PARTICLE SPACING PART 1

Transcription

1 PARTICLE SPACING PART 1 Name(s) ACTIVITY #1 COMPRESSIBILITY OF LIQUIDS AND GASES Obtain two syringes. Fill one of the syringes about half full of water. Remove any air bubbles left inside. Fill the second syringe about half full of air. In a moment (NOT YET!) you will attempt to depress (push in) the syringe plungers as far as you can while covering the openings of the syringes so that no water or air can escape. In this manner, you will be attempting to decrease the volume of the contents of the syringes. plungers water air Predictions 1) By how much (what fraction) do you think you will be able to decrease the volume of the water? 2) By how much (what fraction) do you think you will be able to decrease the volume of the air? Now see how far in you can push the plungers. Remember to cover the openings of the syringes as tightly as possible so that no water or air can escape. 3) Estimate by how much you were able to decrease the volume of the water. 4) Estimate by how much you were able to decrease the volume of the air. II-5

2 2-2 PARTICLE SPACING 5) Do you think these results would hold true for all liquids and gases, or just for water and air? In order to check this, try filling your syringes with other liquids and gases and see what happens. LIST HERE ANY OTHER LIQUIDS YOU TRY: LIST HERE ANY OTHER GASES YOU TRY: 6) What conclusions can you draw about the compressibility of liquids and gases? 7) Many elementary school age children think that molecules in a liquid are always farther apart than the molecules in a solid. Do you agree? Why or why not? How can you use the model for the particulate nature of matter to explain these results? Discuss this question with your group. Based on your conclusions, prepare a share sheet to display to the rest of the class which addresses the following: A. Why could the volume of the water be decreased as much (or as little) as it was? B. Why could the volume of the air be decreased as much (or as little) as it was? C. Why doesn't the air (or the water) in the syringe push the plunger out even before you attempt to push it in? ****************************************************************** *********** 8) After the class has finished discussing the share sheets, summarize your thoughts concerning the spacings of the particles making up liquids and gases. II-6

3 2-2 PARTICLE SPACING ACTIVITY #2 PHASE CHANGE FROM LIQUID TO GAS Wear safety glasses during this activity. Obtain a small (6 oz.) styrofoam cup full of liquid nitrogen (Liquid nitrogen is abbreviated LN.) Observe the contents of your cup for a few moments. 1) When LN is exposed to room temperature air, the LN boils. Utilize a particulate model of matter to define what we mean by the term "boil". 2) The cloudy mist that appears above the LN is condensed water vapor, not nitrogen gas. Nitrogen gas can not be seen with your eyes. Where is the water coming from? 3) Explain the change that has occurred to this water from a particulate point of view. 4) Why do you think you can see water when it is in the liquid phase but not when it is in the gas phase? II-7

4 2-2 PARTICLE SPACING Your LN was placed in a Styrofoam cup because Styrofoam is a good thermal insulator. A thermal insulator is a material which keeps cold things from warming up too quickly, and it keeps hot things from cooling off too quickly. In order to check that Styrofoam is indeed a good thermal insulator, try pouring some of your LN onto the floor and watch how much more quickly it boils than the LN which is still in the cup. Refill your cup of LN so that it is about 3/4 full. Obtain a large plastic garbage bag and place it on the floor or on the table. Being careful not to spill any LN, put your upright cup of LN at the bottom of the garbage bag. Carefully squeeze out as much air from the garbage bag as you can. Finally, tie the open end of the garbage bag closed as tightly as possible. (Tie it near the end!) upright cup of LN bag tied at end When your garbage bag is securely tied, tip over the cup of LN inside the bag and observe what happens. 5) What has filled up the bag? 6) Explain what you just observed based on your model of the amount of space between the particles making up a liquid and a gas. 7) Using a ruler, estimate the volume of the LN in your cup before it turned into a gas. II-8

5 2-2 PARTICLE SPACING 8) Using a meter stick, estimate the volume of the LN in the bag after it turned into a gas. 9) Based on the two volume estimates you just made, estimate how far apart the particles in a gas are on average. (Be careful. This is tricky.) If you get stuck, your instructor may suggest a way to find an approximate answer. PHASE CHANGES FROM GAS TO LIQUID APPLICATION #1 Place a small, inflated balloon in a small plastic dish (cutout containers made from milk cartons work well for this). Wearing safety goggles and being sure to keep your face away from the balloon in case of splashing, pour LN over the balloon until the balloon completely deflates. Observe the balloon for a few more minutes after the LN has all boiled away. Explain your observations of the balloon based on your model of particle spacing in liquids and gases. II-9

6 2-2 PARTICLE SPACING APPLICATION #2 A. Obtain an empty aluminum pop can, some tongs, a hot plate, and a container of cold water. Use masking tape to reduce the hole size by about 50%. B. Pour about 10 or 15 ml of tap water into the pop can and begin heating it on the hot plate. Position the container of cold water next to the hot plate. C. Heat the can until the water inside boils and fills the can with steam. Give sufficient boiling time to insure that the can is completely filled with steam, but do not let all the water boil away. D. Grasp the middle of the can with tongs and, in one quick and smooth motion, invert the open end of the can into the container of cold water. Observe what happens. 1) Explain your observations based on your models of particle spacing in liquids and gases. 2) What would happen if you heated a completely empty pop can, and then inverted it into the container of cold water? If you are not sure, try it with a new can and see. II-10

7 2-2 PARTICLE SPACING PART 2 PARTICLE SPACING PART 2 The previous activities have indicated that gases tend to take up more space than liquids. This is apparently due to the fact that the particles of a gas are fairly spread out, and the particles of a liquid seem to be in contact with each other. However, even if the particles in a liquid are in contact with one another, one may wonder whether or not there still might be some empty space within a liquid. The investigations in Part 2 will address this question, as well as some others. Activity #1 DISSOLVING Obtain a few spoon fulls of mystery substances A, B, and C. Determine which, if any, of these substances will dissolve in water, and indicate your results below. 1) What do you think happened to the substances which dissolved? (Respond to this question by utilizing a particulate model of matter.) 2) Suppose you combined 10 ml of water with 10 ml of the substance which did not dissolve. What do you think would be the total combined volume of these two substances: greater than, less than, or equal to 20 ml? (Give a reason for your response.) 3) Now combine 10 ml of water with 10 ml of this substance. Stir, and then measure the resulting volume. Report on any discrepancies between what you predicted above and what you found. If there are differences, suggest an explanation for your results. II-11

8 2-2 PARTICLE SPACING PART 2 4) Suppose you combined 10 ml of water with 10 ml of one of the substances which did dissolve. What do you think would be the total combined volume of these two substances: greater than, less than, or equal to 20 ml? (Give a reason for your response.) 5) Now combine 10 ml of water with 10 ml of one of the substances which dissolves. Stir, and then measure the resulting volume. Report on any discrepancies between what you predicted above and what you found. If there are differences, suggest an explanation for your results. 6) Check your results to question #5 by combining 10 ml of water with 10 ml of the other substance which dissolves. Do you obtain consistent results? 7) If a solid substance seems to disappear when it is combined with a liquid, we say that the solid has dissolved. The substance is called a solute. If a solid does not dissolve when it is put in a liquid, we say that a suspension has been made. Explain, using a particle model, the difference between a suspension and a solution. II-12

9 2-2 PARTICLE SPACING PART 2 8) When a substance dissolves, many children think that it is completely gone and that it no longer exists. What might you say to someone who thinks this? 9) Suggest a way you might retrieve a substance which has dissolved in water. ACTIVITY #2 RETRIEVING SOLUTES Fill 2 Styrofoam cups each with about 200 ml of hot water heated to about 80 or 90 C. Dissolve as much of one of the dissolvable substances as you can in one of the cups, and dissolve as much of the other dissolvable substance as you can in the other cup. Label the cups so that you can tell them apart. For each of the solutions do the following: 1. Obtain a little more of each of the two substances. 2. Obtain two strings, each with a length about equal to the height of your cups. 3. Separately rub each string in one or the other of the substances so that crystals of the substance stick to it. (It is helpful if the strings are damp.) 4. Tie one end of each string around a different pencil or straw, and lower the free ends of the strings into the cups having the same solute as you rubbed onto the strings. 5. You may now rest the pencils or straws on the rims of the cups. It will take a few days for something noticeable to happen. ACTIVITY #3 LIQUID PARTICLE SPACING It should be clear by now that the volume of a liquid plus the volume of a solid may not equal the sum of the original volumes. We will now investigate if this may also be true for the addition of two different liquids. Obtain two 100 ml graduated cylinders. Wearing safety glasses, carefully pour 50 ml of water into one of the cylinders, and 50 ml of alcohol into the other cylinder. (Note: the alcohol should have a little dye added to it so that you can make measurements more easily.) II-13

10 2-2 PARTICLE SPACING PART 2 1) In a moment you will combine the contents of the two cylinders. What do you think the combined volume of these two substances will be: greater than, less than, or equal to 100 ml? (Give a reason for your response.) 2) Pour the water into the alcohol, and measure the resulting volume. Is the volume you measure consistent with your response to #1 above? 3) Give a particulate explanation for the results of this experiment. 4) How can all of these activities help explain the fact that liquids expand and contract when they are heated and cooled off, even though the sizes of the particles do not change? II-14