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Watch Glass | What are the Types, Uses, functions & Applications?

Watch glass is a glass material widely used in chemistry and biology laboratories, which consists of a concave laminar disk, on whose surface solids or liquids can be placed during countless analyzes or studies. As its name suggests by itself, its physical form is reminiscent of that of pocket watch glasses, or of specimens hanging on the walls.

The solids that are placed on the watch glass can be due to various activities: weighing within the analytical balance, drying of samples, handling of corrosive solids, introduction of capillaries for melting point measurements using a fusiometer, among others. .

Watch glass coated by deposited para-benzoquinone crystals during sublimation. Source: Wikichemistry via Wikipedia.

One of the most common applications of watch glasses is to serve as lids or covers for beakers. For example, if a sublimable solid is heated in a beaker covered by watch glass, the vapor will settle under the watch glass (its convex face) in the form of crystals (top image).

In addition to the aforementioned uses, watch glass is used to contain samples inside flasks, at moderately high temperatures, or inside cryogenic trays, below 0 ºC.

Watch glass features

Watch glasses of different sizes. Source: Navpreet.arr, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Watch glass has the following general and specific characteristics:

-As seen in the image above, in laboratories they are normally acquired in different sizes according to the analysis and the amount of solid to be manipulated.

-In chemistry laboratories, at least at the teaching and business level, they are made of Pyrex glass.

-They are highly resistant to thermal shocks, to attack by acids and alkalis, and to slight mechanical shocks.

-Despite the above, some watch glasses are very sensitive to breaking and scratching.

-They are made of glass, but also acrylic and sapphire, the latter being generally used for making expensive watches.

-They have a concave and a convex face.

-They are very thick.

-They are transparent.

Regarding plastic watch glasses, as they are made of acrylic, they are lighter, resistant to UV radiation, withstand cryogenic temperatures (close to -57 ºC), and are also disposable.

What is a watch glass for?

Spatula and chemical powder in a watch glass

Generally speaking, a watch glass is used to carry solids, carry liquids (small volumes), and cover beakers during heating or magnetic stirring.

Carry solids

Watch glass, unlike beakers, has a concave surface, on which it is easy, due to its width and diameter, to place solid samples of different kinds. These solids can in turn be covered by another watch glass or filter paper, or taken to a desiccator so that they are not too contaminated by air.

Carry liquids

Watch glass is capable of withstanding a remarkable amount of solid. But the same cannot be said for liquids. Only small volumes of liquids are placed on the surface of the watch glass, this for very specific purposes: to carefully observe the precipitation or crystallization processes.

In other words, the transparency of the watch glass and its greater surface area allow direct and real-time visualization of the formation of the crystals, or of a precipitate when a pair of reagents is mixed. This crystallization can be followed much more closely if it is observed with a microscope.

Cover beakers

Another of the most important functions of the watch glass is to serve as a lid for beakers. When the solution in a beaker is stirred magnetically, it is sometimes convenient to keep it as pure as possible, so it is covered with a watch glass to prevent dust or other particles from falling into its contents.

Uses/applications

Specific uses of watch glass will be mentioned below, which however rely on the three functions just described above.

Sample weighing

Heavy magnesium oxide sample on a watch glass. Source: Adam Rędzikowski, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

The watch glass can be placed inside the analytical balance, its mass tared, and used as a support for solid sample weighing; especially those used for the preparation of dissolution or reaction media. When it comes to standards, weighing paper is preferred or using a beaker directly.

Capillary filling

Solids can be carried in watch glasses and their melting points will be determined by using a flushometer. To do this, some capillaries must be sealed, and then proceed to fill them at one end.

The watch glass facilitates this process by pressing the capillary to be filled on it. The operation is repeated several times until the capillary is properly filled with solid.

Solid drying

Solid samples can be placed on the watch glasses to be dried. Once this is done, the watch glass is taken to a desiccator, with the sample protected inside a filter paper, or directly to a muffle with a temperature not exceeding 150 ºC. The watch glass with the sample to be dried can also be placed under a stream of dry air.

Control the evaporation of liquids

The solutions that are heated inside the beakers are also covered in order to slow down the evaporation of the liquid, and thus, ensure better heating and a higher-quality crystal formation.

Solid sublimation

The intro image showed para-benzoquinone crystals deposited under a watch glass. This operation can be repeated with any other sublimable solid so that the watch glass serves for its collection and purification. For example, it is common to do this in iodine sublimation practices.

Demonstrations 

The watch glass can be used in demonstrations of chemical reactions since on it the reagents to be used can be profiled without a problem. For example copper and lithium salts are weighed in different watch glasses and heated on top with fire to witness the colors (green and pink) of their respective flames.

Ingredients

Outside of the laboratory, watch glasses are also used to carry solid ingredients such as seeds, sugar, coffee, fruits, etc. However, this is an unconventional use.

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