Name Biology 137 Water Station Lab Purpose: In the following pages you will find 7 different stages that are designed to help you see and understand the unique features of water listed below. Remember that these properties stem from the fact that water is polar. That means that water has a slight negative charge on the oxygen side and a slight positive 0(r charge on the hydrogen side. o o!yo+ 0+ H.0+ ~ 0- o 0- W-* 0+ H 0+, 0+ ' 0 H 0+ HI "~ 0+ 1. Cohesion: Water molecules are attracted to other water molecules. They form hydrogen bonds between each other. You can see this any time you see water beading up on a surface. Without the hydrogen bonds between the water molecules, they would not be able to form those domes. Car Surface 1.!oII1ru1fS lilsloe 3 \\ "tef drop lin attracted in 1111 dlfl'ctictls, Drops IlI1 UlQ :i'jrfa.:~ orn alttacllld to OK> sidv'. nnd ~?Il1l(d 2. Adhesion: Water molecules are attracted to other types of atoms. This about when you dip a paper towel into a glass of water. If you just leave the paper towel hanging over the edge, the water will creep up the paper towel because the water molecules are attracted to the positive and negative charges in the paper towel. 3. Surface Tension: As the water molecules bond together at the surface, they form a film like surface that can withstand a small amount of force. This is why bugs are able to walk on water-it is pretty cool to watch. Your Turn: Describe each of the terms in your own words: Cohesion: Adhesion: Surface Tension: One final note: Soap messes all of this up. It has one side that will attract the water but the other side doesn't attract the water. That means it will get in the way of these hydrogen bonds.,\ ~~, '0\ - ' oj \.-,.\;)1)a., ~ ~#). ~ <t..'..;) t'l,<e Q" -; " '-"vi>, ~- "....;. ~~ ~,)
Station 1: Detergent Propelled Boat 1. A paper sheet of paper boats 2. A shallow tray 3. A beaker of soap. 1. Rinse out the tray and fill it with water. 4. Observe what happens. 2. Cut out a paper boat and float it on the 5. Try adding a second drop of soap and see surface of the water. what happens. 3. Add of a drop of soap to the area right 6. Rinse out the tray really well for the next behind the boat. group. 1. Why does the paper boat only move once the soap touches the water? 2. What did the soap do to the cohesive forces between the water molecules? 3. Would it work without the hole in the back of the boat? 4. What would happen if the soap was put to the side of the boat? 5. How many times did you get this to work? 6. Why does it stop working?
Station 2: Pouring Water Sideways 1. Two beakers 2. A water absorbent string 1. Fill one beaker % of the way full with water 2. Wet the string thoroughly 3. Stand a book upright between beaker A and beaker B. 4. Transfer the water from one beaker to the next without moving the book. 5. Hold one end of the string in beaker A and the other in beaker B. Then pour the water slowly along the string. 1. Why did the string need to be wet? 2. What held the water to the string? 3. What other materials could be used in place of the string?
Station:3: Floating Cork 1. A cup 2. A cork that floats on water 1. Fill the glass halfway with water and float the cork on the surface. Note where it is floating. 2. Now add more water and fill it as much as you can. Observe where the cork floats now. 3. Try to push the cork to the side. Observe what happens. 1. Where does the cork float when the cup is only half full? Why? 2. Where is the water level highest in the half filled glass? Why? 3. Where does the cork float when the glass is full? Why? 4. Where is the water level highest in the full glass? Why? 5. Why can you fill the glass more than full without spilling the water?
Station 4: Metal Floats! 1. A needle or paperclip 2. A beaker 3. A piece of wax 1. Fill the beaker with water. 2. Drop the needle vertically in the water. Observe what happens. 3. Fish the needle out of the water and dry it off or use a different needle. 4. Now hold the needle CLOSE to the surface of the water and try to drop the need perfectly horizontal onto the surface of the water. 5. If it does not work, fish the needle out, dry it off and rub the needle on the wax and try it again. 1. What does the needle do when you drop it in vertically? Why? 2. Why do you have to dry off the needle before you drop it horizontally? 3. Why will rubbing the needle on wax help? 4. What happens when the needle is dropped horizontally? WHY? 5. How can you make the floating needle sink without touching it?
Station 5: Penny Drops 1. Pennies 2. Water with a dropper 3. Liquid Soap 1. Make sure that your penny is clean and dry. 2. Begin to count how many drops of water you can put on the surface of a penny before the water falls off. 3. On a second penny try the same thing, only this time add a drop of soap before you begin. 1. How many drops of water fit on the clean dry penny? 2. Draw a picture of the penny with the water on it: 3. Why does the water look like that? 4. How many drops of water fit on the penny that had soap on it? WHY?
Station 6: Detergent Fearing Powder 1. A Petri Dish 2. Liquid soap and a dropper 3. Pepper 1. Rinse the petri dish well. Then fill it about half full with water. 2. Sprinkle the pepper onto the water. 3. Place one drop of soap in the center of the petri dish. Observe what happens! 4. Try to drop the experiment again without rinsing out the petri dish. 1. What happened when you added the soap? WHY? 2. What does the detergent do to the surface tension? 3. What forces are acting between the water molecules on the water surface? 4. Does this experiment work the second time? Why or why not? 5. Why might more pepper sink after you add the soap?
Station 7: Capillary Tubes 1. Different sized capillary tubes (size of the diameter changes) 2. A beaker filled with colored water 1. Hold all of the capillary tubes vertically in the water. 2. Observe where the water level is in each tube. 1. Will the water level in the capillary tubes change when the tubes are moved either higher or lower in the beaker? 2. What makes the water goes up the tubes in the first place? 3. Why does the water move up higher in the narrower capillary tubes? 4. Where can we find an application of this in daily life? 5. What is the force between water and other molecules of matter called?
Each Station EXPLAINED! Detergent Propelled Boat: There are hydrogen bonds (COHESION!) between the water molecules in front and in back of the boat as well as on both sides of the boat. All of these bonds are pulling on the boat the same amount. When the soap hits the water behind the boat, the bonds behind the boat are broken. Because these bonds are no longer there, the boat gets pulled forward. Think about tug of war from all sides as pictured below. When they all pull with the same amount of force, the rope will be stuck in place. If the people in the back suddenly let go or are broken, the rope will move forward-just like our boat moved forward. This would work on a boat of any shape. It would also work no matter where you put the drop of soap. The only difference would be that instead of moving forward, it would move away from where you put the soap. This will continue to work until a majority of the hydrogen bonds (which are resulting in cohesion) are broken. Pouring Water Sideways In this demo, both adhesion and cohesion are at work! The string had to be soaked in water so that the water molecules could stick to the string. Water sticking to something else like string is an example of adhesion. Then when you poured the water along the already wet string, the water molecules could stick to the water molecules that were already there. This is where you saw the cohesion at work because water molecules were sticking to other molecules of water. This demo would not have worked if the string had been dry or if it had been some kind of material that does not stick to water. Other materials could have been used instead of the string. In fact anything that is water absorbent (able to stick to water) like cotton, cloth, paper, and wood would have worked. If you tried it with a material that does not absorb water like nylon or wool, the experiment would not have worked. Floating Cork When the glass is half full the water is at the highest level along the sides of the glass. This is because the water is creeping up the side of the glass due to adhesion between the water molecules and the side of the glass. The cork floats at the highest level! When the glass is filled all of the way, the cork floats in the center because that is now the highest level. You are able to fill the glass more than full because the water molecules at the surface stick together (COHESION!) and the surface tension creates a film allowing water to dome at the top of glass. The center of the dome is the highest part so that is where the cork will always float. Metal Floats! When the needle is dropped vertically, it pierces the water's surface. The surface tension at the small area of the needle point is not strong enough to hold the needle up. However, when the needle is dropped horizontally there is a much more surface area. The water molecules that are sticking together next to and below the needle by cohesion are creating a surface tension large enough to keep the needle float. The wax simply makes it so that the water does not interact with the needle. If the needle is wet when you drop it, the water molecules from the needle will interact with the water molecules at the surface momentarily breaking the surface tension and causing the needle to sink.
Penny Drops As we talked about on the first page, the cohesion between the water molecules and the surface tension creating a film at the surface, allow the water to dome on top of the penny. When the soap is added, the dome won't form because the soap molecules get in the way of the cohesive bonds. That means that the surface tension can't form and without surface tension, the water will overflow much sooner. Detergent Fearing Powder On the surface of the water, the water molecules are in a tug of war standoff. Each water molecule is pulling equally on all of those around them with cohesive forces. When you add the soap, a few of these forces are broken so the standoff ends. The pepper flies to the edges of the petri dish because the water molecules on the outer edges win the tug of war and you get to see this movement shown by the pepper because the pepper particles get pulled along by the water molecules. This will not work a second time because the cohesion is weakened by the soap that is in the petri dish and the pepper tends to sink more after the soap is added because the surface tension is also weakened by the soap. Capillary Tubes When the tube is dipped in water, the adhesion between the water and glass of the tube starts to work. It is so large that the water is pulled upwards. Since the smaller tubes have more glass per water molecules, the adhesion is stronger and the water can be pulled up higher. Everyday plants are able to bring water from the soil to the leaves in the same exact way. The water is attracted to the cells in the veins of the plant and crawls upward. We can see adhesion in general in many parts of life. Adhesion is anytime two different types of molecules are attracted to each other. It explains why paint can stick to surfaces and why your jeans get really wet when you are walking through puddles.