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Erlenmeyer flasks from the Argonne National Laboratory glassblowing shop.

Laboratory flasks are vessels (containers) which fall into the category of laboratory equipment known as glassware. In laboratory and other scientific settings, they are usually referred to simply as flasks. Flasks come in a number of shapes and a wide range of sizes, but a common distinguishing aspect in their shapes is a wider vessel "body" and one (or sometimes more) narrower tubular sections at the top called necks which have an opening at the top. Laboratory flask sizes are specified by the volume they can hold, typically in metric units such as milliliters (mL or ml) or liters (L or l). Laboratory flasks have traditionally been made of glass, but can also be made of plastic.

At the opening(s) at top of the neck of some glass flasks such as round-bottom flasks, retorts, or sometimes volumetric flasks, there are outer (or female) tapered (conical) ground glass joints. Some flasks, especially volumetric flasks, come with a stopper or cap for capping the opening at the top of the neck. Such stoppers can be made of glass or plastic. Glass stoppers typically have a matching tapered inner (or male) ground glass joint surface, but often only of stopper quality. Flasks which do not come with such stoppers or caps included may be capped with a rubber bung or cork stopper.

Flasks can be used for making solutions or for holding, containing, collecting, or sometimes volumetrically measuring chemicals, samples, solutions, etc. for chemical reactions or other processes such as mixing, heating, cooling, dissolving, precipitation, boiling (as in distillation), or analysis.

There are several types of laboratory flasks, all of which have different functions within the laboratory. The most common types of flask are:

Erlenmeyer flask or conical flask.	Round-bottom flask — a flask with a spherical body and one or more necks with ground glass joints.	Retort — a spherical vessel with a long downward-pointing neck.	Florence flask — a flask with a round body and one longer neck without a ground glass joint.
Büchner flask or sidearm flask — a thick-walled conical flask with a short hose-connection tube on the side of the neck.	Volumetric flask — for preparing liquids with volumes of high precision. It is a flask with an approximately pear-shaped body and a long neck with a circumferential fill line.	Dewar flask — a flask with a hollow space in the wall to hold a vacuum.