette uses size specific tips that should fit snugly with minimal pressure. NEVER attempt to draw up liquid without a tip!!!!!! 4. When picking up a solution, depress plunger to the first stop before inserting the tip into the solution and stay below the surface when drawing up the solution. 5. Change tips when changing solutions or after combining solutions. 6. Small volumes should be directly expelled into larger volumes or onto the side of the tube. 6

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7. Depress the plunger to the second stop to expel all volume from the pipette. Let’s assess your understanding of the micropipette displays before getting hands on experience. Given the following displays on the corresponding pipettes (top row), what volume would you expect to expel from each? (fill in the appropriate volume in the bottom row of the table below) P-10 P-20 P-200 P-1000 0 0 1 0 5 6 4 8 3 7 8 3 µL µL µL µL Pipetting Practice Protocol: 1. Cut three 1 cm wide strips of Parafilm (4 students will be pipetting onto the same strips, three or more times). 2. Use the P-20 to deliver a 5 µL droplet of colored water onto a piece of the Parafilm from step 1. Have the other 3 students at the lab bench also deliver a 5 µL droplet of colored water onto the same piece of Parafilm. Try to get them in a neat row, and don’t allow them to touch! 3. Use the P-10 to deliver a 1 µL droplet of colored water onto a different piece of Parafilm from step 1. Have the other 3 students at the lab bench also deliver a 1 µL droplet of colored water onto the same piece of Parafilm. Try to get them in a neat row, and don’t allow them to touch! 4. Use the P-200 to deliver a 75 µL droplet of colored water onto a different piece of Parafilm from step 1. Have the other 3 students at the lab bench also deliver a 75 µL droplet of colored water onto the same piece of Parafilm. Try to get them in a neat row, and don’t allow them to touch! 5. Compare your droplets to the droplets of your peers. Do any of your droplets seem too big or too small? Try again if you are not satisfied with the precision and accuracy of your group's droplets… much of the work in this lab heavily depends on the delivery of precise µL amounts of reagents. Part 4: Tidy Up 1. Dispose of slime in regular trash. 2. Wash 250mL beaker, stir rod, and glass cylinders with soap and water. Rinse and dry. 3. Do not dispose of Silly Putty, pens and highlighter, unused newspaper, or supplies for the next lab. 4. Dispose of micropipette tips into the labeled trash tip beakers on each lab bench. 5. Return all micropipettes to the holders on your lab bench. 6. Reset materials for the next lab section. 7