Name ______________________________ Exploring Properties of Liquids Background: Use this information to help you answer the questions on the lab sheet after you have completed the experiment! Liquids have some different properties than solids or gases. Cohesion: The attractive force of liquid molecules to each other. This attraction is especially strong in water molecules. The oxygen end of water has a negative charge and the hydrogen end has a positive charge. We call this a polar molecule. The hydrogen atoms of one water molecule are attracted to the oxygen from other water molecules. This attractive force is what gives water its cohesive property. Adhesion: The attractive force of liquid molecules to other surfaces. Because water is a polar molecule, it is also attracted to some other surfaces and will “stick” to them. This is why we get “wet” when liquids touch us. Liquids will stick to us due to their adhesive properties. Surface Tension: Surface tension is the name we give to the cohesive forces of water molecules at the surface of water. The cohesive property of water molecules forms a surface "film" or “skin”. Some other substances may reduce the cohesive force of water, which will reduce the strength of the surface “skin” of the water, breaking the molecules apart. Capillary Action: This is the ability of a liquid to flow in narrow spaces, sometimes against the force of gravity. This is due to the surface tension caused by the cohesive forces of the liquid, and the adhesive forces between the liquid and whatever it is touching. Activity #1: Moving Water Materials: 4- 50mL beakers. 2 sheets of paper towel, each rolled into a tight tube, food coloring 1. Fill one of the beakers ¾ full of cold tap water. 2. Fill the second beaker ¾ full of hot tap water. 3. Place one drop of food coloring into each of the beakers. Without stirring, how does the food coloring spread out differently in the hot and cold beakers? _____________________________________________________ ________________________________________________________________________________________ Why do you think this different movement happens? _____________________________________________ ________________________________________________________________________________________ 4. Roll 2 paper towel sheets into tight tubes. Place one end of a tube in the beaker of hot water and the other end of the tube in an empty beaker placed next to the hot water beaker. Observe the movement of the water. 5. Repeat step #4 with the second paper towel tube into the cold water beaker, placing the other end into a second empty beaker. Observe the movement of the water through the paper towel. 6. Complete the other lab activities before coming back to observe the beakers. 7. After observing how the water moved from the full to the empty beakers, use the background information to describe why this happened. ________________________________________________________________ ________________________________________________________________________________________ 8. What was different between the hot and cold beakers?_____________________________________________ Activity #2: Does water slide or roll? When a drop of water is held on a sloped surface, does the water drop slide down or roll down the slope? Write your prediction here. I think the water will ____________________ when placed on a tilted piece of paper. 1. Using one of the water drops on the waxed paper, sprinkle a few grains of pepper over the drop. 2. Tilt the waxed paper and observe the movement of the water drop. What do you observe? ________________ _________________________________________________________________________________________ 3. Why do you think this happens? ______________________________________________________________ _________________________________________________________________________________________ 4. Was your prediction correct? Why or why not? __________________________________________________ ____________________________________________________________________________________________ Activity #3: Cut and Connect Water Drops Try to split a water drop into smaller drops, and put the small water drops back together to learn more about the surface tension of liquids. Materials: Beaker of water pipette toothpick waxed paper straw soap solution Procedure: 1. Use the pipette to gently put several drops of the water onto a sheet of waxed paper. Carefully observe the shape of the drops. Describe the shape here. ___________________________________________________________ 2. With a toothpick, try to cut one water drop in half. How many smaller drops can you make? _______________ 3. With a drinking straw, blow gently to try to put two water drops together. Do they rejoin? _________________ Why?_____________________________________________________________________________________ 4. Dip the end of the toothpick in soap solution. Poke the water droplet with the soapy tip. Describe what happens to the drop. _________________________________________________________________________________ Why does this happen? ________________________________________________________________________ What Happened? The cohesive force of water pulls the water molecules toward each other forming a drop. The molecules in the outer layer are drawn in toward the center of the drop, giving the drop its round shape. The surface tension that holds the water in that shape affected how the water acted when you pushed on it with the toothpick and the straw. Activity #4: How many water drops can fit on a penny? Take a Guess: How many drops of water can fit on one side of a penny? _______ Procedure: 1. Each group should get one penny per person for the testing. 2. Rinse each penny in tap water and dry completely with a paper towel. 3. Place the dry penny on a flat paper towel. 4. Use the pipette to slowly place drops of WATER on the penny (one at a time) until ANY amount of water runs over the edge of the penny. 5. Count the number of drops for each student testing and record in the data table. 6. Repeat Steps 1 - 4 for each group member before calculating your average. Student 1 Student 2 Student 3 Student 4 Average # of drops # of water drops Answer each question related to the experiment. 1. Using the terms of cohesion and surface tension, explain what happened in the experiment. __________________ ___________________________________________________________________________________________ ___________________________________________________________________________________________ 2. How do your results compare to the other groups in your class? Provide at least 2 possible reasons for any similarities and differences you identified between your data and others data. _____________________________ ___________________________________________________________________________________________ ___________________________________________________________________________________________ What Happened? You just saw three important forces tugging on the water: gravity, cohesion, and adhesion. Gravity pulls down on the drop causing it to flatten the droplets, cohesion attracts the particles to each other and holds the droplets together, and adhesion attracts the drops on the surface of the coin. The cohesive force is the pull of the water molecules on themselves. Each successive drop sticks to the water that’s already on the coin. We often call this cohesive force “surface tension”. It’s what makes water drops look like they’re wrapped in invisible skins. Soap reduces the cohesive forces of water, and breaks the surface tension. Soapy water makes smaller drops than plain water. Since soapy drops are smaller, more soap drops will fit on a penny than plain water drops. Go back to observe Activity #1, Moving Water and finish question #7. Water Drops: Cohesion and Adhesion In the back of the book Science Instruction in the Middle and Secondary Schools (2002), the authors Eugene Chiapetta and Thomas Koballa suggest that "if you place a drop of water on a piece of wax paper and ask the question, Does the drop of water roll or slide across the paper? you will have a puzzle that needs to be resolved and explained." The book then goes on to describe how the exploration might proceed, explaining that some chalk dust or pepper in the drop will help students see that the drop rolls rather than slides. By way of explanation, the authors state that, "In this situation, we observe the cohesive effects of water molecules permitting a drop of liquid to form and maintain a spherical shape that will roll on a surface without breaking apart." What does this really mean? Is this explanation enough? I decided to play around with this system and see what things I could come up with. Let's start with the drop of water on the wax paper: It is easy to see that the drop seems to have a "skin" holding it into a sort of flattened sphere. It turns out that this surface tension is the result of the tendency of water molecules to attract one another (called cohesion). The lowest energy state for this drop occurs when the maximum number of water molecules are surrounded on all sides by other water molecules -- meaning that the drop should have the minimum possible surface area, which is a sphere. The effect of gravity flattens this ideal sphere into the shape we see. Surface tension can be shown with many other demonstrations, including one in which you add coins to a full glass of water and note how the surface rises well above the rim of the glass: Though teachers often talk about surface tension being like a "skin", this is not really an accurate model, as there is no other compound other than water in the glass. In addition, the surface does not react to being poked with a stick as a skin would. In fact, the water not only sticks to itself, it sticks to the wood as you can see by looking at the arrows in the picture. This is called adhesion because the attraction is to a different substance. In all systems within which water interacts with another surface, both adhesion and cohesion are factors. When cohesion is more of a factor, the water forms spherical droplets; when adhesion is more of a factor, we get sheets of water. water drop on oiled plastic wrap (cohesion of water and only a little adhesion of water to plastic) water drop on untreated plastic wrap (cohesion of water molecules to one another as well as strong adhesion of water to plastic) The wax on the wax paper, it would seem, acts like the oil. You might have read somewhere that wax and oil are hydrophobic and might think that the wax and water are repelling one another. However, this is not quite true, as you will see if you turn a piece of wax paper with droplet on it upside down: Since the water drops stay on the wax paper, there must be some sort of attraction between these materials. It turns out the force of attraction between the water and the wax is actually quite strong, but it is less strong than the water-water (cohesive) force. For a good short discussion of a common misconception about hydrophobic materials, check out the Bad Chemistry page at http://www.princeton.edu/~lehmann/BadChemistry.html#hydrophobic. On clean glass, the forces of adhesion between water and the surface are stronger than they are on oil or wax. The following image is of a mirror half of which I treated with butter (right) and half of which I left alone (left). I then put the mirror in the freezer to cool it and harden the butter, removed it, and dropped some water on it. Notice that the water on the left side (where cohesion and adhesion were both major factors) formed a sheet. On the right, where cohesion won out over adhesion, the water formed drops: So now we have a mental model of what is going on with a drop of water on a surface, and this should help us think about the original problem. The water drop is close to spherical because of the cohesion of the water molecules. (Gravity squashes it, however.) This ball of water, of course, is also somewhat attracted to the wax paper (adhesion); thus, when it is tilted, the surface of the water "sticks" to the wax, and the drop rolls. For more discussion of intermolecular forces, a good webpage is Intermolecular Bonding -- VAn Der Waals Forces, at http://www.chemguide.co.uk/atoms/bonding/vdw.html Now let's look at the glass again: Like the water drop, the surface tension on the water on this glass will be lost if you add some soap to the water. One end of each soap molecule is attracted to the water molecules and comes between them. If one floats a paper clip on the water (bend a bit of the clip up so it is easy to put it on the surface carefully), the effect of soap can be observed easily. This principle is behind the trick of putting soap on your bathroom mirror to keep it from fogging. The fog on a mirror is caused by many tiny drops of water condensing on the mirror and reflecting light in a variety of directions. The soap disrupts the cohesion of the water molecules, causing the water to form sheets (through which the image can be seen relatively easily) rather than drops. By the way, if we come back to our original drop of water on the wax paper, we can break the surface tension by adding soap, and we'll see that the drop changes into a flat puddle. Notice the relatively flat profile: When we reduce the force of cohesion (by introducing soap), the drop changes shape as now the forces of adhesion and cohesion are more nearly the same. One of the problems with an activity like the original water drop experiment is that some students feel like there is one right answer and stop wondering about the phenomenon once this answer is found. Moreover, many people will feel that finding out that the drop rolls is what this is all about, when this is really only the beginning. (In fact, even the book does a poor job explaining why it rolls and leaves one with more questions than answers.) A good teacher, by looking at a phenomenon more carefully herself before class, will have many questions and avenues of research for those students who get to a place where they need a challenge.