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Name: Gabby S.
Status: student
Grade: 9-12
Location: US
Country: USA
Date: N/A


Question:
Hello, I have been researching the properties of Glass, I am aware that it is basically formed from SiO2, but in order to make it cheaper to produce other ingredients are added. Commonly Sodium Carbonate and limestone. Once these components are added, how does this affect the molecular structure, bonds or properties?


Replies:
Hi Gabby,

The "glass" made from pure silica (SiO2) you refer to is in fact called "Fused Silica" or "Fused Quartz" It is rather inconvenient to process, since it melts at around 2000°C (almost 4000°F). As you have stated, other compounds are added, primarily Sodium Carbonate and Lime (not "Limestone" as you stated) in order to reduce its melting point to the area of 1000°C. (The difference between lime and limestone is that Lime is CaO, and Limestone is CaCO3). This type of glass, which is the most common type available, is called "Soda-Lime Glass".

Both Fused Silica and Soda Lime glasses are both amorphous "glasses" (there, the word "glass" refers to the molecular structure, which it totally amorphous, just like a liquid). These materials are in fact called supercooled liquids, because like normal liquids, they actually flow at room temperature.... albeit very, very slowly and imperceptably.

Comparing their properties, Soda-Lime glass will block UV radiation, whereas Fused Silica does not. Fused Silica also can withstand much higher temperatures than "normal" Soda- Lime glass, which is why it is used as an envelope for halogen light bulbs that operate at temperatures high enough to soften Soda-Lime glass. Otherwise, these two types of glass are very similar.

Regards,

Bob Wilson.

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