# Why is a volumetric flask shaped the way it is?

A volumetric flask does not look like an Erlenmeyer flask, but has more of a pear shape. Is it just to distinguish more easily between them while they are on the shelf?

A volumetric flask is designed to store a relatively great volume of liquid with great accuracy. If you want to reach a determinate volume of liquid in a large backer when you add liquid to it so the liquid height increases a little bit the volume increase a lot. What you measure with your eyes for determining the volume is the height of the liquid, so the error in the volume measure is directly linked with the error in measuring the liquid height. The relation between the increase of the height ($$\Delta h$$) and the volume for a cylinder is very simple:

$$V= \pi r^{2} \Delta h$$

So suppose that you make a systematic error $$\Delta h$$ of 0.5 mm, that is quite a good esteem of average max human accuracy in measuring the liquid height in a cylinder with naked eyes. If you make this error in glassware with different radius you will change the accuracy of your measurements, according to the previous equation:

So if you use ten times greater radius you will have a 100 times greater error!

# Why the body has not the conical shape of the Erlenmeyer?

This actually is not so important. Generally, the spherical surface more evenly distributes stress across its surface and is less prone to fractures. This is why we use round-bottom flasks. It doesn't seem there are a lot of differences in the production of both Erlenmeyer and volumetric flask, so it's probably mainly due to a convention. In fact, you can find also volumetric flask with Erlenmeyer style. Like in the image below.

• This explains why a narrow neck is needed, but it doesn't imply that the body shouldn't be Erlenmeyer shaped. – cbeleites Jul 10 at 9:52
• @cbeleites thanks for the comment you are right, I think that this is the main motivation. I've expanded the answer. – G M Jul 10 at 13:31
• Speaking of roundbottom flasks, those things are just absolute tanks. I've dropped them in the past after washing them, straight to the floor, and had them just bounce instead of shattering, not even any visible crack. – AmphotericLewisAcid Jul 10 at 14:38
• @AmphotericLewisAcid the magic of spherical shape or you might consider investigating the properties of the compounds you were producing maybe you discover a glass reinforcement compound – G M Jul 10 at 17:50

When the glassware was made by hand, the bulb at the bottom varied in size, so each flask had to be filled with the correct volume before the line could be etched on the glass. A tube was used for the neck since the volume filling the tube would change at a consistent rate. So you have a bulb holding most of the liquid, and a tubular neck to indicate when you have the correct volume. The flask looks like a pear shaped container.