CH 112 Special Assignment #2 Density Layers and Lava Lamps PRE-LAB ASSIGNMENT: Make sure that you read this handout and bring the essentials to lab with you. Here are the pre-lab questions for this week. Include your sources. 1. What is density, and how do you calculate it? 2. On a hot summer day, Bob and Rock go jump in the lake. Rock sinks like a stone, but Bob floats peacefully. Who has the greater density? Explain. 3. What is the density of pure water at 20 C? Given this information, estimate Bob s density. INTRODUCTION: Your customer is a senior, Lana Lang, who hopes to start up her own business after graduation: Lana s Lava Lamps. Lana wants to market kid-friendly do-it-yourself lava lamp kits but is unsure what liquids to use. That s where we come in. Density is defined as the mass of a material divided by its volume. Most often, the density of a liquid is expressed as grams per milliliter. A liquid with a low density will float on a liquid with a higher density, which is why oil floats on water. Even if two liquids are quite miscible (that is, capable of being mixed), it often requires significant stirring to comingle the molecules when the liquids have different densities. Therefore, it is often possible to layer miscible liquids with differing densities. Other liquids may not be miscible, despite the most vigorous effort at mixing them, and will therefore layer easily. An example is oil-andvinegar salad dressing that can initially be mixed with shaking but then settles back out into distinct layers when left undisturbed. A solid object in a series of liquid layers will float in a layer that is of the same density or will float between two layers of flanking densities.
LEARNING GOALS: Be able to: Understand the concept of density on a molecular level. Create density gradients by mixing materials with different densities. Calculate the densities of several common household fluids. Estimate the density of a material given its position in a density gradient. Predict the position of a material in a density gradient given its density. PROCEDURE: Part 1. Observing a density gradient 1. Measure out 25 ml of corn syrup and add 2 drops of green food coloring. Mix well a glass stirring rod or wooden stick until uniform. Pour into a 150- ml beaker. 2. Measure out 25 ml of milk and carefully pour it on top of the corn syrup. 3. Measure out 25 ml of lamp oil and carefully pour it on top of the milk. 4. Record your observations. What can you conclude about the relative densities of each liquid? Part 2. Factors that affect density: temperature 1. Add ~50 ml water to each of two 150-mL beakers. 2. Add a few drops of red food coloring to one of the beakers and heat it on a hot plate for ~5 minutes. Do not boil the water. 3. Add a few drops of blue food coloring and a few ice cubes to the other beaker. 4. After a few minutes, pipette ~10 ml of the blue water into a 25-mL graduated cylinder 5. Carefully pipette ~10 ml of the red water into the graduated cylinder. Pipette slowly to prevent the water from mixing. Two distinct layers should form. 6. Record your observations. What can you conclude about the relative densities of the hot versus the cold water? Part 3. Factors that affect density: concentration 1. Add ~60 ml corn syrup and ~20 ml water to a 150-mL beaker. Mix the two until thoroughly combined, and then add a few drops of food coloring. 2. Add this solution to ~80 ml regular corn syrup in a 150-mL beaker. 3. Record your observations. What can you conclude about the relative densities of the diluted versus undiluted corn syrup? 2
Part 4. Calculating the densities of various household fluids 1. Measure and record the mass of a clean, dry 10-mL graduated cylinder. 2. Fill the cylinder to just below the 10-mL mark with one of the various household fluids available in the lab. Read the exact volume to 1 decimal place. 3. Measure and record the mass of the cylinder and the fluid. 4. Repeat the above for at least seven different fluids available in the lab. 5. Calculate the density of each of the fluids. 6. Try mixing 1 ml of each of your test fluids with 1 ml of each other fluid. Which ones mix well with one another? Part 5. Forming a novel density gradient 1. Decide what liquids to use for your density gradient. Your final gradient should have at least six layers and be as colorful as possible. Many of the non-oil-based ones can be colored by adding a few drops of food coloring. Try not to pick fluids that mix easily for adjacent layers. Predict what your density gradient will look like in your lab notebook, using the crayons available in the lab to sketch it. 2. Create your density gradient by adding 25 ml of each layer slowly to your bottle. Start with the fluid of the highest density, adding each successive layer in turn. 3. Record your observations. Did your density gradient agree with your prediction? 4. Make a prediction about what will happen when you sprinkle table salt onto your density gradient. Record this prediction in your lab notebook, then try this. Record your observations. 5. Pick three of the solids available in the lab to add to your density gradient. Record where they fall among the layers, using this information to estimate their densities. References: Hassell, Marshall, Hill (2004) Chemical Investigations for Changing Times, Prentice Hall publishers. Todd, Allie (2010) Potion Density Layers. Colby College CH 151 website. http://www.colby.edu/chemistry/ch151_2010/ch151_2010/activities_1.html Lee, Lyoe (2010) Rainbow Density Layers. Colby College CH 151 website. http://www.colby.edu/chemistry/ch151_2010/ch151_2010/activities_2.html Mahmood, Aqsa (2010) Density. Colby College CH 151 website. http://www.colby.edu/chemistry/ch151_2010/ch151_2010/activities_3.html 3
CraC Special Assignment #2 REPORT SHEET NAME PARTNER DATE DATA SUMMARY Density ranking for milk, corn syrup, lamp oil: > > Density values for household liquids tested: Liquid Density (g/ml) 1. 2. 3. 4. 5. 6. 7. Density ranking for solids tested: > > Approximate densities of these solids: g/ml g/ml g/ml Sketch your final density gradient in the adjacent figure, labeling and coloring each layer appropriately. POST-LAB QUESTIONS 1. Explain your results for the hot and cold water. Why did they layer? 4
2. The water in lakes often layers according to temperature ( stratification ). Where would you expect to find the coolest water? Explain. 3. Explain your results for the diluted and undiluted corn syrup water. Why did they layer? 4. For your density bottle, what would have happened if you had poured the liquids in a different order? Explain. 5. How could you directly measure the densities of the solid objects, rather than inferring them? CONCLUSIONS Use the space below to report on your experiment to Lana Lang, making sure that you address the original goals of this work. 5