Kitchen Chemistry Test 1

Transcription

1 Roy1 Penny water-droplet lab Kitchen Chemistry Test 1 Question: How many drops of water will fit on the surface of a clean, dry penny? Make a hypothesis, and then continue. Hypothesis: I think that if I place drops of water on a penny, that 50 drops of water will fit. Materials: Eyedropper, glass of water, penny, paper towel Procedure: 1. Place a clean, dry penny on a paper towel on a stable surface. 2. Using the eyedropper, carefully place and count the number of drops of water that will fit on the penny before it spills off the penny. 3. How many actually fit? In my experiment, 40 drops of water fit.

2 Roy2 Conclusion: 1. What property of water accounts for the difference between your prediction and the actual results? a. b. The property of water that accounts for the difference between my prediction and results is the property of cohesion. Cohesion is the attraction of like molecules, which in this experiment are the water molecules. One of the main properties of water is that it exhibits strong cohesion tension. This allows for the water molecules to attract each other and form the dome around the penny as more droplets are added. The bubble/dome shape is an outcome of the water molecules clinging to one another in one ideal shape. The bubble does not break due to the surface tension formed from the strong cohesion of the molecules. This explains why 40 drops of water were able stay on the penny before the water spilt off the penny. 2. Think of 2 examples in which humans use this property, or in which this is found in nature. a. One example of this property found in nature is the drops of water that collect on leaves of a tree, even traveling up tall trees to reach the highest leaves due to the strong cohesion of the water molecules. b. Another example of this property is shown through if you gradually overfill a glass of water, the molecules will stick together; avoiding the act of falling off until the force of gravity overcomes the force of cohesion.

3 Roy3 Kitchen Chemistry Test 2 Penny water-droplet lab with Soap Background: Surface tension of water is a force exerted by the surface of water on the particles below. It results from the attraction of water molecules to other water molecules. The force of surface tension tends to pull drops of water into spherical shapes. Question: What happens to the shape of a water drop if soap is added? Hypothesis: I think that if touch the water droplet bubble on the penny with the soap that this will cause the water droplets to separate and flow off of the penny. Materials: toothpick, eyedropper, glass of water, penny, paper towel, liquid soap Procedure: 1. Place 10 drops of water on the penny from Test Draw or describe the shape of the water droplet when viewed from the side. a. The water droplets are in a spherical form on top of the penny, all collected together in one big bubble shape. 3. Dip a toothpick into the liquid soap so that a tiny glob hangs off the toothpick. 4. Touch the soap from the toothpick to the water drop while viewing the water droplet from the side. 5. Be careful not to pike the water with the toothpick-only touch the soap to the water. 6. How did the drop of water change with the addition of the soap? a. The addition of the soap caused the water on the penny to deform from the spherical shape of the droplets, to a flat shape on the penny.

4 Roy4 Conclusion: 1. What caused the drop of water to change shape (I want mechanics, not just the soap )? a. The soap molecules collect together and stick out at the surface of the water, forcing their nonpolar tails to stick out from the water. This then causes a decrease in surface tension of the water droplets by the interference of the dipole-dipole interaction between the water molecules (when the partially negative portion of one of the polar molecules is attracted to the partially positive portion of the second polar molecule). 2. What can you conclude about the effects soap on the surface tension of water? a. Overall, I can conclude that the addition of soap on the surface tension of the water will cause the surface tension of the water to drastically decrease (as it can be seen in the pictures above from the high surface tension of the spherical form before the addition of soap, and the low surface tension of the flat form after the addition of soap).

5 Roy5 Kitchen Chemistry Test 3 Aluminum Foil on Water Lab Question: Will the shape of two equally sized pieces of foil effect whether or not they float? Hypothesis: I think that if I take 2 equal pieces of aluminum foil (1 crumpled and 1 flat) that the flat piece will float and the crumbled piece will sink. Materials: two beakers of water, two, 3x3 cm squares of aluminum foil, scissors, ruler, glass of water Procedure: 1. Cut out two sheets of aluminum foil, each 3x3 cm 2. Keep one of the sheets perfectly flat, but crumple the other into a ball (be sure to get any air pockets out) 3. Place each piece of foil carefully on the surface of the water. 4. What results did you see? a. The piece of Aluminum that was flat floated on the top of the water, whereas the crumbled piece of Aluminum sank to the bottom of the container.

6 Roy6 Conclusion: 1. What is this property of water called? a. Density of water b. The density of the flat piece of aluminum foil is less dense than that of the crumbled piece. Since the flat piece is expanded over the water, it allows less pressure to be placed on the surface tension of the water; allowing it to float (not breaking the surface tension) due to its lower density than the water. The crumbled piece has a higher density as it is not expanded, applying more pressure to the surface of the water; causing it to sink due to its higher density than the water. 2. How is this property of water seen or used by humans? a. Humans use this property through the careful construction of boats and rafts to make sure that they will not sink when in action or use by other humans. 3. Where is this property found in nature? a. An example of this is when a body of water freezes over in the winter time. The low density of ice allows it to float on the surface of the water, creating insulation for the body of water under it. This then allows the aquatic environment to thrive. ((If ice was a higher density than the water, the aquatic life would be killed due to the fact that it would sink))

7 Roy7 Conclusion and Application Questions 1. Distinguish between hydrogen bonds and Van der Waals forces a. Hydrogen bonds occur when hydrogen is covalently bonded (usually with nitrogen or oxygen) and they are then attracted to the electronegative part of the other molecule (giving hydrogen a partial positive charge and the other a partial negative charge). Whereas Van Der Waals forces state that as electrons move attraction points form due to temporary polarity. 2. Using the term electronegativity distinguish between nonpolar and polar covalent bonding. a. Nonpolar bonding involves the bonding between 2 nonmetal atoms that have the same electronegativity resulting in the equal sharing of the electrons. Polar-covalent bonding involves the bonding between 2 nonmetal atoms with different electro negativities resulting in the unequal sharing of electrons. 3. Cohesion vs. Adhesion. Relate this to how water moves up plant roots and tissues. a. Cohesion is when water molecules are attracted to themselves, whereas adhesion is when water molecules are attracted to other polar substances. b. When transpiration occurs, the water molecules stick to each other and move up plant roots representing cohesion. Adhesion is represented by the water molecules also sticking to the sides of the plant and moving up the plant. 4. Fill in the following: Type of Bond How do you predict when it will form? What holds the bond together? Ionic result from the transfer of electrons (nonmetal to metal) The opposite charge of the ions attracts them to one another therefore

8 Roy8 Covalent Hydrogen when atoms share electrons (nonmetal to nonmetal) when hydrogen is covalently bonded (usually with N or O) and they are then attracted to the electronegative part of the other molecule holding the bond together. The electrons in their outer shells are attracted to the positive charges of the nuclei of each atom. The partial positive charge of the H and the partial negative charge of O (or N) attracts one another; holding the bond together. 5. Fill in the following: Property of Water What causes it? Why is it important? Universal Solvent High Heat Capacity Ice floats The high polarity of water molecules causes it to dissolve all polar and ionic substances. The high heat capacity is produced by the hydrogen bonding within water molecules (when heat is absorbed, the energy is spent breaking the bonds so the average kinetic energy doesn t increase easily). Ice floats because when water freezes it expands due to the tetrahedral shape of the molecule as it freezes; this then causes the ice to become This allows substances to be broken down easily contributing to chemical equilibrium within organisms. This can help to provide stable environmental temperatures in large bodies of water (also in the bodies of organisms to keep a stable body temperature). This helps aquatic environments thrive; as if the ice did not float it would kill the aquatic life found within the environment. Also the

9 Roy9 High surface tension Capillary action less dense and fully hydrogen bonded. It is causes by the strong forces of cohesion found in water molecules, therefore allowing the molecules on the surface (with no molecules completely surrounding them) to be more attracted to its neighbors on the surface. The combined forces of cohesion and adhesion (allowing water to be sticky) ice insulates the water below, helping the organisms to survive. It allows insects such as spiders to travel across water without breaking the surface, contributing to their survival. This contributes to the movement of water allowing organisms such as trees and plants to prosper.