Density and Buoyancy (Larissa Karan)

525 Lab demonstration Assignment:

Larissa Karan

Title:

Eggbert and the Situation in Eggtown! Comparing Densities of Different Liquids Challenge!

Principles investigated:

• The density of liquids affects the buoyancy of objects submerged in those liquids.
• Students will be able to recognize that liquids have different densities

Standards:

8^th grade physical science standards set 8: All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:

8a. Students know Density is mass per unit volume

8b. Students know how to calculate density of various substances

8c. Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object displaced.

8d. Students know how to predict whether an object will sink or float.

Materials:

3 1000mL (or larger) beakers

1 raw egg (with face drawn on it with a sharpie marker)

water

Kosher Salt

A spoon to scoop the egg out of each beaker

Student Prior Knowledge:

This demonstration takes place in the middle of an inquiry-based density buoyancy unit, where students have the understanding of the concept of density, and have a background with measuring the densities of solids, as well as testing whether these objects sink or float in water.

Procedures:

This is an adapted demonstration from a colleague, which the main ideas can be found in Dr. Herr’s Hands-on Physics Activities book, on page 143 (section 3.2.5: Fluid Density and Buoyancy). The demonstration can be used as an introduction to comparing densities of various fluids, as part of a Density/ Buoyancy unit for 8^th grade physical science.

1. Days before the demonstration, you have the option of writing “Eggbert is Coming” somewhere on your board. Students may have many questions about who Eggbert is, which increases student engagement for the activity.
2. Fill the first beaker with water. Label beaker with a post-it as “bottom-swim pool”.
3. Fill the second beaker with water, and add kosher salt. Let the kosher salt dissolve. Kosher salt works best for this lab, as kosher salt will allow the water to remain clear once the salt has dissolved. Label beaker with a post-it as “Top-swim pool”
4. The third beaker is filled halfway with salt water and then fresh water is poured on top carefully, not to mix the layers of the 2 different liquids. Label beaker with a post-it as “middle-swim pool”
5. Review prior knowledge of previous labs, where students have tested objects in water to see if they float or sink.
6. If you are going to tell the story of Eggbert, I have a video attached of my telling students the story of Eggbert and the town of Eggtown as well as the PowerPoint I used in the video attached.
7. As you place Eggbert in the Bottom-swim pool, he will sink to the bottom of the beaker. As students what this tells us about the density of the egg vs. the fluid in the beaker.
8. Take Eggbert out of the bottom swim pool with a spoon, place Eggbert in the beaker labeled “top-swim pool”. This is the beaker with salt water, and the egg will float at the top of the beaker. Again, discuss what this tells us about the density of the egg vs. the density of the fluid in the beaker.
9. Take Eggbert out of the top-swim pool, and then place him in the beaker labeled “middle-swim pool”. This time, the egg should be suspended somewhere in the middle of the beaker. Again discuss what this tells us about the density of the egg by where it is located in the beaker.
10. You can then introduce a challenge of trying to reproduce a top-swim. Bottom-swim, and middle-swim pool at home using only edible products that can be found in your kitchen… depending on how many clues you want to provide, or share the secret of how you design the 3 “pools” with the class and invite them to challenge their families at home!

Explanation/ Summary of events: A denser fluid will also have a greater uplift, or buoyancy. Salt makes water denser, and an egg that would sink in freshwater will float in salt water because salt water is more dense than the egg and/or the freshwater. The middle swim pool, shows an example of how fluids of different densities separate, and objects with densities that are in-between the 2 fluids can appear to be suspended in the middle of the beaker, taking their place in the density ordering of objects/ fluids.

Questions and Answers:

1. Why did adding salt to the water create a “top-swim” pool for Eggbert?
2. Eggbert sinking to the bottom of the beaker filled with freshwater tells you what about the egg’s density?
3. Would you float higher in freshwater or seawater? Why?

Applications to the Real World:

1) Would a large Navy ship ride higher in seawater or freshwater? Well, the same principles that we saw with Eggbert apply here. The higher density of seawater due to salt content, gives seawater an average density of 1.027g/mL over freshwater’s density of 1.0 g/mL. This means that the ship will float higher in the salt water over freshwater.

2) Many food products say “shake well before opening”. Although we usually follow these directions without questioning them, the different densities of the components in the product are the reason why we shake them up before opening them. Without mixing, portions of the products ingredients will not be released evenly, such as in many salad dressings.

3) Paints also are composed of different ingredients that have different densities. That is why when you go to the hardware/ paint store, oil-based and water based paints have different densities as well as different ingredients. This is why you need to know what type of paint you used already if you are trying to repaint holes or other imperfections on the walls of your apartment!