In chemistry, glass is an amorphous solid, meaning its atoms are arranged in a disordered, non-crystalline pattern. Unlike table salt or diamond, where atoms lock into neat, repeating grids, the atoms in glass are frozen in a random arrangement, more like a liquid that has been caught mid-flow and solidified. This distinction between ordered crystals and disordered glasses is one of the most fundamental concepts in materials science, and it explains nearly everything about how glass behaves.
How Glass Differs From Crystalline Solids
Most solids you encounter, from ice to metals to gemstones, are crystalline. Their atoms or molecules stack into a repeating three-dimensional lattice, the way bricks form a wall. Glass skips this step entirely. When molten glass cools, it solidifies before the atoms have time to organize into a crystal. The result is a rigid material with no long-range order.
That said, glass isn’t completely random at the atomic level. Atoms can’t overlap or sit arbitrarily close together because their electron shells repel each other. So glass does have short-range order: each atom has a predictable number of nearest neighbors at a predictable distance. Beyond that first shell of neighbors, though, the pattern dissolves into disorder. Think of it as a crowd of people standing at arm’s length from each other but with no rows, no columns, no grid.
The Glass Transition Temperature
Glass doesn’t melt or freeze at a single sharp temperature the way ice does at 0 °C. Instead, it passes through a range called the glass transition temperature (often written as Tg). Above Tg, the material behaves like a thick, viscous liquid: molecules can slide past one another slowly. Below Tg, molecular motion essentially stops, and the material becomes rigid and brittle.
What changes at Tg is the amount of free volume between molecules. As temperature drops, molecules pack more tightly, free volume shrinks, and eventually there isn’t enough room for molecules to rearrange. The material locks into whatever random configuration it happens to be in at that moment. Properties like stiffness, heat capacity, and thermal expansion all shift dramatically across this transition. It’s not a phase change in the traditional sense (no latent heat, no crystal formation), but it produces an equally dramatic change in how the material feels and performs.
This is also why glass is sometimes called a “supercooled liquid.” Technically, the atoms never reorganized into a crystal, so the material is a liquid that cooled past its melting point without crystallizing. In practice, below Tg it is rigid enough to shatter, so calling it a liquid can be misleading. It’s more accurate to call it an amorphous solid.
Why Glass Is Transparent
The transparency of glass is one of its most useful properties, and it comes down to how light interacts with electrons. When a photon hits an electron, it can only be absorbed if its energy matches the gap between the electron’s current energy level and a higher one. In metals, electrons occupy a continuous band of energy states, so photons of almost any wavelength get absorbed. That’s why metals are opaque.
In silicate glass, the energy gap between electron levels is larger than the energy carried by visible light photons. Because no electron can absorb a visible photon (the energy doesn’t match any available jump), the light passes straight through. Ultraviolet light, which carries more energy per photon, can bridge the gap, which is why ordinary glass blocks some UV radiation. The specific chemistry of the glass determines the size of that gap, and therefore which wavelengths pass through and which don’t. Adding metal oxides to the mix introduces new electron energy levels, which is how colored glass gets its color.
What Glass Is Made Of
Silicon dioxide (SiO₂) is the backbone of most common glasses. Pure silica glass exists and is used in fiber optics and laboratory equipment, but it requires extremely high temperatures to melt. To make glass production practical, manufacturers add other compounds that lower the melting point and modify the material’s properties.
Soda-Lime Glass
This is the glass in your windows, bottles, and drinking glasses, accounting for roughly 90% of all glass produced. A typical soda-lime composition, based on a National Bureau of Standards reference material, is about 71.7% SiO₂, 14.0% sodium oxide (Na₂O), and 11.5% calcium oxide (CaO) by weight. The sodium oxide (from soda ash) lowers the melting point dramatically, making the glass easier and cheaper to produce. The calcium oxide (from limestone) improves chemical durability so the finished glass doesn’t slowly dissolve in water.
Borosilicate Glass
Borosilicate glass replaces much of the sodium and calcium with boron trioxide (B₂O₃). A standard borosilicate composition is roughly 73% SiO₂, 10.7% B₂O₃, 6.4% Na₂O, and just 0.7% CaO. The key advantage is thermal resilience: borosilicate glass expands about one-third as much as soda-lime glass when heated. That low thermal expansion means it can handle sudden temperature changes without cracking, which is why it’s used in laboratory beakers, baking dishes, and vacuum flasks.
Specialty Glasses Beyond Silica
Not all glasses are built on silicon dioxide. The amorphous structure that defines glass can be achieved with many different chemistries, and some of the most interesting modern materials push well beyond the traditional window pane.
Bioactive glasses are designed to interact with living tissue. The original formulation, developed by Larry Hench in the 1960s and known as 45S5 Bioglass, contains 45% SiO₂, 24.5% CaO, 24.5% Na₂O, and 6% phosphorus pentoxide (P₂O₅). When placed in the body, this glass gradually dissolves and stimulates new bone growth at its surface. It has been used clinically since the 1980s in dental surgery, middle ear replacement, and repair of bone defects. A modified version even appears in some toothpastes as an ingredient that helps remineralize tooth enamel and reduce sensitivity. Researchers adjust the ratio of metal oxides in bioactive glasses to control how quickly they dissolve and how strong they are, tailoring them for specific medical applications.
Metallic glasses (also called amorphous metals or bulk metallic glasses) form when molten metal alloys are cooled so rapidly that the atoms can’t arrange into a crystal lattice. The result is a metal with the disordered structure of glass. These materials can be remarkably strong and elastic compared to their crystalline counterparts, making them useful in high-performance applications like surgical instruments and phone casings.
What Makes Glass Chemically Durable
Silicate glass resists most acids, solvents, and biological fluids, which is why it has been the container of choice for chemicals and medicines for centuries. This durability comes from the strong covalent bonds between silicon and oxygen atoms in the glass network. Each silicon atom bonds to four oxygen atoms in a tetrahedral shape, and these tetrahedra link together into an extensive three-dimensional web. Breaking into that network requires significant energy.
The weak point is the modifier ions, like sodium. Water can slowly leach sodium ions out of the glass surface through ion exchange, which is why very old glass sometimes develops a cloudy, iridescent layer. Borosilicate glass, with far less sodium, resists this process better than soda-lime glass. Pure fused silica is the most chemically inert of all, with no modifier ions to leach, but its high melting temperature (around 1,700 °C) makes it expensive to produce.
The composition of a glass, then, isn’t just a recipe. It determines thermal behavior, optical properties, chemical resistance, and even biological compatibility. In chemistry, “glass” is less a single material than a structural category: any solid whose atoms are frozen in disorder rather than locked in a crystal. That one structural distinction opens the door to an enormous range of materials, from the pane in your kitchen window to the scaffold rebuilding a patient’s bone.