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A Tale of Two Rivers

The Blue Spring sample Attendance Sheets below depict how far up into the Run that the dark waters of the St. John's River come on several sample days. What characteristic(s) is there in the dark River water that causes it to come up into the Run? Why does the dark water extend farther up into the Run on some days more than on others?

In your Journal, write down all the possible reasons you can think of to explain why the dark water moves to different points in the Run from day to day. After you've done this then conduct the experiment below. You can revsit your Journal again after the Experiment.

This Activity gives you an experiment to explore how water of one temperature interacts with water of a different temperature. The different temperature waters are distinguished by using food color.

Dark St. Johns River

Clear Blue Spring Run water

Ranger Wayne's Attendance Sheets
(Click on image to enlarge and print)

As you look at the sample attendance sheets here, focus in on Ranger Wayne's drawing of how far the dark water comes into the Run. Find the sheet where the dark water comes in the farthest, and find the sheet where the dark water comes in the least. Now look at the River water temperature data for those two days. Which sheet shows the warmest River water temperature? The coldest?





Materials Needed
  • 2 clear glasses
  • A small bowl or cup
  • Tap water
  • Ice
  • Food coloring (See color caution below)
  • A spoon
  • Paper or old towels for clean up
  • A clock or watch with second hand so you can time 15 seconds

Color Caution: In this experiment you are going to be using food coloring, which can color fingers, clothing, the carpet, your dog, the kitchen counter, etc. This experiment should not be messy, but accidents can happen. Wear old clothes or an apron and work over the sink. Clean up with paper or old towels instead of the good kitchen towels.

  1. Put about 10 spoonfuls of room temperature water into the small bowl. Add about 10 drops of food coloring. You can use any color--we used blue. Mix the water and the food coloring well.

  2. Fill one of the glasses with very cold water.(If you use ice to chill it, be sure to remove the ice cubes before starting this experiment). Fill the second glass with hot water from the faucet. Place both glasses in the sink, or someplace where they will not be disturbed and where it won't make a mess if you spill some.

  3. Place the bowl with the food coloring in the sink or near the glasses. Fill the spoon with colored water and VERY GENTLY add the colored water to the glass of cold water. Be sure you don't pour the coloring in. Instead, slowly lower the spoon into the water and then gently pull it out from under the coloring. You do not want to stir or disturb the water any more than necessary. Watch carefully and observe what happens for about 15 seconds.

  4. Now do the same thing with the glass of hot water. Being very careful to gently release the food coloring into the hot water. Again, watch for about 15 seconds. Compare the two. With both glasses, the coloring will begin to mix with the water, but there is a noticeable difference. With the hot water, the coloring will sink quickly towards the bottom. Some of it will mix with the water along the way, but most of it winds up near the bottom of the glass. With the cold water, the coloring tends to stay towards the top and slowly begins to drift downwards. Even after a minute, both the hot and cold water will still have areas where the color is concentrated and areas where the water is clear.

Once we understand and think about hot and cold water, this makes good sense. One spoonful of cold water weighs more than one spoonful of hot water. Cold water is denser than hot water and it tends to sink. When you put the colored water into hot clear water, the coloring sank towards the bottom because it was cooler and denser and heavier. This denser, heavier cool colored water displaces the hot clear water. As the coloring warmed up, its density was nearer to that of the hot water, letting some of it mix upwards.

When you put the colored water into cold clear water, it tended to float near the surface because it was warmer and less dense. The warmer colored water floats, just as cooking oil does on the cold clear water. Both are less dense than cold, clear water. As the colored water cools down, its density becomes more like that of the cold clear water around it and it is easier for it to mix.

  • From the River and Run temperatures on the Attendance Sheets, which experiment, #3 or #4 above, do you think most closely simulates the conditions at Blue Spring? Discuss how the results of that experiment apply to the question of why the dark River water comes up into the clear Run water.

If you have laboratory equipment (thermometer, hot plate etc..), try using waters of a specific temperature to simulate conditions at Blue Spring. For instance, look at temperatures noted on one of the Attendance Sheets above. Make the glass of hot clear water the same temperature as the Run. Then make the colored water the same temperature as the River . Do your results with these specific temperatures differ from the experiment in Step 4 above?

Try This! Journaling Question
  • Why do you think the water of the St. John's River is dark and the water of the Blue Spring Run is clear?
  • What is the source of water for the Run ? The River?
  • What is the length of the Run? The River?
  • Does anything grow in the Run? The River? Why or why not?

(This Experiment was adapted from Robert Krampf's Experiment of the Week email service)

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