After this activity, students should be able to:

• Define density and viscosity.
• Compare and contrast density and viscosity.
• See that visualizing a fluid flow can both show the physics that are occurring, and be beautiful.

Two properties of fluids that are often easily confused are density and viscosity. What is density? What is viscosity? Density is the measure of mass per unit volume, or how heavy a fluid is. Viscosity is a measure of how stiff a fluid is, or how easily, or quickly, it flows.

It is easy to measure the density for liquids; just see how much it weighs. What is interesting is when fluids of different densities get together; the densest fluid sinks to the bottom and the less dense fluid stays above it. You may have seen this before if you have noticed how motor oil floats on top of water in a puddle.

Within a single fluid such as air or liquids, the cooler the fluid, the denser it gets. The idea that 'heat rises' comes from the fact that hot gases are less dense than their cooler neighbors, so up they go. In the atmosphere, this effect is responsible for cumulus clouds, which form from rising heated air. In the ocean, salty water is denser than fresh water. The dense, saltier water stays deeper in the ocean until other forces make it move. On a grand scale, the ocean is denser than the atmosphere, so it stays below the air around us. Imagine if it was the other way around!

When dense fluids stay below less dense fluids, we end up with stratified fluids. In the density rainbow activity, we will layer liquids of different densities and colors. The densest ones go in first, on the bottom of the glass. Stratified fluids are pretty stable, so the rainbow we create will last until you stir it up. If you try to pour a heavier fluid on top, your combination becomes unstable, and the new, denser fluid sinks to the bottom. This is called the Rayleigh-Taylor instability.

Viscosity measures the resistance of a fluid to flow. For example, it takes longer to pour corn syrup out of a bottle than to pour water from a similar bottle. This shows that corn syrup is more viscous than water. Now, corn syrup is both denser and more viscous than water, but viscosity and density do not always go together. Absolute viscosity is independent of density, but because dense fluids usually are more viscous, the two properties are often confused. In the Great Viscosity Race, we will compare the speeds of two fluids, and then compare their densities. Will the less dense fluid win by being less viscous too? Let's find out!

Before the Activity

• Divide the class into teams of two or three students each.
• Gather materials and make copies of the Student Instructions Handout, one per group, and the Density Rainbow and the Great Viscosity Race Worksheet, one per person.
• Set out the chocolate syrup and ketchup to warm to room temperature.
• Prepare one station or table for each group, as described below.

Figure 2. Use two pieces of white paper to create a white background. click for copyright

1. To set up the stations, choose table top surfaces that are cleanable. For the best observations, position the back of each table against a wall.
2. On the wall behind each station, tape a piece of white paper so that one edge of the paper aligns with the edge of the tabletop (see Figure 2).
3. Tape another piece of white paper on the table so that the edges of this paper align with the one already taped to the wall (see Figure 2).

4. Set 5 disposable cups at each station and fill each one quarter full with a different fluid (corn syrup, strawberry syrup, shampoo, cooking oil and isopropyl alcohol). Label each cup with the fluid it contains.
5. Place glass container, food coloring and a spoon at each station.

With the Students

Part 1: The Density Rainbow

• Dye the corn syrup, alcohol and shampoo different colors by dropping 1 or 2 drops of food coloring into each cup. Make sure not to dye a fluid pure red because the strawberry syrup is already that color. You can also experiment with mixing different colors of food coloring to obtain colors such as purple and orange by dropping in drops of two different colors into a fluid. It is important that no more than 2 drops of blue are dropped into the fluid because too much food coloring makes a very dense color which, in turn, looks black. Record on your worksheet what color is used for each fluid.
• Stir each dyed fluid thoroughly with the spoon/mixing rod. (Remember to rinse the spoon or mixing rod so the food colors do not mix, or use disposable plastic spoons or stirrers.)

1. Answer questions 2-3 on your worksheet.
2. On your worksheet, predict which fluid will be the densest, and which will be the least dense.

5. As a team, decide on the order of fluids, from the most to least dense. Then pour what you think is the most dense fluid into the glass container first, being careful not to drip any on the sides of the container. Slowly pour the next densest fluid on top of the previously poured fluid. If poured slowly, no mixing should happen between them and the layers will remain visible. If the container is very tall, use a spoon to ease the fall (see Figure 3). Although some mixing will probably happen, your fluids will settle after a while (see Figure 4). Keep pouring in the fluids in order of density (the most dense at the bottom, and the least dense at the top).
6. When you poured in the fluids, did they stay in the order you poured them? Or did some that you thought were denser move up a layer? Why did the fluids set in that order? Answer question 5 on your worksheet.
7. The rainbow takes at least five minutes to settle out and have distinguishable layers (see Figure 7). After Part 3, all materials can be poured down the drain for disposal.

Figure 3. The 'spoon-method' of pouring green-dyed shampoo into a density rainbow. click for copyright

Figure 4. A rainbow created by fluids of different densities settles after pouring of the fluids. click for copyright

Part 2: The Great Viscosity Race

1. Our two racing fluids are chocolate syrup and ketchup.
2. First, see which is denser. Pour about 1/3 cup (78 ml) of chocolate syrup into the clear 8 oz (.24 l) drinking glass.
3. Pour ketchup into the glass, above the syrup. If you pour it in slowly, it should stay above the syrup, showing that the ketchup is less dense than the syrup.
4. Answer questions 6-8 on your worksheet.

5. Place a white piece of paper on the table, and at one end of the paper, place a small spoonful of ketchup next to a small spoonful of syrup. Make sure that they are not touching (see Figure 5).
6. Based on your observations so far, which liquid do you think will flow faster down the sheet? Answer question 9 on your worksheet.
7. Tilt the piece of paper to a vertical orientation. Observe which fluid flows faster down the paper (see Figure 6). The winner is the less viscous fluid.

Figure 5. Drops of chocolate syrup and ketchup placed on a white piece of paper lying on a table. click for copyright

Figure 6. Tilting the paper to compare the flow of chocolate syrup to ketchup. click for copyright

8. Compare the ranking of viscosity to the ranking of density for the ketchup and chocolate syrup. Was the least dense also the least viscous? Answer questions 10-12 on your worksheet. After Part 3, all materials can be poured down the drain for disposal.

Part 3 (optional): The Density Rainbow Revisited

1. Return to your density rainbow container and observe how the fluids have separated (see Figures 1 and 7).
2. Position a black background behind the density rainbow.
3. Experiment with the clip light and camera position so that the light glows through the density rainbow while the rest of the image is dark. Adjust your camera to make the rainbow fill the composition. Use the camera to capture stunning pictures of the rainbow.
4. Clean-up: All materials from the rainbow and race can be poured down the drain for disposal.

Figure 7. The result: A rainbow created by fluids of different densities. click for copyright

• Warn students that the food coloring can stain their clothes and fingers
• Warn students that alcohol stings if comes in contact with cuts or eyes.
• Remind students that the shampoo and isopropyl alcohol are not for drinking.
• In general, foods and hair/skin products can be safely mixed together, but household cleaners (cleaners for glass, laundry, floor, etc.) should not be mixed with anything.
• Materials from the rainbow and race can be disposed of down the sink.

Pre-Activity Assessment

Activity Embedded Assessment

Post-Activity Assessment