Understanding Buoyancy

Understanding Buoyancy ~20 Minutes

Objectives: To investigate the buoyancy of eggs in fresh and salt water

Outcomes:
-identify the properties of solids, liquids, and gases (grade 2)
-investigate the interactions of liquids and solids (grade 2)

Materials: Coarse Salt, Water, Eggs, Beakers

Doing it:
A1. Students in groups of 3, send one student from each group to collect materials: 3 eggs and 4 beakers. Number each beaker.(2 min)
A2. Students mix fresh water and salt. Fill beakers 1 & 2 with 500mL of fresh water and beaker 3 and 4 with 250mL of fresh water. Add 100mL of salt to beaker #2 and 50mL of salt beaker #3. Stir until the salt dissolves.(5 min)
A3. Fill in the POE sheet for beakers 1 and 2. (2 min)

Fig 1.1 - Eggs in the fresh water(left) and salt water(right).

B1. Place eggs in beakers 1 and 2. What do you notice? Were your predictions right? (4 min)
B2. Fill in the POE sheet for beaker 3(2 min)
B3. SLOWLY add fresh water from beaker #4 to beaker #3 with the egg. It helps to slowly pour the water onto the top of the floating egg as close as you can get. What do you notice? How is the egg floating in the middle of the water?(4 min)

Fig 1.2 - The egg in the salt water once the fresh water was added. The egg is suspended between the layers. Some green food colouring was added to the fresh water before it was added.

C1. Explain what is happening at each step to the students. For the mysterious 'floating egg' in beaker 3, it is easy to explain by repeating that step in front of the class but adding some colouring to beaker #4 before they are combined. (2 min)
C2. Any questions and a short closing discussion about density and how it affects floating. (4 min)

Notes:

  • PDF Worksheet
  • Explanation:
    This experiment deals with density. The egg's density is somewhere in between the fresh water's and the salt water's, such that when the egg is in salt water it has a lower density and will float above the water. This occurs since gravity is pulling harder on the water than the egg. The reverse occurs in the fresh water, gravity is pulling harder on the egg than the water, and the egg sinks. With the mixture of salt and fresh water, the same logic holds. Gravity pulls hardest on the salt water, then the egg, then the fresh water, which is the order that the objects are observed in.