Buoyancy & Salt Water Demonstration

Grade: 8, Unit A

Created By: Heather Denkhaus

Science concept:

Bouyancy, specifically (from the grade 8 curriculum) “changes in buoyancy resulting from increasing the concentration of salt in water” (Alberta Learning, p 31).

Questions and Answers:

Prior to demonstration

• What does buoyant mean? Something is buoyant if it can float, ie buoyant means able to float.
• Can you name some buoyant objects? Answers will vary.

Following demonstration

• What is wrong with the question, “Can you name some buoyant objects?” You don’t know if an object will be buoyant or not, unless you know what substance you are putting it in. That is, the buoyancy of the object may change depending on the material you are placing it in.
• Would the egg float or sink in oil? Explain. The egg sunk in fresh water. Oil floats on water, therefore the egg will sink in oil.
• Which is more dense: fresh water or salt water? Explain. Salt water is more dense, the density of the water increased when salt was added causing the egg to float in salt water but not fresh.

Required materials:

• Table salt, 2 cups
• Measuring cup (1/2 cup)
• 1 egg
• Large, clear container (at least 2L capacity)
• Stirring spoon

Description of the Demonstration:

Fill the large container ¾ full of water. Have students predict whether the egg will float or sink when it is placed in the water-filled large container. Place it in the large container full of water. Remove the egg. Add ½ cup of salt to the water in the large container. Stir. Ask students for a prediction re: whether egg will float or sink in the salt water. Reintroduce the egg into the now salty water. It should float. If it does not, continue to add ½ cup of salt until the egg floats.

NOTE: Practice ahead of time, it’s nice to know ahead of time approximately how much salt will be required to make the egg float.

Explanation:

Adding salt to the water increases the density of the water. It increases the density enough, so that the egg floats. The salt water becomes, on average, more dense than the egg.

Reference:

VanCleave, Janice. (1991). Rising Bottle. Janice VanCleave’s physics for every kid: 101 easy experiments in motion, heat, light, machines, and sound. (pp. 58-59). New York: Wiley.