Unit #1

Activity 19
Don’t be So Dense - You Might Sink to the Bottom, or,
How Do Whales Dive So Deep?

Activity Overview

Density is a description of how much mass is crammed into how small a space.

Part A: The students observe several plastic soda bottles filled with various substances and discuss the varying density of those substances. After being given the numerical density of each substance the students observe while some are placed in an aquarium tank filled with water. After the first observations, students are asked to predict whether the next couple of bottles will sink or float.

Part B: Students use the computer to measure the density of various combinations of atoms with varying mass and varying space between the atoms.

Learning Objectives

Students will:

• Describe how changing the mass or volume of a fixed number of atoms will affect the density of those atoms.
• Predict if something of known density will float in another substance of known density.

Conceptual Prologue

Macro-Micro Connection

The key factor in why a hot air balloon floats has to do with the density of the air inside as compared to the density of air outside the balloon. Less dense substances float in more dense substances. The bigger the difference in density and the larger the volume of the low density substance, the greater the buoyant [upward] forces will be. Heating the air causes the molecules to spread further apart, lowering the density of the gas inside the balloon.

Other macro connections:

• We can explain or at least predict why certain things float or sink.
• Boats made of very dense metal float because the air sealed inside becomes part of their volume in the measurement of density.
• Whales can dive or surface without swimming by changing the density of a large organ in their head. When the density is high they sink, when low, they float, allowing them to conserve energy and not have to swim to change depths.

Science Concepts

To calculate density, one needs an object, or if measuring the general density of a substance, a sample of that substance. Calculating density involves knowing the mass and volume of that object or sample. Conceptually, density is a description of the concentration of mass, or the ratio of mass to volume. If a lot of mass is crammed into a small space, then a high density is measured. If the mass is spread out over a large volume, then the density is low.

When comparing two substances, imagine that you have a sugar cube sized piece of each substance. The one which is more dense will weigh more because it has more mass crammed into the same volume (both sugar cube sized pieces are the same size). Because gravity will pull harder on the piece that has more mass, less dense things or substances tend to float on more dense substances.

Balloons that float do so by having an overall density that is less than the air surrounding them. This can occur in two ways:

• Fill the balloon with a gas that is less dense than air. Helium gas is typically used in blimps and balloons associated with parties.
• Heat the air inside the balloon so that the atoms move faster, bumping into each other harder, and causing them to spread apart more. If the same mass (all the bumping gas atoms) takes up more space, then the density will be lowered.

Naive Conceptions

• Density relates only to the size or mass of an object.

Students almost always interchange the word heavy with dense. They don't understand that a very tiny piece of gold which is not heavy is still one of the most dense substances on Earth. Density is the ratio of mass to size (or volume), so something which is very small, but dense should have a high mass when compared with something else of the SAME volume.

Activity Design and Execution

Major Science Concepts:	• Density • Buoyancy
Assumed Previous Knowledge:	• That everything consists of atoms. • The atomic level structure of liquids, solids, and gasses.
Time:	• Part A: approximately 30 minutes • Part B: approximately 30 minutes
Materials:	• Several clear soda bottles of the same size and shape filled with the following: - Ethanol - Water - Air - Iron Filings - Vegetable Oil - Baking Powder
Advanced Preparation: (if any)	• None.

Investigative Question: What is density and when will things float?

Part A:

1. Show students the various bottles filled with different substances. Pass them around to let them get a feel for the mass of each bottle.
2. Ask them what is the same and what is different about the bottles.
3. Ask them if they could predict which one would be heaviest if all the bottles were half the size.
4. Talk about density as a way of describing how much something of a specific size weighs. That it is a number which describes the concentration of mass into a specific volume. The more mass crammed into a smaller space, the higher the density.
5. Give them numerical numbers for the density of everything except the vegetable oil.
6. Put the bottles containing air, water, and iron filings in the water. Ask the students what patterns they notice about which things float and which sink.
7. Ask them to predict if the ethanol and baking powder will float. Then try these.
8. Then put the vegetable oil in and ask the students to explain what they know about the density of vegetable oil.

Part B:

1. Using the Flash plugin already installed in your browser, have students bring up the "Density" simulation.
2. The software will show students a bunch of atoms and a way of choosing which state of matter to study - solid, liquid, or gas - and a way to choose the mass of the atoms displayed. Graphs of mass, volume, and density will also be displayed.
3. If gas is chosen as the substance to study, then a volume adjustment would also be made available.
4. The software will ask them to try experimenting with the various controls and describe what they notice about the relationship between mass, volume, and density.
5. The software will then ask them why they were only allowed to control the volume for the gas state of matter.
6. The computer will then ask them to come up with two different ways to make something very dense and two ways to make something with low density.
7. The software will then present them with a large density and ask the students to find two different ways to achieve this density.
8. Have a discussion about what they learned about density and relate the computer pictures to the various substances used in the demonstrations. Maybe even have them try to construct a computer model that looks like the most dense and least dense substances used.

Assessment

Have students write several things in their notebooks:

1. Why does a block of concrete sink, but a canoe made out of concrete floats?
2. Some whales can dive or surface without swimming. How must they be able to change their bodies in order to do this?
   

Internal Notes:
•See computer lab H for the mock up of part b of this activity.