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Question Video: Stating What the Volume Is and How the Volume Should Be Measured Chemistry

The image below shows a volume of liquid in a graduated cylinder. Where should the volume of liquid be read from? What is the volume of the liquid in the graduated cylinder?

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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.

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