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Is glass a liquid or a solid? This is an interesting question, and I'm not surprised that you have received different answers. A physicist, I would guess, would almost certainly call glass a fluid. I am a metallurgist/material scientist type and I would call glass a solid. Physicists classify gasses and liquids into a category called fluids, where a fluid is defined as something that can flow. Physicists draw little distinction between liquids and gasses because if temperature and pressures are changed the right way, one can turn the liquid into a gas without the appearance of a meniscus or boiling. My rationale in calling glass a solid is based upon training in materials and the definitions of solids, liquids and gases from their respective atomic structure. In the treatment of fluids by physicists, there is no reference to atomic structure, but only to the glasses ability to flow, although they do admit that because flow is so slow at ambient temperatures, the material behaves like a solid. In materials science we frequently measure a property called creep, which really represents how rapidly a material will flow under a defined load at a specified temperature. At ambient temperatures, most metals flow (under an applied load) much more readily than does glass. Does this mean that steel and aluminum should be considered fluids as well? Clearly there is something lacking in physicists definitions of a fluid, and their classification of glass as a fluid.In materials science, (and physicists would agree), there are three phases of matter; solids, liquids and gases. Gases and liquids have no atomic structure, and their atoms are randomly distributed throughout whatever confines them. Solids have an orderly arrangement of atoms in some form or another. You can have crystalline solids, which are made up of small crystals each having all its atoms in the same arrangement (all metals fall into this classification). There are also non-crystalline solids such as plastics, and glasses. Plastics are molecular solids and can have an amorphous arrangement of there molecules, but each molecule has a precise atomic structure. Glasses have initially been thought to have a totally random/amorphous structure, (which might be why they were/are considered by some to be fluids), but in reality, they too have a short-range atomic order to their atoms (physical dimensions here being on the order of 50-100 angstroms). Studies with x-rays have shown this to be true. This is why glasses are considered a non-crystalline solid by those in the materials community. There are many good reference books about the structure of materials, and your high school should have some available. If you have an interest in materials and such, I would encourage you to pursue it. It is a fascinating career choice. And let's face reality, without materials scientists and engineers, you would not have any of the high tech goodies such as computers, CD's and such that we all enjoy today. There will always be a need for materials engineers and scientists.
Greg Dries
Admin Note: This is the third answer about the state of glass that we have received in our archives, and it turns out that the physicist agrees with the materials scientist. Please read the physics answer to see another explanation.
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