Activities
Modeling Molecules8
To balance chemical equations, students must know how to count atoms, understand the difference between coefficients and subscripts, and identify reactants and products. Balancing equations reinforces understanding of the law of conservation of mass as students keep in mind that the same number and types of atoms must be on the reactant and product sides of the chemical equation. 8
In this activity, students build models of compounds to help them visualize the reactants and products in chemical reactions and determine the coefficients in chemical equations. 8 This activity is appropriate for a class of middle school students working in pairs.
Materials 8
For this activity students will need 60 gumdrops (5 different colors), toothpicks, and plastic bags. 8
Procedure 8
1.) For each pair of students, place gumdrops (12 of each color) in a small plastic bag. Each student pair also needs the toothpicks.
2.) Divide your class into pairs and distribute the materials.
3.) Give an overview of the activity—explaining that the pairs are to construct molecular models using the gumdrops and toothpicks. They are to model chemical reactions, assigning a color of gumdrop to a specific element. Still using their models, they balance each reaction. Then, they record their final data before moving to the next reaction.
4.) Have students build models of the reactants and products, using the gumdrops and toothpicks for each equation. The gumdrops represent the atoms, and the toothpicks, the bonds. For the purpose of this balancing exercise, it is not important that students model correct bond angles; numbers and types of atoms are the important things.
5.) Have students lay their models out and group them so that they know which models represent the reactants and which represent the products. It may be helpful to have students crease a sheet of notebook paper in half and label the left side “reactants” and the right side “products.”
6.) Once the molecules are built and the reaction is laid out, let students know that in order to balance the reactions they must add complete molecules—not individual gumdrops. Reinforce the difference between a coefficient and a subscript. The coefficient is the number in front of the chemical formula in a chemical equation, indicating the number of molecules. (Absence of a coefficient is understood to indicate 1 molecule.) A subscript indicates how many atoms of an element are in each molecule of a compound. Students should understand that once they build a molecular model, the defined subscripts are unchangeable. Only the coefficients may change in balancing the chemical equation.
7.) Have students count the number of atoms of each element present on the reactant side and compare it with the number of atoms of that element on the product side.
8.) If those numbers are unequal, students must build additional molecules until the numbers match. Then, the number of models of each compound on each side provides the coefficients needed to balance the equation. Have students place the coefficients in the equation and record the final total of each type of atom on the product and reactant side.
Sample Reactions to Balance 8
__N2 + __H2 -> __NH3
__Fe + __HCl -> __H2 + __FeCl3
__CH4 + __O2 -> __CO2 + __H2O
__K + H2O -> __KOH + __H2
__HCl + NaOH -> __NaCl + __H2O
__FeS + HCl -> __H2S + FeCl2
__C2H4 + __O2 -> __CO2 + __H2O
Floating Spinach Disks 7
Spinach leaves contain a lot of chlorophyll, making them perfect for demonstrations of photosynthesis. Students can watch as photosynthesis causes spinach disks to rise and fall when placed in a baking soda solution. 7 The spinach leaves take in carbon dioxide from the baking soda solution causing them to sink to the bottom when placed in a cup of water. 7 However, when they are exposed to the light, the disks use the carbon dioxide and water to produce oxygen and glucose. 7 When oxygen is released from the leaves tiny bubbles are formed causing the disks to float. 7 In this experiment, students will get to watch spinach leaves perform photosynthesis right before their eyes.
Materials 7
The materials needed for this demonstration include: fresh spinach leaves, single hole punch, baking soda, liquid dish detergent, plastic syringe (no needle, 10 cc or larger), clear cup, and a light source (bright sunlight, lamp, or flashlight). 7
Experimental Procedure 7
1.) Create a bicarbonate solution by mixing 6.3 grams (about 1/8 teaspoon) of baking soda in 300 milliliters of water. (Note: This bicarbonate solution will act as a source of dissolved carbon dioxide for photosynthesis.)
2.) In another container, prepare a dilute detergent solution by stirring a drop of dishwashing liquid in 200 milliliters of water.
3.) Add about half of the baking soda solution to a cup. Add a drop of the detergent solution to this cup. (Note: If the solution forms suds, add more baking soda solution until you stop seeing bubbles.)
4.) Use the hole punch to punch 10-20 disks from your spinach leaves. (Note: It is best to avoid the edges of the leaves or major veins. The demonstration works best with smooth, flat disks.)
5.) Remove the plunger from the syringe and add the 10-20 leaf disks.
6.) Put the plunger back into the syringe and slowly push it down to get rid of as much air as possible, without crushing the leaves.
7.) Place the syringe in the baking soda/detergent solution and fill it with about 3 cc of liquid. Then tap the syringe to suspend the leaves in the solution.
8.) Push the plunger to expel excess air, then place a finger over the end of the syringe and pull back on the plunger to create a vacuum.
9.) While keeping the vacuum with your finger, swirl the leaf disks in the syringe. After 10 seconds, remove your finger to release the vacuum. (Note: It is best to repeat the vacuum procedure 2-3 more times, to ensure the leaves take up carbon dioxide from the baking soda solution and are purged of oxygen bubbles. The disks should sink to the bottom of the syringe when they are ready for the demonstration. If the disks do not sink, try using new spinach disks and a solution with a higher concentration of baking soda and more detergent.)
11.) Pour the spinach leaf discs into the remaining cup of baking soda/detergent solution. (Note: Make sure to dislodge any disks that stick to the side of the container. Initially, the disks should sink to the bottom of the cup.)
12.) Expose the cup to light using bright sunlight, a lamp, or a flashlight. (Note: As the leaves produce oxygen, bubbles form on the surface of the disks causing them to rise. If the light source is removed from the cup the leaves will sink again.)
Extension Ideas 7
Students can experiment to see what happens when you return the disks to the light. Students can also experiment with the intensity and duration of the light and its wavelength. 7 One way to set up a control for such an experiment, for comparison, is preparing a cup containing water with diluted detergent and spinach leaf disks that have not been soaked in the bicarbonate solution. 7
The Lights Plants Need 9
In this activity, students will observe and compare three types of light sources. They will then engage in an experiment to determine how different colors of light affect plant growth. Some essential questions that can be asked throughout this activity include: What effect does light have on plants? What effect does light have on other living things? How do different colors of light affect plant growth? How does technology advance science? 9
Materials 9
The following materials will be needed in order to conduct that activity: 60 watt white incandescent light, 6 watt white LED light, 4 bean seeds, 4 250 mL foam or plastic cups, 400 mL potting soil, 8-12 small rocks, 1 plastic milk or water jug full of tap water, 1 250 mL beaker, 1 100 mL gradated cylinder, 1 30 cm x 45 cm piece of plastic wrap or cellophane in four colors (red, blue, green, and clear), 4 wooden dowels, and 1 thin metric ruler. 9
Engage 9
Ask students what they already know about light bulbs and light. Have them complete a KHWL graphic organizer to record what they KNOW, HOW they know this information, and what they WANT TO KNOW about light bulbs and how they produce light. Discuss students’ answers. 9
Show students the incandescent light and the LED light without during either on. Ask students to record the differences between the two lights. Turn the classroom lights off and turn both lights on and ask students to record their observations. Show students the NASA video segment Real World: Space Lighting (5:31) which can be viewed on the NASA eClips YouTube channel (https://www.youtube.com/watch?v=z9hq92u0sl0). After watching the video, students should discuss what type of light bulb they think each of the lights are. 9
Explore 9
Organize students into teams and have each team plant four seeds in each cup. Have them follow the below steps when planting:
1.) Place a rock at the bottom of the plastic cup to help the soil drain.
2.) Use the beaker to measure and fill each of the cups with about 100 mL potting soil. Pour 45 mL of water over the dry soil and let it absorb overnight.
3.) Soak bean seeds in a beaker of water labeled with your team number for 24 hours.
4.) In groups, discuss ways to set up the four colored plastic sheets so that once your plants are positioned near the window or designated light source they will receive equal light through only the appropriate sheet which acts as a light filter. Have students present their solutions to the class and vote on the method that is both secure and allows for the efficient watering and measuring of the plants.
5.) Plant each seed 3 cm deep in each cup.
6.) Lightly cover the seeds with soil. Don’t compact the soil.
7.) Use the graduated cylinder to provide 15 mL of water into each cup. Pour off any excess water.
8.) Place the cups near a window or other designated light source. Arrange the cups so that each receives the same amount of light.
9.) Set up the plastic sheets as decided in step 4.
10. Planting day is Day 0. Water plants with 15 mL of water every other day (or every Monday, Wednesday, and Friday) for the next 20 days. If the plants appear to be too wet or too dry, discuss and agree to changes to the amount of water as a team. Share your changes and reasons with the class. Discuss what to do about long weekends.
10.) Record plant observations. Check daily to be sure to note the date the seeds first sprout. Once the plants have germinated, measure heights with a metric ruler with a flat “foot” made by folding a narrow piece of index paper and taping it closed around the bottom of the ruler. This will prevent the ruler from sinking into the soil. Measure plant height to the nearest tenth of a centimeter.
Explain 9
Ask students to analyze the plant growth data and create a line graph to represent the data.
Extend 9
For an extension activity, students can act as engineers to design a new plant experiment to grow the healthiest plants in the shortest amount of time.
Evaluate 9
Have students review and update their KHWL chart by adding what they LEARNED. Ask students to journal about their response to this question: How do different colors of light affect a plants growth? 9
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