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.)
#1: What Floats, What Sinks?
Slice of soft white bread (the cheapest kind works best)
Large dishpan filled with water
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.
lay the slice of bread on the surface of the water in the pan. What
the bread and squeeze into a very tight, small ball. Place the bread-ball
on the water. Ask students to note what happens.
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.)
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.)
#2: Water's Pushing Power
2 glass drinking glasses
1 raw egg in the shell
Hot water (Salt dissolves better in hot water than cool water.)
- 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.
the egg in the glass of water without salt. What happens?
the egg and place it in the glass of saltwater. What happens?
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.)
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?
This! More Activities
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.)
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.
students may want to find out what other adaptations enable animals
to live in saltwater.
Science Education Standards
- Use data
to conduct a reasonable explanation.
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.
can exist in different states?solid, liquid, and gas.
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