Name: ____________________________ Block: ____
Bio 2014
Water Lab APPLICATION QUESTIONS ANSWERS Each of the following biological phenomena related to specific water properties you witnessed in this lab. For each of the following, NAME the specific property (or properties) involved and EXPLAIN HOW it relates to each phenomenon given. (Use your notes and readiing for reference.) 1. Water strider insects are able to “walk” on water. Why?
• Water molecules in the middle of the container are pulled equally in all directions by H-­‐bonds with surrounding water molecules. Water molecules at the surface are only pulled to the sides and down, resulting in a net downward pull on the surface water molecules & creating water’s high surface tension. • Therefore, the H-­‐bonds at the surface of the water act to support the distributed weight of the insect. • Additionally, the legs of the insect are coated with a nonpolar waxy substance that is not attracted to water (does not H-­‐bond). This aids the water strider insects in walking “on” the water. http://en.wikipedia.org/wiki/Surface_tension 2. Oil spills in the ocean result in a “slick” of oil on the water’s surface. Why?
• Polar water molecules are attracted to each other through H-­‐bonding, which makes water cohesive. • Comparatively, nonpolar oil is not attracted to water. As such, oil is excluded by water and remains separate from cohesive water, which continually acts to maximize H-­‐bonds between water molecules.
• Note that water & oil are NOT repelled from each other. They simply are not attracted to each other AND water is attracted to itself, which results in the passive exclusion of oil.
3. Many plants have a waxy coating on their leaves that help keep moisture in and allows
raindrops to roll right off the top surface of the leaves. Why/how does this happen?
• Polar water molecules are attracted to each other, but NOT to the waxy coating on the leaves, which is nonpolar like the oil discussed in Q.2. • As such, water forms spherical droplets which maximize H-­‐bonding between water molecules and minimize the surface area of the water droplet. These water droplet roll easily because of the lack of attraction for the surface of the leaf. http://en.wikipedia.org/wiki/Surface_tension 4. The most effective way to wash greasy dishes or hands is with water and soap. Why?
• Soap is an amphipathic molecule, meaning that it has a significant polar region and a significant nonpolar region. • Water is polar and grease is nonpolar. • Water is not attracted to the grease on it’s own, but add amphipathic soap, and something fantastic happens! • Polar water is attracted to the polar heads of the soap molecules and itself. The polar heads are also attracted to other polar heads. • The non polar tails of the soap and the nonpolar grease are excluded by the water and polar heads, and will be very weakly attracted to one-­‐another through Van der Waals forces. • Now, water is attracted to polar heads of soap, the polar heads are covalently bonded to the nonpolar tails of soap, and the nonpolar tails of soap are weakly attracted to the nonpolar grease. This chain of attractions allows the grease to be “grabbed” by the combination of soap & water. Water alone would not be able to “grab” the grease.
Soap molecules http://nsb.wikidot.com/c-­‐9-­‐5-­‐5-­‐3 5. When you go on vacation, you put your potted plants in a tray of water. You make sure the tip
of the roots in the soil is below the water line so the plant can take up water from the tray.
How is the plant able to draw the water up its stem?
• The tube that transports water inside the stem of a plant is either polar or charged, and thus attracts the polar water molecules.
• This attraction results in the water climbing the tube or stem – a phenomenon called capillary action.
• Capillary action is due to the adhesion of water to the sides of the tube and the cohesion of water molecules for each other. The force of the adhesion & cohesion is greater than the force of gravity pulling down on the water molecules.
http://hyperphysics.phy-­‐astr.gsu.edu/hbase/surten2.html#c5