Activities for Episode Seven, It’s in the Water Activity 7.1: Understanding Density
In this exercise, you will conduct kitchen experiments to gain an understanding of the effects of temperature and salinity on the density of seawater.
I. Mind Experiments Before we jump into the kitchen, it’s useful to put to the test what we know about density and its relationship to temperature and salinity. Make sure you review the episode and read the relevant sections of your textbook before you attempt this activity.
Complete the four mind experiments below. In each experiment, you will find three samples of seawater (represented as boxes) at different temperatures and salinities. In the blank boxes, place each sample from top to bottom according to its density (which you should be able to figure out based on its temperature and salinity characteristics). Hint: the densest sample should be at the bottom, the least dense at the top. Mind experiment #1 Sample A Sample B Sample C ______T = 20˚ C T = 20˚ C T = 20˚ C S = 33 ppt S = 31 ppt S = 34 ppt
______
______
Mind experiment #2
Sample B Sample C ______T = 10˚ C T = 20˚ C S = 35 ppt S = 35 ppt ______
______
Mind experiment #3
Sample A Sample B Sample C ______T = 15˚ C T = 10˚ C T = 20˚ C S = 33 ppt S = 35 ppt S = 31 ppt ______
______
Mind experiment #4
Sample A Sample B Sample C ______T = 18˚ C T = 10˚ C T = 5˚ C S = 35 ppt S = 33 ppt S = 31 ppt ______
______II. Mind Experiment Discussion
1. Defend your answers for each mind experiment. Why did you choose the order that you did for the samples? What makes one sample more dense or less dense than another sample? 2. What is the “problem” with Mind Experiment #4? How would you solve this problem?
III. Kitchen Experiments Now that you have density in your mind, try the following kitchen experiments to put into practice what you have learned.
In this experiment, you will create samples of water with different temperatures and salinity and place them in a container of fresh water. Before each experiment, you should predict what you will see and write your predictions on the data sheet below. Perform the experiment, make observations and compare your predictions with what you observe. Then complete the discussion questions below.
What you will need 1. At least two colors of food dye. Any color will do but blue and red are best. 2. A large clear container, like an aquarium. A clear pitcher will work okay, a jar is probably too small. You need to be able to put your hand inside it and you need to be able to see inside it. 3. Salt 4. Ice cube trays and some way to freeze water 5. The rubber bulb of a turkey baster, a film canister, or some small rubber object into which you can place water. 6. A glass (plastic or glass). 7. Aluminum foil
Prepare the Experiments 1. Fill an ice cube tray with water. Add food dye. Freeze the water. 2. Add a teaspoon of salt each to two glasses of water. Mix it until all the salt dissolves. Add food dye. 2. Fill another ice cube tray with water and add a few pinches (10-15 grains) of salt into each cube receptacle. Dissolve the salt by stirring. Add a different color of food dye (so you can tell the fresh water ice cubes apart from the salt water ones). Freeze the salt water. 3. Prepare hot fresh water (but not so hot that it burns your hand!). Add food dye. 4. Prepare hot salt water (but not so hot that it burns your hand!). Add food dye. 5. Fill your aquarium or receptacle with room temperature tap water. Perform the experiments Note: make sure you empty the main container and refill it with fresh tap water between each experiment. 1. Fresh cold water in room temperature water Place one frozen freshwater ice cube in the container of tap water. Describe in detail what you see. How does the colored meltwater from the ice cube move in the container? Up? Down? Swirling? Write your detailed observations on the data sheet. 2. Salty room temperature water in fresh room temperature water Slowly pour a few ounces of salt water into the container of tap water. Describe in detail what you see. How does the colored water move in the container? Up? Down? Swirling? Write your detailed observations on the data sheet. 3. Salty cold water in fresh room temperature water Place a saltwater ice cube in a fresh container of water. Describe in detail what you see. How does the colored meltwater from the ice cube move in the container? Up? Down? Swirling? Write your detailed observations on the data sheet. 4. Salty cold water in fresh cold water Place a freshwater ice cube and a saltwater ice cube at different ends of the container at the same time. Describe in detail what you see. Compare the movement of the meltwater from the two cubes. How does the meltwater from each cube move in the container? Up? Down? Swirling? Which meltwater moves fastest? Which moves deepest? Write your detailed observations on the data sheet. 5. Fresh hot water in fresh room temperature water. Slowly pour colored hot fresh water into the room temperature water. Describe in detail what you see. How does the colored water move in the container? Up? Down? Swirling? Write your detailed observations on the data sheet. Repeat the experiment, only this time place the hot freshwater in the turkey baster bulb (or film canister), seal the opening with a piece of aluminum foil, slowly lower it into the main container and remove the foil. Describe in detail what you see. How does the colored water move in the container? Up? Down? Swirling? Write your observations on the data sheet. 6. Salty hot water in fresh room temperature water. Slowly pour colored hot salty water into the room temperature water. Describe in detail what you see. How does the colored water move in the container? Up? Down? Swirling? Write your detailed observations on the data sheet. Repeat the experiment, only this time place the hot salty water in the turkey baster bulb (or a film canister), seal the opening with a piece of aluminum foil, slowly lower it into the main container and remove the foil. Describe in detail what you see. How does the colored water move in the container? Up? Down? Swirling? Write your observations on the data sheet. Data Sheet for Kitchen Experiments
Predictions What do you predict will happen when you perform the following:
1. Fresh cold water in fresh room temperature water?
2. Salty room temperature water in fresh room temperature water?
3. Salty cold water in fresh room temperature water?
4. Salty cold water in fresh cold water?
5. Fresh hot water in fresh room temperature water?
6. Salty hot water in fresh room temperature water? Data Sheet (continued) Observations
1. Fresh cold water in fresh room temperature water?
2. Salty room temperature water in fresh room temperature water?
3. Salty cold water in fresh room temperature water?
4. Salty cold water in fresh cold water?
5. Fresh hot water in fresh room temperature water?
6. Salty hot water in fresh room temperature water? Discussion 1. How well did your predictions match your observations? Did your observations in each experiment agree with scientific theory? Write a brief statement comparing your predictions and observations for each experiment. Where your results were not expected, explain why. 2. What did you learn about the relationship between temperature, salinity, and water density from your experiments? What do your experiments tell you about the nature of “theory” versus “real-life”? 3. What do the results of this experiment predict about movements of water in the ocean? Explain how changes in temperature and salinity create layering of the ocean’s water masses. Discuss how temperature and salinity drive the circulation of the oceans.