Why is glass so chemically durable?

Who among us hasn’t broken a window or shattered a glass container in our childhood? Yet, glass stands out for its extremely strong chemical stability, finding use from nuclear waste storage to chemical laboratories. Miriam Koppel, a junior researcher in physical chemistry at the University of Tartu, explained to Novaator why glass is such a durable material.

Glass is a mixture of silicates, typically described by its oxide content. Although glass is widely used in various fields and there are many types of glass with different chemical compositions, for clarity, I will focus on quartz glass (SiO2).

Quartz glass is chemically stable because the chemical bond between silicon and oxygen atoms is very strong. This means that a lot of energy is required to break it. Simply put, a silicon atom is much more suited to form a chemical bond with an oxygen atom than with atoms of other elements.

The chemical bond between two atoms is formed by the electrons located in the outer electron shells of the atoms. The size and electron arrangement of silicon and oxygen atoms are such that the two atoms are at an optimal distance from each other. Additionally, the outer shell electrons can form a very strong chemical bond.

Silicon and oxygen atoms form a network where each silicon atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. All the formed chemical bonds are extremely strong. In this arrangement, oxygen atoms act as bridges between silicon atoms. This atomic arrangement makes glass very inert and durable.

In which fields is the chemical stability of glass particularly important?

The chemical stability of glass is especially important in chemical laboratories, where borosilicate glass is used. This type of glass contains B2O3 (boron oxide), which reduces thermal expansion. Borosilicate glass is used to make funnels, test tubes, beakers, flasks, and measuring cylinders that come into contact with chemicals.

In practice, it is crucial that chemicals do not react with the glass and that the substances in the glass do not enter the chemicals. Therefore, some chemicals cannot be stored in glass containers. For example, hydrofluoric acid and high-concentration sodium hydroxide react with glass.

Glass is also used in pH meters, which measure the acidity of a solution. The tip of the pH meter, which is immersed in the solution, is made of special hydrogen ion-permeable glass. In this case, too, it is important that the glass does not react with the solution.

Due to its chemical stability, glass is used in the pharmaceutical industry for packaging and storing medicines. For example, equipment used to produce certain chemicals is coated with glass on the inside. This ensures the inertness of the environment where the chemical reaction takes place.

Glass can also be used for storing nuclear waste.

What are the new trends in the use of glass?

On the innovative side, bioactive glass is used to help heal bone fractures. Additionally, hollow glass microspheres with a porous coating, ranging from 20 to 100 micrometers in diameter, are being researched for use in cancer treatment.

Of course, window glass is also being developed. For example, some types of window glass have solar panel properties. These windows allow about 70 percent of light to pass through, while the remaining light is directed to solar cells, where it is converted into electrical energy.