Sink or Float?
Investigating Buoyancy

Background
Some of the biggest mammals ever found on the planet are the sea mammals that live in the oceans. A gray whale, for example, weighs 20-40 tons (18-36 metric tons). The blue whale is even larger! Such massive animals have never existed on land. So what makes it possible for animals so heavy to easily float and move in water? These investigations into density and the properties of salt water can help shed light on that question.

(Activities adapted from Beakman & Jax and used with permission.)

Activity #1: What Floats, What Sinks?

Materials
Slice of soft white bread (the cheapest kind works best)
Large dishpan filled with water

Exploration

1. Before you begin, ask students to free-write for three minutes and then pair up to say what they think they know about what makes whales float. Then share ideas on one large chart. Next, work together to turn students' ideas into questions.
2. Carefully lay the slice of bread on the surface of the water in the pan. What happens?
3. Remove the bread and squeeze into a very tight, small ball. Place the bread-ball on the water. Ask students to note what happens.

Making Connections

• Discuss this question: Why did the slice of bread float, but the wadded-up bread sank? (If necessary, explain that floating is like a pushing contest between the water and objects in the water. Water can push up harder than the soft, porous slice of bread can push down, so the bread slice floats. Squeezing bread into a wad changes its density. Bread that's denser pushes down harder than the force of the water pushing back up, so the ball sinks.)
• How does this help explain how a large sea mammal floats? (Although a whale or manatee is big and heavy, it is not as heavy as the water it pushes away. That's why it floats.)

Activity #2: Water's Pushing Power

Materials
2 glass drinking glasses
1 raw egg in the shell
1/3-cups salt
Hot water (Salt dissolves better in hot water than cool water.)

Exploration

1. Fill both glasses about two-thirds full with hot water. Stir about 1/3-cup salt into one of the glasses. Stir vigorously to make the salt dissolve.
2. Place the egg in the glass of water without salt. What happens?
3. Remove the egg and place it in the glass of saltwater. What happens?

Making Connections

• Why do you think the egg sank in the plain water but floated in the saltwater? (The salt, which you cannot see because it dissolved, changed the water's density. Saltwater is denser than fresh water. Denser water can push up on the egg harder than the egg can push down, so the egg floats. It may help to explain how density is like a suitcase. An empty suitcase is less dense than one that is packed for a vacation. Both suitcases are the same size, but one has more matter, or stuff, in its space.)
• How might the density of saltwater help explain why more immense animals are found in saltwater habitats than in land habitats?
• Look at the list of questions you had when you started. How can you find answers to some of your other questions?

Try This! More Activities

• Create water as salty as an ocean to help you understand saltwater's density relative to fresh water. Here's how: Pass out a cup or glass to each student. To create a solution roughly equal in salinity to ocean water, use these proportions: 4 ounces (118.8 cubic centimeters or milliliters) of warm water mixed with 5/6 teaspoon (4.1 cubic centimeters or milliliters) of salt. Have students stir until the salt dissolves. They can taste (but not drink) the solution. They can then try to float different objects in this water and in the same volume of fresh water. (For a large-scale comparison, explain that ocean water contains about 3.5 pounds (1.6 kilograms) of salt in every 100 pounds or 45.4 kilograms (around 12 gallons or 45.4 liters) of water.)
• Design an experiment to find out how the salt in ocean water affects the temperature at which it freezes. Discuss what this means for the life cycle (feeding and breeding or calving) of gray whales and other sea mammals that spend part of the year in the cold arctic seas.
• Interested students may want to find out what other adaptations enable animals to live in saltwater.

National Science Education Standards

• Use data to conduct a reasonable explanation.
• Objects have observable properties, including size, weight, shape, color, temperature, and the ability to react with other substances. Those properties can be measured using tools, such as rulers, balances, and thermometers.
• Materials can exist in different states?solid, liquid, and gas.