Video Transcript
Students drop an egg in a cup of
fresh water. They find that it sinks to the
bottom of the cup. Then, they take that same egg and
drop it in a cup of salt water. The egg ends up floating on the
surface of the salt water. Which table correctly describes the
density of objects in this experiment?
For this question, we need to
describe the density of the objects, which are the salt water, egg, and fresh
water.
Before we discuss density, let us
talk about matter. All of the particles around us,
such as the fresh water, salt water, and the egg in this problem, are considered
matter, since they have mass and occupy space. Since matter has mass and occupies
space, it must also have density. The density of a substance is
expressed as mass per unit volume. The reason matter must have density
is because both matter and density have mass associated with it as well as volume,
which is nothing more than the space a substance occupies.
Now, let us go to the image in this
problem to help us answer this question. The density of water is one gram
per centimeter cubed. If an object were to float in fresh
water, it would be less dense and therefore have a density of less than one gram per
centimeter cubed. And if an object were to sink in
fresh water, it would be more dense and therefore have a density of greater than one
gram per centimeter cubed.
Notice how the egg sinks in fresh
water, so it must be more dense than fresh water. This means that fresh water cannot
have the highest density. Therefore, option (E) is
incorrect.
Now, let us take a look at the salt
water. Notice how the egg is now floating
at the surface of the salt water. Since the egg is floating, it is
less dense than the salt water. Since the egg is less dense
compared to salt water, answer choices (A) and (C) are incorrect, since they both
state the egg has the highest density.
Looking at answer choices (B) and
(D), they both state that salt water has the highest density, which we can infer
since the egg floats in salt water, but not the fresh water. Since the egg floated in the salt
water but sunk in the fresh water, it has a density that is in between the density
of fresh water and salt water. This means the egg has the
intermediate density, not the lowest density.
Therefore, the table which
correctly describes the density of objects in this experiment is answer choice
(D). The salt water has the highest
density, the egg has the intermediate density, and the fresh water has the lowest
density.
