Density Lab. If you get stuck or are uncertain, please ask questions and/or refer to the hints at the end of the lab. Name: Section: Due Date:

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1 Name: Section: Due Date: Lab 01B-1 If you get stuck or are uncertain, please ask questions and/or refer to the hints at the end of the lab. Density Lab Density is an important concept in oceanography, because the density of an object (a ship, an animal, some water, etc.) determines whether it floats or sinks in ocean water. 1. Names of Group Members: The key rule of density is: The higher density substance sinks and the lower density substance floats. The formal definition of mass density is: Mass Weight Density = = Volume Size When you think about what density means, think of the difference between a densely-packed room at a party and an empty one. Or think about the difference between a dense, heavy cake and a light, airy one. Density is a measure of how much stuff is packed into a particular space Now, imagine how you could make something less dense. The most obvious thing to do would be to remove stuff. For example, in the case of a densely-crowded room, you could remove people from the room. However, you have another option: you could increase the size of the space! For example, if you are worried that your party is going to be crowded, why not hold the party in a bigger room? That would also make it less densely packed too. This is what water does when it is heated: it expands (in other words, it takes up more space); the fast-moving water molecules spread out so that they are not as densely packed.

2 Lab 01B-2 Since warm water molecules take up more space, warm water has a lower density than roomtemperature water, so it rises above and then floats on room-temperature water. Cold water takes up less space, so it is has a higher density than room-temperature water and therefore sinks to the bottom 1 below room-temperature water. Warm Atoms Cold Atoms When you add salt to a glass of water, you are adding both weight and size to the water. (The level of the glass will rise. The salty water will take up more space than the fresh water.) The weight increases the density, but the extra volume (size) reduces it. Salt atoms are heavier than water molecules, though, so you are adding more weight than size. Therefore, salty water has a higher density than fresh water. 1 Ice is an exception. As explained in topic 4A, water molecules can only bond with one another in certain directions to form solid ice. This creates gaps or holes between the water molecules. Thus, ice (solid water) has a lower density than liquid water.

3 Stamps Lab 01B-3 Activity #1: Density, Floating, and Sinking Write your name in this box. At this station, you will practice calculating the density of objects and substances, and you will see that it is the density of an object not just its weight that determines whether the object sinks or floats in water. 1. Calculate the density of: the block of wood a penny using the instruments at the station (ruler & scale). Yours answers should be in grams per cubic centimeter (g/cm 3 ). Show your work in the boxes below (i.e., write down your observations and mathematical calculations). Hints: A penny has a mass of about 2.5 grams. The volume of a cylinder is π (radius) 2 (height). The radius of a circle is the distance from its center to its edge. a penny is a very short cylinder radius cylinder height Mass ( Weight ) of Wood Volume ( Size ) of Wood Mass ( Weight ) of Penny 2.5 grams Volume ( Size ) of Penny Density of Wood Density of Penny

4 Lab 01B-4 Place the block of wood and the penny in the tank of water. 2. Which sinks, the penny or the piece of wood? 3. Which is heavier, the penny or the piece of wood? 4. Which is more dense, the penny or the piece of wood? 5. Which sinks, the heavier object or the higher density object? Water (Density of 1 g/cm 3 ) 6. Sketch the penny and the piece of wood in the tank of water above. Then: write heavier and lighter next to the appropriate objects write the density of the penny next to the penny and the density of the wood next to the wood 7. The density of water is 1 g/cm 3. Based on your observations, under what conditions will an object sink, and under what conditions will an object float? HAVE YOUR INSTRUCTOR CHECK YOUR WORK

5 Activity #2: More Density, Floating, and Sinking Lab 01B-5 8. Determine whether the small plastic canister will float or sink to the bottom of the tank when it contains 4, 8, 12, 16, and 20 pennies. (Don t forget to put the lid back on before putting it in the tank!) Make sure that you keep the inside of the plastic canister and the pennies dry. (Dry them if they re wet!) Number of Pennies 4 Sinks or Floats? Density (in g/cm 3 ) Do not calculate the density until you have answered question Calculate the volume of the small plastic canister. Answer in cubic centimeters (cm 3 ). Remember, that the volume of a cylinder is π (radius) 2 (height), radius and the radius of a circle is the distance from its center to its edge. Measure the radius of the bottom of the cylinder. (Show your work. Be clear. Circle your answer.) cylinder height 10. A penny has a mass of about 2.5 grams & the small plastic canister has a mass of about 5 grams. Using this information and the volume of the cylinder, calculate the combined density of the film canister with the pennies inside in each experiment. Your answers should be in grams per cubic centimeter (g/cm 3 ). Show your work in the space provided above. 11. The density of water is 1 g/cm 3. Based on your observations, under what conditions will an object sink, and under what conditions will an object float? HAVE YOUR INSTRUCTOR CHECK YOUR WORK

6 Activity #3: The Cartesian Diver Lab 01B-6 At this station, you will consider and examine how the size of an object affects its density. 12. Squeeze the bottle with the eyedropper floating in it. What does the eyedropper (i.e., the Cartesian diver ) do? when bottle is squeezed Sketch the diver s behavior into the picture on the right. 13. Now, carefully observe the inside of the eyedropper. What happens to the bubble inside the eyedropper when you squeeze? Why does the bubble change size? If you cannot see the bubble, come get me. Another group may have broken the diver. Sketch the bubble into the pictures above. 14. Why does the eyedropper (i.e., the diver ) sink when you squeeze the bottle? The eyedropper sinks, because its density becomes (A) than the density of the. Why does the eyedropper s density change when the bottle is squeezed? Hint: I had you carefully look at the bubble for a reason. How the bubble changes is an important clue. Possible Answers for A: Higher, Lower HAVE YOUR INSTRUCTOR CHECK YOUR WORK

7 Activity #4: Thermal Expansion and Contraction Lab 01B-7 In this experiment, you will see how a traditional mercury (or other liquid) thermometers works. put water in the pot if necessary (it should be about 1/3 full) using the beaker, and set the stove burner to high. The water is ready when you see steam or there are some bubbles at the bottom of the pot, but the water should not be boiling; once the water reaches this temperature, turn the heat down to medium-low. if the level of the red water in the straw is between 1/3 to 2/3 of the length of the straw, then you are ready to begin the experiment. Otherwise: (a) if the level is too low, push the stopper down into the bottle a little bit (DO NOT make it impossible to get it out again) or take out the stopper, add a little water & red dye, and push the stopper into the top (b) if the level is too high, remove the stopper, pour out a few drops of water, and put the stopper back into the bottle again EVERYONE IN THE GROUP SHOULD WATCH THE LIQUID IN THE TUBE as one student places the glass bottle into the pot of hot water 15. What happens to the level of the red water at first when you put the bottle into the pot? (What happens the instant that you put the bottle into the pot?) Does it go up or down? 16. Continue watching the red water in the tube. Does the level of the red water eventually begin to rise or fall? 17. Well BEFORE the red water reaches the top of the straw, remove the bottle from the pot. What happens to the level of the red water at first when you take the bottle out of the pot? (What happens the instant that you take the bottle out the pot?) Does it go up or down? 18. Continue watching the red water in the tube. Does the level of the red water eventually begin to rise or fall?

8 Lab 01B A few moments after you put the bottle into the pot, the level of the water began to rise. Why did the water level go up? Select the ONE bullet that correctly answers the question, and complete the statement. Change in the Density of the Water: The water (A), causing the water to (B). This (C) the density of the water at the bottom of the bottle, so the water rises upwards. The water at the top of the bottle sinks down to take its place. Change in Pressure on the Water: The bottle and water (A), causing to exert (D) pressure on the water and thus force it up the straw. Change in the Size of the Bottle: The bottle (A) and this caused the bottle to (B), creating (D) space inside the bottle. The water level went up because the water did not have enough space in the bottle and had to go somewhere. Change in the Size of the Water: The water (A), causing the water to (B) The water level went up because the water no longer had enough space in the bottle and had to go somewhere. 20. Why did the water level go DOWN at first when you put the bottle into the pot? Select the ONE bullet that correctly answers the question, and complete the statement. Change in the Density of the Water: The water (A), causing the water to (B). This (C) the density of the water, so the water sinks downwards. Change in Pressure on the Water: The bottle and water (A), causing to exert (D) pressure on the water and thus force it down the straw. Change in the Size of the Bottle: The bottle (A), and this caused the bottle to (B), creating (D) space inside the bottle. The water level went down because gravity pulled the water downwards to fill the extra space inside the bottle. Change in the Size of the Water: The water (A), causing the water to (B) The water did not need as much space, so gravity pulled it down to occupy the bottom part of the straw. Possible Answers: A: gained heat (warmed), lost heat (cooled) C: increased, decreased B: expand, contract D: more, less

9 Lab 01B Sketch pictures illustrating your answers to the two previous questions. Use a red colored pencil for the red liquid. Show how the level of the liquid and the size of the glass bottle changes. (The dotted line shows the original size of the bottle.) In the Beginning Original Level At First in the Pot Later on in the Pot 22. If global warming continues, the oceans will get warmer. Will the warming of the ocean cause sea level to rise or fall? Explain your reasoning. Suppose that the ice at the Earth s Poles does not melt. Evaporation is not a significant factor. There is an important exception to the rule that warm things expand and cold things contract. When water freezes (goes from a liquid to a solid), it expands, because water molecules form a lattice of open hexagons when they freeze, as shown in figures 5.9 and 5.12 on pages 135 and 138 of your textbook. 23. Does the density of liquid water get higher or lower when liquid water freezes into solid ice? Does this mean that ice floats in water or sinks towards the bottom? This only happens when it freezes; water does contract as it cools until it reaches the freezing point, and also it contracts after it freezes too. 24. What evidence have you seen in this experiment that water contracts (gets smaller) when it cools? HAVE YOUR INSTRUCTOR CHECK YOUR WORK

10 Activity #5: Density, Temperature, and Salinity At this station, we will examine how temperature and salinity affect the density of water. Begin the experiment by doing the following: fill the large tank with water as shown in the picture below heat about 200 ml of water in the GLASS JAR for a minute in the microwave add several drops of RED food coloring to the warm water stir the water until it is well-mixed fill the small, black canister to the brim with warm, red water GENTLY lower the black canister beneath the surface of the water in the tank, and place it on the bottom. (The figure below shows you what to do.) Lab 01B What does the red water do? Sketch the red water s behavior into the Figure below. (Use the red-colored pencil!) Ice Cube Warm Salty Now, place an ice cube in the tank. Heat will begin flowing from the room-temperature water in the tank to the ice cube, melting the ice and cooling the water near the ice cube. Place a drop of BLUE food coloring ON the ice cube. 26. Describe what happens to the cooled water next to the ice cube once the dye flows off of the ice cube. Sketch the blue water s behavior into the figure above. (Use the blue-colored pencil!) Next, let s examine the density of salt water: stir a heaping spoon-full of salt into about 200 ml of water add a couple drops of GREEN food coloring stir the salt water until it is well-mixed fill the small, black canister full of green, salty water GENTLY lower the black canister to the bottom of the tank, and tip the canister over on its side (all the way) 27. What does the green, salty water do? Sketch the green water s behavior into the figure above. (Use the green-colored pencil!) EMPTY THE TANK, RINSE IT OUT, AND RE-FILL IT FOR THE NEXT GROUP.

11 Lab 01B Based on your observations, fill in the following table: Water Warm Cold Salty float or sink? Is it more or less dense than room-temperature fresh water? Why is its density higher or lower? The warm water has a (A) density than room-temperature, fresh water because gaining heat makes the water (B). The cold water has a (A) density than room-temperature, fresh water because losing heat makes the water (B). The salty water has a (A) density than room-temperature, fresh water because adding salt makes the water (B). A: Higher, Lower B: Heavier, Lighter, Expand, Contract 29. The side-view below shows the 3 major layers of the ocean. Which layer is the warmest? coldest? least salty (freshest)? saltiest? Write warm, cold, least salty (freshest), and most salty into the appropriate layers below. 100 milliters of water has a mass of 100 grams. 100 milliliters of seawater has a mass of grams. 100 m (330 ft) Mixed Layer 1000 m (3300 ft) Thermocline (or Halocline) Transition Layer To the bottom... ( 3000 m (10,000 ft) ) Deep Ocean Layer HAVE YOUR INSTRUCTOR CHECK YOUR WORK

12 Activity #6: Density and the Buoyancy Force Lab 01B-12 At this station, you will see that the density of a fluid is one factor (among others) that determines how much support an object receives (how hard the fluid pushes upwards on it). 30. Write oil and water in the appropriate layers of fluid in the picture on the right. In other words, which floats on top, oil or water? 31. Which substance floats on top, the higher density fluid or the lower density fluid? Put the hydrometer (the straw with a bit of clay in the bottom) into the fresh water, and carefully note how high the hydrometer floats out of the water. Then, dry it off gently, place it into the oil, and note how high it floats out of the oil. After finishing your comparison, TAKE THE HYDROMETER OUT of the liquids, dry it off, and place it in its holder. Do not leave it in the oil or water!!! 32. Does the hydrometer float higher in the oil or in the water? (In other words, in which case is more of the straw above the surface of the fluid?) Oil Fresh Water 33. Sketch hydrometers into the picture above showing the difference in their behavior. 34. Which fluid is more dense, oil or water? 35. Which fluid gives more support, oil or water? 36. Do eggs float in fresh water or salty water? 37. Sketch eggs into the picture on the right showing the difference in their behavior. Fresh Water Salty Water 38. Which is more dense, fresh water or salty water? 39. Which gives more support, fresh water or salty water?

13 40. Based on your observations fill in the following table: Lab 01B-13 Fluid Oil Density: highest? lowest? in the middle? Support: most, least, in the middle? Fresh Water Salt Water 41. Examine the results in your table. Which provides more support, a high density fluid or a low density fluid? (In other words, which pushes up harder on an object?) Phytoplankton are tiny marine organisms that are incapable of swimming against ocean currents and rely on flotation to keep themselves from sinking to the seafloor. Like plants, they make their own food using sunlight as an energy source. 42. Would phytoplankton get more support from a hypersaline (very salty) lagoon or a freshwater lake? Would phytoplankton get more support from the warm surface waters of the tropics or the cold surface waters of the poles? Explain your reasoning. 43. Where do phytoplankton prefer to live, in fresh or salty water? Where do phytoplankton prefer to live, in warm or cold water? Why? Note: This question is not the same as the question above. The question above is about which water provides more support. This question is about why they need to float: What does floating help them get? Why do they need it? HAVE YOUR INSTRUCTOR CHECK YOUR WORK

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15 Density Lab (1B) Review: Hints & Advice Lab 01B-15 Activity #1 Do NOT use the formula for the volume of a cylinder for the piece of wood, because it is not a cylinder. You should know the formula for the volume of a rectangular piece of wood. If you are not sure, you should confirm your idea with your instructor. Do not forget to show your work. In other words, you need to show your calculations. If you just write down the answer that someone else tells you (or if your work is inconsistent with your final result), you will be marked wrong. Do not forget to include the UNITS after EVERY number: grams (g), cubic centimeters (cm 3 ), or grams per cubic centimeter (g/cm 3 ). Do not forget to write heavy and light next to each object in the picture. Do not forget to write the density of each object next to each object in the picture, including the UNITS! Activity #2 After you calculate the volume of the cylinder, you will use volume of the cylinder to find the density in each experiment, because the size of the cylinder is the same in every trial of the experiment; only the weight is changed in each trial. Do NOT calculate the density by adding the densities of the cylinder and pennies together. This is the wrong way to calculate the density. Instead, you need to calculate the density of the combined object (cylinder and pennies together) by determining the weight and size of the combined object during each trial of the experiment. Do not forget to include the UNITS after EVERY number: grams (g), cubic centimeters (cm 3 ), or grams per cubic centimeter (g/cm 3 ). Activity #3 The air in the bubble does NOT leave the eyedropper when you squeeze the bottle. (The air does NOT go out the top, because the top is sealed; only the bottom is open.) The same amount of air is in the eyedropper at all times, both when you are squeezing the bottle and when you are not squeezing the bottle. Also: the air bubble does not rise when the bottle is squeezed; it is already at the top of the eyedropper owing to its low density, right? When answering question 14, you must explain why squeezing the bottle changes the density of eyedropper. On exams and labs, it will NEVER be satisfactory to answer because the density changes, or the because it became more dense or because it became less dense; you must ALWAYS explain WHY the density changed. In other words, you need to discuss how or why the weight or size of the object changes, and how these changes affect the density of the object. (The density formula contains both weight and size, so a change in either one will change the density.) There are several good answers to question 14. If your group cannot agree on an answer, you should discuss your ideas with your instructor. (Continued on the Next Page)

16 Activity #4 Lab 01B-16 The answer does NOT involve a change in density. It is true that water at the bottom of the bottle is warmed and rises, but when the warm water rises, the cooler water at the top of the bottle will sink and switch places with the warm water. In other words, the water in the bottle will go in a circle, so the change in density which causes this circulation will not change the level of the water. If density is NOT the answer, what does the answer to question 19 involve? Read the title of Activity #4. (This is ALWAYS a good strategy, because the title tells you what the key idea for the activity is.) In your answer to question 19, you need to (1) describe how water changes when it is heated, and (2) explain why this changes the level of the water in the bottle. For question 20, you need to (1) describe how the glass container changes when heated, and (2) explain why this changes the level of the water in the bottle. Do not forget to show how the size of the glass container changes in your sketches for question 21. Activity #5 Answers like because it rose to the top, or the because it sank to the bottom are NOT satisfactory answers to the why questions in question 28, because they do not answer the question that is being asked. Instead, these are answers to the question How can you tell that that water X has a higher or lower density than roomtemperature, fresh water? When answering question 28, you must explain why heating, cooling, and adding salt changes the density of the water. In other words, you need to discuss how or why the weight or size of the water changes, and how these changes affect the density of the water. (The density formula contains both weight and size, so a change in either one will change the density.) The middle layer of the ocean the thermocline is a transition layer between the warmest and coldest water, and between the least salty and saltiest water. Do not write anything in the transition layer: it is NOT the warmest, coldest, least salty, or saltiest layer; its temperature and salinity are always in between the temperature and salinity of the mixed layer and the deep ocean layer. You need to write one temperature description ( warm or cold ) and one temperature description ( least salty or most salty ) in both the mixed layer and deep ocean layer; leave the thermocline blank. Activity #6 DO NOT MOVE THE EGGS OR MIX THE LIQUIDS! Whether or not oil floats or sinks in water is common knowledge. For example, what happens to the oil when there is an oil spill in the ocean? Alternatively, think of your experiences making Italian salad dressing or a vinaigrette. Do not forget to discuss BOTH the temperature AND the salinity of the water when answering questions 42 and 43. The answers to questions 42 and 43 are similar, but there are important differences. In your answers to both questions, you need to discuss where phytoplankton will float better: in warm water or cold water, and in fresh water or salty water. Question 42 asks why phytoplankton float better in some places. Question 43 asks why phytoplankton need or want to float. It is VERY IMPORTANT that you carefully read questions and that your answers address them; you will be marked wrong no partial credit on labs and exams if you do not answer the question that is asked, even if you provide true but irrelevant information.