Glass is a
- (a)liquid
- (b)colloid
- (c)non-crystalline amorphous solid
- (d)crystalline solid
Correct — C, non-crystalline amorphous solid. Glass is rigid, holds its shape and bears weight, so it is a solid; but its silicate units are frozen in a disordered arrangement, with order among near neighbours and no repeating pattern over long distances, so it is not a crystal. That single structural fact explains its behaviour. A crystalline solid melts sharply at one fixed temperature, because the same amount of energy breaks every part of its regular lattice; glass instead softens gradually over a range of temperature, because the disordered network has bonds of many different strengths. Materials of this kind are called amorphous, which is precisely what option (c) says.
- (a)liquid — Glass at room temperature does not flow and has a definite shape of its own, so it is not a liquid. The familiar phrase calling glass a super-cooled liquid is a description of how it was formed — a melt cooled too fast for its units to arrange themselves into a lattice — not a statement that it is liquid now.
- (b)colloid — A colloid is one substance dispersed as fine particles through another, such as milk or fog, and shows the Tyndall effect. Ordinary glass is a single homogeneous phase with no dispersed particles in it.
- (d)crystalline solid — A crystalline solid has a long-range ordered lattice, a sharp melting point and definite geometric shape. Glass has none of these — its structure is disordered and it softens over a range rather than melting at a point.
Solids come in two structural families. Crystalline solids have long-range order, a sharp melting point and anisotropy in some physical properties. Amorphous solids such as glass, rubber and many plastics have only short-range order, no fixed melting point, and behave the same in every direction. Because an amorphous solid is made by cooling a melt faster than its particles can settle into a lattice, it is often called a super-cooled liquid, which is a statement about its origin rather than its state.
The examiner sets this item on the tension between the two descriptions, so answer by comparing the options offered. NDA itself has treated 'glass is often said to be a super-cooled liquid' as a correct statement in a later paper, and here it marks the amorphous solid. Both fit the same physics, but when a paper asks flatly what glass is and offers both liquid and non-crystalline amorphous solid, the precise answer is the amorphous solid. It is also worth knowing that the popular claim that old window panes are thicker at the bottom because the glass has slowly flowed down is not accepted by materials scientists — the uneven thickness comes from the way those panes were made.
- Glass has short-range order only and no long-range repeating lattice, which makes it an amorphous solid.
- An amorphous solid has no sharp melting point and softens over a range of temperature.
- Crystalline solids melt at a fixed temperature and can show different properties along different directions.
- Amorphous solids are often described as super-cooled liquids because they are melts cooled too quickly to crystallise.
- Rubber, most plastics and quartz glass are other common amorphous solids, while common salt and quartz crystal are crystalline.
- Answering 'liquid' on the strength of the super-cooled liquid phrase.
- Assuming every transparent, hard material is crystalline.
- Believing that glass in old windows has flowed downward over the centuries.
NDA asks what glass is, or sets a list of statements about glass and asks which one is not correct, so keep the softening range and the amorphous structure in mind.
Which one of the following statements about glass is not correct?
- (a) Glass is often said to be a super-cooled liquid.
- (b) Glass has no definite melting point.
- (c) Soda glass is harder than pyrex glass.
- (d) Boron is present in pyrex glass.
Answer(c) Soda glass is harder than pyrex glass.
Its first two options carry the same two facts used here, that glass is loosely called a super-cooled liquid and that it has no definite melting point, which is the hallmark of an amorphous solid.
- practice — not a real PYQ
Which one of the following is an amorphous solid?
- (a)Common salt
- (b)Diamond
- (c)Rubber
- (d)Quartz crystal
Answer(c) Rubber — rubber lacks a long-range ordered lattice, whereas common salt, diamond and quartz crystal are crystalline.
- practice — not a real PYQ
Unlike a crystalline solid, an amorphous solid
- (a)melts at a sharp fixed temperature
- (b)softens over a range of temperature
- (c)always conducts electricity
- (d)has a regular repeating lattice
Answer(b) softens over a range of temperature — its disordered structure has bonds of many strengths, so there is no single melting point.