Which one of the following statements about glass is not correct?
Glass is often said to be a supercooled liquid.
Glass has no definite melting point.
Soda glass is harder than pyrex glass.
Boron is present in pyrex glass.
Answer is Right!
Answer is Wrong!
This question was previously asked in
UPSC NDA-2 – 2019
The correct option is C) Soda glass is harder than pyrex glass. This statement is incorrect. Pyrex glass (borosilicate glass) is generally harder and more durable than soda-lime glass (soda glass).
A) Glass is an amorphous solid, lacking a long-range ordered crystalline structure. Its structure is similar to a liquid’s, and it becomes increasingly viscous as it cools, without a sharp transition like freezing. Hence, it is often described as a supercooled liquid or an amorphous solid. This statement is generally considered correct.
B) Due to its amorphous nature, glass does not have a specific melting point. It softens gradually over a range of temperatures. This statement is correct.
C) Soda-lime glass (soda glass) is the most common type of glass (window panes, bottles). Pyrex is a brand of borosilicate glass, which contains boron oxide (B₂O₃) in addition to silica. The addition of boron oxide significantly changes the properties, making it more resistant to thermal shock and chemical corrosion, and generally harder and more resistant to scratches compared to soda-lime glass. Therefore, soda glass is *not* harder than pyrex glass; it is softer.
D) Pyrex glass is a type of borosilicate glass, meaning it contains boron (specifically in the form of boron oxide). This statement is correct.