Density in a Straw Sandra J. Walton

Transcription

1 SCIENCE EXPERIMENTS ON FILE Revised Edition Density in a Straw Sandra J. Walton Topic Density of solutions Time 2 hours preparation; 50 minutes for experimental procedure! Safety Please click on the safety icon to view the safety precautions. Be careful not to spill the colored solutions: Food coloring can stain clothes permanently. Materials 1 cup table salt food coloring: red, green, blue, yellow seven drinking glasses 1-qt or 1-L measuring cup one clear-plastic drinking straw one large pitcher spoon Procedure You will need a helper for part of this experiment. PREPARATION Make a very concentrated solution of saltwater in the pitcher by mixing 3 cups of warm water with 1 cup of table salt. Stir the mixture well, then let it sit for 2 hr. You will need a helper for one step in the following procedure. 1. You will notice that some of the salt in the pitcher has not dissolved, but has settled to the bottom. When salt dissolves in water, it takes up spaces in between water molecules, and there is a limit to the available space. The water in the pitcher has absorbed as much salt as it can, and the remainder has settled to the bottom. The liquid at the top of the pitcher is the most concentrated saltwater solution possible at this temperature. It is called a saturated solution. Pour off 2 cups of the clear liquid from the top of the pitcher into the measuring cup. This will be your most concentrated solution. By adding water to it, you will make a series of progressively more dilute solutions. 2. Line up five glasses on the table. Turn the glasses upside down, and on the bottom of each one, put a small piece of masking tape. Number the glasses 1 to 5 on the tape, and turn them right side up again. Keep them in numbered order. 3. Pour 1 cup of the salt solution from the measuring cup into the first glass. 4. Add 1 cup of warm tap water to the solution remaining in the measuring cup and stir.

2 SCIENCE EXPERIMENTS ON FILE Revised Edition 5. Pour 1 cup of this more-dilute solution into the next glass, and again add 1 cup of tap water to the solution remaining in the measuring cup. 6. Repeat step 5 until you have five salt solutions in the five water glasses, each one more dilute than the last. 7. Have someone else add about 4 drops of food coloring to each of the glasses to make each solution a different color. Use red, green, blue, yellow, and purple (two red drops and two blue drops). Then have your helper mix up the order of the glasses, so you do not know which colors are in which solutions. 8. Using a clear-plastic drinking straw, try to stack two colors so that they will stay in the straw without mixing. For this to work, the least-dense solution must be at the top of the stack and the most dense solution at the bottom. The stacking is done by dipping the straw about a 1 2 cm into one of the solutions, putting your finger over the top, withdrawing the straw, inserting the straw into a different color solution, and then repeating the process (see figure). Place finger over top opening of straw. "stacked" solution 9. Keep testing pairs of colors. On data table 1, keep track of which one is less dense. Each open box on the table represents a pair of colored solutions: one in the horizontal list above the boxes and the other in the vertical column to the left of the boxes. For example, the box in the upper-right-hand corner of the DATA TABLE 1 Color Blue Red Purple Yellow Green Blue Red Purple Yellow Green

3 SCIENCE EXPERIMENTS ON FILE Revised Edition table is for the green and blue pair; the box directly below that is for the green and red pair; the one below that is for the green and purple pair; and so on. In the box for the pair of solutions you are testing, write the color of the solution that was less dense that is, the color on top of the pair if the colors did not mix, or the one on the bottom of the pair if the colors did mix. Dispose of liquids in the spare glass. Be careful not to mix the original solutions. 10. When you have filled in all the spaces on data table 1 by testing pairs, look over your findings, and list on data table 2 the solutions in order of density by color, from least dense to most dense. DATA TABLE 2 Density Color Number Least dense Most dense 11. To check that you have listed them correctly, stack all five colors in your straw, beginning with the least dense and ending with the most dense. If you have listed them correctly, the colors will stay in five separate bands in the straw. 12. Now you have ranked your solutions by color in order of density. Do you think there is a relationship between the density and the concentration of a solution? Consider the definition of density (defined in Connections), and the process you used to dilute each solution from the previous one. Next to the list of colors in the spaces provided on data table 2, write the numbers that you think correspond to the concentrations of the colored saltwater solutions you have listed according to density in step 10 above. Assign number 1 to the solution you think is the most concentrated and number 5 to the least concentrated. Then check the numbers written on the tape under the glasses to see if you were correct. 13. Explain your reasoning for the order you made in step Each glass contains the same volume of solution: 1 cup. Yet each has a different density. What must be different in the solution for this to be the case? Recall the definition of density. 15. If you have had a chance to go swimming in saltwater (such as the ocean) and freshwater (pools and lakes), you will have noticed that you can float more easily in saltwater. Why do you think this is the case?

4 SCIENCE EXPERIMENTS ON FILE Revised Edition What s Going On The numbers will go from 5 to 1, reading from top to bottom. The number 1 glass contains the most-concentrated and the most-dense solution, and number 5 the least concentrated and least dense. The most-concentrated solution is the most dense because it has the most solid material mixed into the water. When the salt dissolves in the water, salt molecules take up some of the empty space between the water molecules. The more concentrated the solution, the more salt and the less space -which also makes it a denser liquid. A dense liquid has more material in the same amount or less space than a less-dense liquid. The leastconcentrated (most-dilute) solution is the least dense because it has the least amount of solid dissolved in the liquid. The mass of the solutions is different. Density = mass/volume. If there is an increase in density when the volume is the same. it must be because the mass has increased. You float more easily in seawater (saltwater) because it is more dense and thus supports your mass more easily. Connections The density of a liquid is defined as its mass per volume. A dense liquid has more material in less space than a liquid that is less dense. Liquids that are less dense will float on top of denser ones: This is why oil floats on water. A solution is a mixture of a solid dissolved in a liquid. A concentrated solution has more of the solid mixed into the liquid than a dilute solution. In this experiment, you explored the relationship between concentration and density in solutions of saltwater.

5 Safety Precautions READ AND COPY BEFORE STARTING ANY EXPERIMENT Experimental science can be dangerous. Events can happen very quickly while you are performing an experiment. Things can spill, break, even catch fire. Basic safety procedures help prevent serious accidents. Be sure to follow additional safety precautions and adult supervision requirements for each experiment. If you are working in a lab or in the field, do not work alone. This book assumes that you will read the safety precautions that follow, as well as those at the start of each experiment you perform, and that you will remember them. These precautions will not always be repeated in the instructions for the procedures. It is up to you to use good judgment and pay attention when performing potentially dangerous procedures. Just because the book does not always say be careful with hot liquids or don t cut yourself with the knife does not mean that you should be careless when simmering water or stripping an electrical wire. It does mean that when you see a special note to be careful, it is extremely important that you pay attention to it. If you ever have a question about whether a procedure or material is dangerous, stop to find out for sure that it is safe before continuing the experiment. To avoid accidents, always pay close attention to your work, take your time, and practice the general safety procedures listed below. PREPARE Clear all surfaces before beginning work. Read through the whole experiment before you start. Identify hazardous procedures and anticipate dangers. PROTECT YOURSELF Follow all directions step by step; do only one procedure at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure that there is adequate ventilation. Do not horseplay. Wear an apron and goggles. Do not wear contact lenses, open shoes, and loose clothing; do not wear your hair loose. Keep floor and work space neat, clean, and dry. Clean up spills immediately. Never eat, drink, or smoke in the laboratory or near the work space. Do not taste any substances tested unless expressly permitted to do so by a science teacher in charge. USE EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives and other sharp or pointed instruments with caution; always cut away from yourself and others. Pull plugs, not cords, when inserting and removing electrical plugs. Don t use your mouth to pipette; use a suction bulb. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges. Clean up broken glassware immediately. v

6 vi Safety SCIENCE EXPERIMENTS ON FILE REVISED EDITION Do not use reflected sunlight to illuminate your microscope. Do not touch metal conductors. Use only low-voltage and low-current materials. Be careful when using stepstools, chairs, and ladders. USING CHEMICALS Never taste or inhale chemicals. Label all bottles and apparatus containing chemicals. Read all labels carefully. Avoid chemical contact with skin and eyes (wear goggles, apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING INSTRUCTIONS Use goggles, apron, and gloves when boiling liquids. Keep your face away from test tubes and beakers. Never leave heating apparatus unattended. Use safety tongs and heat-resistant mittens. Turn off hot plates, bunsen burners, and gas when you are done. Keep flammable substances away from heat. Have a fire extinguisher on hand. WORKING WITH MICROORGANISMS Assume that all microorganisms are infectious; handle them with care. Sterilize all equipment being used to handle microorganisms. GOING ON FIELD TRIPS Do not go on a field trip by yourself. Tell a responsible adult where you are going, and maintain that route. Know the area and its potential hazards, such as poisonous plants, deep water, and rapids. Dress for terrain and weather conditions (prepare for exposure to sun as well as to cold). Bring along a first-aid kit. Do not drink water or eat plants found in the wild. Use the buddy system; do not experiment outdoors alone. FINISHING UP Thoroughly clean your work area and glassware. Be careful not to return chemicals or contaminated reagents to the wrong containers. Don t dispose of materials in the sink unless instructed to do so. Wash your hands thoroughly. Clean up all residue, and containerize it for proper disposal. Dispose of all chemicals according to local, state, and federal laws. BE SAFETY-CONSCIOUS AT ALL TIMES