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.
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.
- (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.
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.
- 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.
- 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.
No directly related past PYQ was found.
- 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.