Floating Fruit

Floating Fruit

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Floating Fruit

Concepts Illustrated: Density, buoyancy, displacement, physical properties of matter, absorption and reflection of light, visual interpretation of color.

This demonstration produces quite a discrepant outcome for elementary level students.

Paradox:

Students will be rather amazed to find that when dropped in water, the largest fruit floats while the smallest sinks!

Equipment:

1) A clear tank or aquarium. See the Demonstration Variation section for a version using beakers. A large tub can also be used.

2) Pieces of fruit. The demonstration will be explained using several types of fruit. There is actually a wide selection of fruit to choose from.

3) Water.

Preparation:

Fill the tank, beakers, or bowl with water. Leave enough room at the top to allow for your hands and the fruit to be put into the water without spilling over the rim.

The pieces of fruit are lined up on the desk in the following order: Pumpkin, grapefruit, orange, apple, lemon, lime, strawberry, and grape. Note that this is in order of size, from largest to smallest. Try to select pieces that clearly display this. Students should distinctly recognize that each piece of fruit is progressively smaller from pumpkin to grape.

The Lesson:

Call attention to the fruit lined up on the desk. Ask if anyone recognizes a pattern amongst the fruit. This is a critical step of the demonstration. You want to be certain students identify that each piece of fruit decreases in size from pumpkin to grape, or increases in the reverse order. When lined up in successive order, students are apt to more easily recognize this.

Display the tank and explain that it is filled with plain water. Explain that you will place each piece of fruit into the water, one at a time. Tell them that you will begin with the grape. Pick it up and casually show it. Inform them that you will test each piece of fruit in their successive order on the table, ending with the pumpkin. When you say this, pick up the pumpkin. Be a little dramatic as you do. Struggle with it as you pick it up, reinforcing the stark difference in mass from grape to pumpkin. In comparison to the grape, it is very heavy. Although mass does play a role here, many students think that heavy objects naturally sink while lighter objects float. Ask students to predict whether each piece floats or sinks.

Place the grape into the water. Most students predict that the grape floats, because it is small and light. They are surprised to find that it actually sinks! Many will now think that if the grape sank, the larger pieces of fruit that follow will also sink. But again they are surprised to find that the strawberry which is

Copyright ? August 2020 Vince Mancuso, Ed.D. ()

a little larger than the grape floats! This presents a conflict. Why does the smaller grape sink, while the larger strawberry floats? The contradictions and questions continue with the next pieces of fruit.

The next two pieces of fruit tested present a discrepancy as well. The lime is shown to sink. But the lemon, which is a bit larger than the lime, floats. Even if they are very close in size, most students predict both to float. They are surprised to find that the lime sinks. Many do predict apples to float, and they do. But many then predict oranges, and especially grapefruit, to sink. Yet, they float! The demonstration culminates with the most discrepant observation. Almost all will predict the massive pumpkin to sink. There is great surprise when it floats! The sight of it floating in water is actually rather startling!

Lesson Variation:

The demonstration can be extended by peeling the orange, lemon, and lime. This can serve to launch student investigation, or it can be left entirely to investigation. Students might consider that the rind causes these fruit to sink or float. After removing the rind from each, drop them in water. The outcome surprises most. The lime continues to sink, the lemon continues to float, but the orange now sinks! This phenomenon can lead into some "fruitful" student-led investigation!

The demonstration can also be presented using just limes and lemons. In this case, one 4000ml beaker can be used, or one of each fruit can be separately dropped into two large beakers. A very large bowl could also be used. Select pieces of fruit that are very close in size. When presented with just these two pieces of fruit, most students will predict they both float. They are surprised to find that only the lemon floats, while the lime sinks!

Bananas provide a number of variables to investigate. Use either one half or one third of it in the demonstration. Students will question whether using the whole piece of fruit will affect the outcome. They might also question whether peeling it or removing the stem plays a role in the outcome.

Your demonstration could include a number of other fruit choices. Be creative!

Possible Variables:

1. Different types of fruit. 2. Using fruit slices. This can extend to studying slices of varying thickness. 3. Cutting the pieces of fruit in half. 4. Using wedges cut into various shapes. 5. Removing the rind.

Phenomenon Explained:

The density of an object will determine whether it will float in a fluid. Density is determined by calculating the mass for a given volume, expressed in the formula g/ml, g/cm3, or g/cc. Density is independent of the sample size, since it represents the mass per ml of the sample. A larger volume does not necessarily equate to a higher mass. Fewer molecules packed into a given volume will result in a lower density than many more molecules packed into the same volume, which would increase the density. Objects with a density less than water will float, while those denser than water will sink. The density of water is 1.0g/ml. Whether the pieces of fruit float or sink when placed into water is dependent on their density.

Copyright ? August 2020 Vince Mancuso, Ed.D. ()

The density of each piece of fruit can be calculated by massing it and then finding the volume by placing it into an overflow can, collecting and measuring the water that spills out. Students will discover the densities to be close. Yet their densities do explain why one floats and the other sinks. The density of a lemon is approximately 1.02 g/ml, while the lime is about 1.12 g/ml. Since the density of water is 1.0 g/ml, it makes sense the lemon floats while the lime sinks.

Students might think that differences in the rind cause limes to sink, while lemons and oranges float. Yet when they investigate this by removing the rind from each, they will be surprised. As mentioned, the orange will behave differently once the rind is removed. Air pockets within the rind cause the orange to float. When the rind is removed, the orange sinks. When they see this, most students assume that removing the rinds from the lemon and lime will change their outcome as well. However, their expectations are contradicted when the rinds are removed and the outcomes do not change! The lemon still floats and the lime still sinks!

Since fruits of different colors can be used, the lesson presents the opportunity for a rich discussion regarding the absorption and reflection of light, as well as the visual interpretation of color. Light is made up of a spectrum of distinct wavelengths, each a particular color. Specifically, these are red, orange, yellow, green, blue, indigo, and violet. Objects around you appear to be particular colors because those colors, or wavelengths, are reflected from those objects. All other colors in the spectrum are absorbed. For example, when light hits a green lime it absorbs the entire spectrum of light except for green, which is reflected off the lime. Your retina perceives the color being reflected and transmits that information to the brain for processing. The lime appears green. Essentially, the colors that we see are those wavelengths that are reflected off the object.

Differently colored fruits can also generate a conversation regarding the physical properties of matter. A physical property is a property of a substance that can be immediately observed without changing the identity of the substance. Color is a physical property of matter. It can be immediately observed and it can help identify a substance. When you refer to a yellow lemon, students will know which one you are pointing out. Other physical properties include hardness, shape, size, texture, odor, etc. A few of these can be included in the discussion as the pieces of fruit are referred to.

Standards Alignment:

Next Generation Science Standards (NGSS, Lead States 2013)

Disciplinary Core Ideas in Physical Science

PS1: Matter and Its Interactions

PS1.A: Structure and Properties of Matter

PS3: Energy

PS3.D: Energy in Chemical Processes and Everyday Life

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Disciplinary Core Ideas in Life Science LS1: From Molecules to Organisms: Structures and Processes LS1.A: Structure and Function LS1.C: Organization for Matter and Energy Flow in Organisms LS1.D: Information Processing

Copyright ? August 2020 Vince Mancuso, Ed.D. ()

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