Which one among the following statements is not true for glass?
- (a)Sodium silicate is known as water glass.
- (b)Glass is also called super cooled liquid.
- (c)Crystallinity develops on cooling of the molten mass of the glass.
- (d)Coloured glasses can be made by adding different colouring materials.
Answer
Why
Correct: C, crystallinity develops on cooling of the molten mass of the glass. This is the false statement: glass is an amorphous solid, and the fact that it does NOT crystallise as the melt cools is precisely what makes it glass rather than a ceramic.
Why the others are wrong
- (a)Sodium silicate is known as water glass.: True. Sodium silicate really is known as water glass, sold as a syrupy solution and used as an adhesive and sealant.
- (b)Glass is also called super cooled liquid.: True in the sense school texts intend. Glass is routinely described as a supercooled liquid because it is cooled past its freezing point without crystallising; modern physics prefers 'amorphous solid', but the textbook statement is not the false one here.
- (d)Coloured glasses can be made by adding different colouring materials.: True. Cobalt salts give blue, chromium green, and colloidal gold ruby red: adding colouring materials to the melt is exactly how coloured glass is made.
Concept
A glass is a solid whose atoms never settle into a repeating lattice. On cooling, the melt becomes steadily more viscous until motion effectively stops, freezing in a disordered, liquid-like arrangement rather than crystallising.
If crystals do form in glass (through slow cooling or contamination) the result is devitrification, which clouds the material and weakens it. It is treated as a manufacturing fault, which is why option (c) inverts the defining property. Ordinary soda-lime glass is silica with sodium carbonate and lime added to lower the melting point and stabilise the product.
Key facts
- Glass is amorphous: no long-range crystalline order.
- Crystallisation in glass is called devitrification and is a defect.
- Sodium silicate = water glass.
- Colourants: cobalt (blue), chromium (green), colloidal gold (ruby red).
Option (c) describes the defect and presents it as the norm.
Study next
Common traps
- Reading 'supercooled liquid' as the false statement: school texts use it approvingly.
- Assuming coloured glass is surface-coated rather than bulk-coloured.
Glass items usually plant one statement that inverts the amorphous property; find that and the rest are true.
Related PYQs
No directly related past PYQ was found.
Practice
- practice, not a real PYQ
The formation of crystals in glass, which clouds and weakens it, is called
- (a)annealing
- (b)devitrification
- (c)tempering
- (d)sintering
Answer(b) devitrification: the loss of the glassy, non-crystalline state.