Video Transcript
The densities of water and a
certain oil are given in the image below. An object is found to sink in the
oil but float on water. What can be assumed about the
density of the object? (A) Its density is greater than 0.9
grams per centimeter cubed but less than 1.0 grams per centimeter cubed. (B) Its density is less than 0.9
grams per centimeter cubed but greater than 1.0 grams per centimeter cubed. (C) Its density is greater than 0.9
grams per centimeter cubed and greater than 1.0 grams per centimeter cubed. (D) Its density is less than 0.9
grams per centimeter cubed and less than 1.0 grams per centimeter cubed. (E) Its density is exactly 0.95
grams per centimeter cubed.
Density is a value which we give to
a substance or object when we measure its mass and divide this mass by the
substance’s volume. We usually measure mass in grams,
symbol lowercase g, and volume in centimeters cubed, unit symbol cm to the power of
three.
We can measure the mass of a
substance on a scale or balance. This diagram shows an electronic
scale. And we can use various methods to
measure the volume of a substance or object in centimeters cubed.
Another way to define density is to
say it is the mass per unit volume of a substance. We can see this definition tells us
the same thing as the formula. This is the mass. The divide line means in or
per. And this is the volume. So we have mass per unit
volume. Density is usually measured in
grams per centimeter cubed.
Another way to think about density
is how compact the particles in the substance or object are or how much mass is
squeezed into a certain volume. We know that if we had a balloon
the same volume as a rock, that the rock would be much heavier. The rock would have a greater
mass. This is because the rock particles
are packed very closely together. And there are many more particles
in the rock than there are air particles in the balloon. Rock is much more dense than
air. Rock has a high density compared to
the air in a balloon.
Now, let’s look at the
question. We are given two containers. One container holds oil. Oil has a density of 0.9 grams per
centimeter cubed. The other container holds
water. Water has a density of 1.0 grams
per centimeter cubed. We are told that when an object is
put into oil, the object sinks. And when it is put into water, it
floats. We are asked what we can assume
about the density of the object.
Let’s look at the first
picture. The object sinks in the oil because
its particles are more tightly packed and it has a larger density than the oil. In the second picture, we see that
the object floats on water. This is because the object’s
particles are less tightly packed than water’s particles. The object is less dense than
water, and so it floats. Water has a larger density than the
object. We can see that the object has a
density between the density of oil and water.
Since oil has a density of 0.9
grams per centimeter cubed and since water has a density of 1.0 grams per centimeter
cubed, we can assume that the density of the object is between 0.9 and 1.0 grams per
centimeter cubed. When we look at the answer options,
we can see that this matches answer option (A). The object’s density is greater
than 0.9 grams per centimeter cubed but less than 1.0 grams per centimeter
cubed.
