ACTIVITY 2: The Small Particle Model and Density of Liquids and Solids--KEY

Transcription

1 CYCLE 5 Developing Ideas ACTIVITY 2: The Small Particle Model and Density of Liquids and Solids--KEY Purpose Most of your investigations in Cycle 4 pertained to gases. Recall that scientists Small Particle Model assumes that all materials, not just gases, are composed of small particles. While some ideas that we developed for gases may have some application to liquids and solids, it is important for us to examine how the Small Particle Model distinguishes liquids and solids from one another, and from gases. In this activity we will answer the question: Initial Ideas How does the Small Particle Model describe liquids and solids and density of liquids and solids? By now you have developed a rich description of the Small Particle Model of gases. Draw and describe a model of how you think liquids differ from gases in terms of particles. Possible student answers: may show/describe liquid particles as larger, closer together (but perhaps still not touching), and having less motion than gas particles. Draw and describe a model of how you think solids differ from liquids in terms of particles. Possible student answers: may show/describe solid particles as larger, closer together (perhaps now touching), and with no motion compared to gas and liquid particles. Participate in a whole class discussion. Be prepared to describe your models to the rest of the class PSET 5-19

2 Cycle 5 Collecting and Interpreting Evidence Experiment #1: Is there evidence of particle motion in liquids? You will need: 50-mL beaker Room temperature water Food coloring STEP 1. Place the beaker on a hard, flat surface. STEP 2. Fill the beaker with about 30 ml of room temperature water. STEP 3. Add one drop of food coloring and allow the beaker to sit undisturbed for a few minutes. Do not swirl or stir the water. Observe the water and food coloring in the beaker during this time. Describe what you see happening to the water and food coloring in the cup. The food coloring moves slowly through the water. STEP 4. Allow the cup to sit undisturbed while you complete Experiment #2. Observe the water and food coloring in the beaker again. Describe what you now see. Eventually the food coloring spreads uniformly throughout the water. Do you have evidence for particle motion in liquids? Explain. Yes, the water and food coloring particles must have been free to move, hence the food coloring being incorporated into the water sample. Do you have evidence for space between particles of liquids? Explain. Yes, there must be space between particles of water and food coloring because the food coloring was incorporated into the water sample. 5-20

3 Activity 2: Small Particle Model of Liquids and Solids If particles of liquids did not move or have space between them, what might you expect to happen when the food coloring droplet landed on the surface of the water? If the particles did not move, we might expect that the droplet of food coloring would stay on top of the water s surface, or if it did penetrate the surface, would stay together as a droplet. If there had been no space between water particles then the food coloring particles would not have a place to go, and would not be uniformly distributed through the water. If there had been no space between food coloring particles, water particles could not have penetrated the droplet in order to break it up. Based on your observations, how do you think the motion and spacing of particles might be different in liquids compared to gases? The particle motion in liquids appears to be slower than in gas at the same temperature it takes awhile for the food coloring to be incorporated into the water. The particle spacing appears to be less than gas at the same temperature we can see water (and other liquids) because collections of liquid particles are so close together. We cannot detect gases with our senses unless they are fragrant or vividly colored, because collections of their gas particles are so diffuse that we cannot see them. Experiment #2: Is there space between particles of solids? In this experiment we will use marbles, sand, and water to create physical models of the small particles of solids. You will need: 150-mL beaker 50-mL graduated cylinder Room temperature water Small marbles or spherical glass beads Sand STEP 1. Fill the graduated cylinder with water to the 50-mL mark. STEP 2. Fill the beaker with marbles to the 100-mL mark. STEP 3. Slowly pour water from the graduated cylinder into the beaker of marbles, until the water reaches the top of the marbles. 5-21

4 Cycle 5 Observe the water level in the graduated cylinder. Record the final volume of water in the graduated cylinder in the space below. Initial: 50 ml, Final:10-20 ml (will vary depending on how tightly marbles are packed) What volume of water was added to the beaker of marbles? ml poured into the beaker filled with marbles. If the total volume occupied by marbles and water is 100 ml, calculate the percentage of the total volume in the beaker that was water. Volume of water added x 100 = % water (v/v) 100 ml % water by volume STEP 4. Refill the graduated cylinder with water to the 50-mL mark. STEP 5. Empty the beaker, according to your instructor s directions. Then fill the beaker with sand to the 100 ml mark. As you are filling the beaker, gently tap it on the table to ensure even distribution of sand. STEP 6. Slowly pour water from the graduated cylinder into the beaker of sand, gently tapping periodically, until the water reaches the top of the sand and all of the visible sand appears wet. Observe the water level in the graduated cylinder. In the space below, record the final volume of water in the graduated cylinder. Initial: 50 ml, Final:10-20 ml (will vary depending on how tightly packed sand grains are) What volume of water was added to the beaker of sand? ml poured into the beaker filled with marbles. If the total volume occupied by sand and water is 100 ml, calculate the percentage of the total volume in the beaker that was water. Volume of water added x 100 = % water (v/v) 100 ml % water by volume 5-22

5 Activity 2: Small Particle Model of Liquids and Solids How does the volume of water added to the marbles compare to the volume of water added to the sand? The volume of water added to each system should be fairly close between ml. In both systems, what does the volume of water added represent with respect to the Small Particle Model? In both systems the volume of water added represents the empty space in the small particle model. What do the marbles and sand grains represent with respect to the Small Particle Model? The marbles and sand grains represent the particles in the small particle model. In what way(s) are the two systems similar? In what way(s) are they different? They are different in that the marbles represent larger particles and sand grains represent smaller particles. The space between a few marbles appears to be much greater than the space between the same number of sand grains. They are also different in that fewer marbles (large particles) can fit into the same volume as the sand grains (smaller particles). Yet, they are similar in that they both have about 30% water by volume (30 % void space). Based on your observations, how do you think the motion and spacing of particles might be different in solids as compared to liquids? There is still space between particles but the spacing seems be less in solids than between liquid particles. Solid particles may not be mobile like liquid particles, but because solids have a temperature (which is a measure of the average kinetic energy) they must be in moving in some way. Perhaps they just wiggle or vibrate in place. In this activity, we modeled small particles of solids using marbles and sand. The marbles represent a solid with larger particles, while sand represents a solid with smaller particles. The volume of water that could be added to each represents the empty space (sometimes known as void space ) that exists between small particles of solids. Just as you determined using these macroscopic models, solids are composed of about 30-40% void space regardless of the size of their particles. 5-23

6 Cycle 5 Simulator Exploration #3: How do particle motion and spacing vary in gases, liquids, and solids? Next we will use the Ultrascope to check our preliminary ideas about the motion and spacing of particles in liquids and solids. Before doing so, you should be aware of a few things. The Ultrascope viewer has a magnification of 6,000,000x for liquids and solids as compared to 3,000,000x for gases. Since the viewing field is much smaller for liquids and solids only 1 nm ( of a centimeter) you will observe fewer particles at a time and they will appear to be larger in size than before (even though they are roughly the same size as the gas particles viewed earlier). What might you expect to see in the Ultrascope viewer if you were to observe a gas at a magnification of 6,000,000x? Explain your reasoning. We would probably observe a black background with a particle or two zooming by every now and then. STEP 1. Go to the simulator index page and open Cycle 5 Activity 2 Setup 1. You should see a container that is half full of liquid, and the Ultrascope Viewer: STEP 2. Run the simulator. As you watch the Ultrascope, think about and answer the questions. 5-24

7 Activity 2: Small Particle Model of Liquids and Solids Do all the liquid particles move in the same direction or in different directions? Particles move in different directions. What causes a liquid particle to change direction? Particles change direction when they collide with other particles (or with the container wall). What is between liquid particles? Why do you think so? There is empty space between particles. If the black consisted of other particles that are smaller than liquid particles, we will still expect the liquid particles to change direction constantly, without having to collide with other liquid particles. We do not observe this. Do all liquid particles seem to move at the same speed or at different speeds? How do you know? By watching the Ultrascope closely, we observe that liquid particles move at different speeds, some slow, some fast. Often, collisions make a slow particle move faster and a faster particle move slower. On average, are liquid particles closer together or further apart than particles of gases, relative to the size of the particles? On average liquid particles are closer together than gas particles. On average, gas particles were ~10 particle lengths apart. On average, liquid particles are 1-2 particle lengths apart, although this varies with time just as with gas particles. Do liquid particles undergo fewer collisions or more collisions than particles of gases? What is your reasoning? We would expect them to undergo more collisions than particles of gases. If they are closer to one another, they have to travel less distance before colliding with another particle. Recall that gas particles are not attracted to one another; rather, they bounce off each other after colliding. Look for instances in which liquid particles slide past one another, or form small clusters for short periods of time. These behaviors suggest that liquid particles have some influence over one another. We can use the same simulator to study solids. 5-25

8 Cycle 5 STEP 3. Stop the simulator, then click the Select tool and double click on the inside of the container to open its properties box. Select SOLID State, Apply, and OK. Notice that the solid particles are separated by void space equivalent to 1-2 particle diameters. On average, are solid particles closer together, further apart, or about the same distance as particles of liquids? of gases? Particles of solids are about the same distance or slightly closer together than liquid particles. Unlike liquid particles, this distance does not vary over time. Particles of solids are much closer together than particles of gases. Do solid particles move? If so, describe how the motion of particles in solids is different from the motion of particles in liquids and gases. If not, describe what you observe. Yes, solid particles are moving constantly but they are not mobile like particles of liquids and gases. Particles of solids wiggle or vibrate in place. Notice the uniform organization of solid particles. Unlike gases and liquids, solid particles are fixed distances from each other (usually one to two particles apart) in three dimensions. The organization of gas and liquid particles is more random at any given instant. Like gas and liquid particles, solid particles are also in constant motion. When particles can move over some distance, this motion is called translation. To represent translation on a two-dimensional drawing, you can use a singleheaded motion arrow pointing in any direction:. When particles can only wiggle around a fixed axis (like solid particles), we call this vibration. To represent this type of motion on a two-dimensional drawing, you can use overlapping double-headed arrows: Taken together, the organization and the motion of solid particles suggest that solid particles have some influence over one another. 5-26

9 Activity 2: Small Particle Model of Liquids and Solids Experiment #4: Is there evidence of forces between particles? In the previous cycles you explored two basic types of forces those that occur through direct contact, or touching, and those that occur over a distance (e.g. gravitational, magnetic, and electric charge). When particles collide and bounce apart, as you observed with the Ultrascope simulators, the forces that particle exert on each other are most similar to a direct contact force. We learned in Cycle 4 that gas particles are not attracted to each other, that is, they do not exert forces on one another over a distance. What about liquid and solid particles? If they do exert forces on each other, which force(s) are involved? What evidence do we have? The way that scientists created the small particle model simulators suggests that there are attractions between particles of liquids (they slide instead of collide, they travel in small clusters, etc.) and solids (the attractions are so great particles are held in place). But, since the small particle model was based on experimental evidence, we should be able to collect some evidence in the laboratory for these ideas. You will need: A piece of wax paper Water Ethanol Hexane 3 beakers (or other containers for liquids) 3 eyedroppers, or plastic pipettes CAUTION: Always handle hexane with care. Use in a well ventilated area and never intentionally inhale fumes. Hexane is flammable, so do not use it around a heat source. Hexane should not be disposed of down the drain your instructor will collect your hexane waste for safe disposal. CAUTION: Safety glasses or goggles should be worn. STEP 1. Lay the wax paper on a level, flat surface. STEP 2. Using a dropper, place a small droplet of water on the wax paper. Observe the water droplet from all angles. 5-27

10 Cycle 5 Describe and/or draw what you see. The water forms an almost perfectly spherical droplet or bead. STEP 3. Empty the dropper. Hold the dropper almost horizontal, place its tip in the middle of the droplet, and slowly drag the dropper in any direction. What happens to the water droplet as the dropper is dragged through it does it smear or does it move as an intact droplet(s)? The water droplet or bead moves intact. What evidence do you have that forces, other than forces due to colliding and bouncing, exist between particles of water? If the collection of water particles moves together as an intact droplet or bead, there must be some forces holding the collection together in this formation. Would you describe forces between water particles as attractive or repulsive? Attractive forces they hold the droplet or bead together. If the forces were repulsive, we might expect the droplet or bead to shatter or the droplet or bead to never form at all. STEP 4. Using clean droppers for each liquid, place small droplets of ethanol and hexane on dry parts of the wax paper. Describe and/or draw the appearance of each droplet when first applied to the wax paper. The ethanol droplet spreads out some, forming a puddle on the wax paper rather than a bead. The hexane spreads out a lot and quickly disappears (evaporates) What do the appearances of the liquid droplets indicate about the strength of forces between particles? (In other words, which liquid has the strongest forces, which has the weakest forces?) Explain your reasoning. The strength of attractive forces between particles must be weaker for ethanol and hexane since they do not form spherical droplets or beads like water. Hexane must have extremely weak attractions for it to evaporate.

11 Activity 2: Small Particle Model of Liquids and Solids Your instructor will demonstrate or show you a video clip of an experiment being performed on a stream of water. As you watch, think about the following questions: What kind of force exists between particles of water gravitational, magnetic, or electrostatic? What is your evidence? Electrostatic the water stream is attracted to a charged object but not to a magnet. Imagine that the experiment was repeated with hexane. What do you think you would observe? The hexane stream might not bend as much since the strength of the attractive force was weaker in the previous experiment. Your instructor will now demonstrate or show a video clip of the experiment being performed with hexane. How did your predicted results compare to the experiment results? There was very little attraction of the hexane stream when the electrostically charged rod was brought nearby. Attractive Electrostatic Force and Potential Energy In the last experiment you observed that, indeed, there is a second type of force that can exist between particles an attractive force that is electrostatic in nature and varies in strength depending on the material. We will call this force the attractive electrostatic force. You studied this force in Cycle 3 Activity 3. The electrostatic attractive force between particles gives rise to many of the physical properties of materials, including their physical state (gas, liquid, or solid) at a given temperature. In the rest of this cycle you will investigate the relationship between the attractive electrostatic forces at the particle level and macroscopic, or bulk, properties of materials. 5-29

12 Cycle 5 Attractive electrostatic forces between particles give rise to a new type of system energy at the particle level. Potential energy is the term used to describe the energy associated with the spatial configuration of the particles in a system. Changes in potential energy accompany changes in the spatial configuration of particles (e.g. if the configuration of particles is stretched or compressed, or particles leave or enter the configuration). Since there is no attractive electrostatic force between gas particles (or it is so weak in strength that it can be ignored), particles move relatively independently of one another and have no spatial configuration with respect to one another. Scientists generally do not use the potential energy term when referring to a system that contains only gas particles. Summarizing Questions Discuss these questions with your group and note your ideas. Leave space to add any different ideas that may emerge when the whole class discusses their thinking. S1. Use checkmarks in the table below to distinguish which descriptions apply to each physical state (gas, liquid, and solid). 5-30

13 Activity 2: Small Particle Model of Liquids and Solids 5-31