Video Transcript
The image below shows a volume of
liquid in a graduated cylinder. Where should the volume of liquid
be read from?
Water molecules exhibit strong
intermolecular attractions with other water molecules. In a sample of water, the
intermolecular attractions are felt equally in all directions by most of the
molecules, but not for molecules that are at the surface of the liquid. These molecules have fewer
interactions with the rest of the water molecules in the sample. To minimize the number of molecules
on the surface and maximize the number of intermolecular attractions between the
molecules, water tends to form spherical droplets as a sphere has the smallest
surface area for its volume. Of course, as the size of the
droplet increases, other effects like gravity can cause the droplet to be less
spherical.
When we add water to a glass
container, like a graduated cylinder, the water molecules at the surface have fewer
intermolecular attractions than the rest of the water molecules. So we might expect the surface of
the water to be domed much like a water droplet, but the molecules at the surface
have strong attractions to the glass container. To maximize the number of
interactions between the water molecules and the walls of the container, the surface
of the water won’t be domed or flat but rather concave. We call this a meniscus.
We can see that the liquid in the
graduated cylinder given in the question has formed a meniscus. But where should the volume of
liquid be read from, the bottom of the meniscus or the top? If we read from the bottom of the
meniscus, the volume that we record will be less than the actual volume by the
amount shown in orange. If we read from the top of the
meniscus, the volume that we record will be greater than the actual volume by the
amount shown in green. We can see in this diagram that the
orange region is smaller than the green region. This indicates that reading the
volume from the bottom of the meniscus produces a smaller margin of error and is
therefore more accurate.
In answering the question, we
should read the volume of liquid from the bottom of the meniscus of the liquid.
What is the volume of the liquid in
the graduated cylinder?
We already know that we need to
read the volume from the bottom of the meniscus, so we need to know what volume this
line on the graduated cylinder corresponds to. We can see that the larger numbered
graduations are marked in increments of five milliliters. Between two labeled graduations,
there are five spaces of equal volume. This means that each of the small
graduations corresponds to a change in volume of one milliliter. The bottom of the meniscus is two
hash marks above the 45-milliliter mark. This means that the volume of the
liquid in the graduated cylinder is 47 milliliters.
