Which one among the following statements is not true for glass?
Glass Properties: Identifying the False Statement
This question asks us to identify the statement that is not true regarding glass. Glass is a fascinating material, often described as an amorphous solid. Let's analyze each statement to determine its accuracy.
Statement 1: Water Glass Explained
The statement suggests that sodium silicate is known as water glass. This is generally considered true. Sodium silicate (often represented by formulas like Na₂SiO₃ or Na₂O·(SiO₂)ₓ) is soluble in water, giving it a syrupy or 'watery' consistency, hence the name 'water glass'. It's used in detergents, adhesives, and as a binder.
Statement 2: The Supercooled Liquid Analogy
This statement claims that glass is also called super cooled liquid. This is also often considered true in a descriptive sense. While glass is technically an amorphous solid (meaning its atoms lack a regular, repeating crystalline structure), it lacks the long-range order found in true crystals. Its viscosity is extremely high, and over very long periods (like geological timescales), it can exhibit extremely slow flow. This behavior leads scientists to sometimes refer to it as a 'supercooled liquid' – a liquid that has cooled below its freezing point without crystallizing.
Statement 3: Crystallinity and Glass Formation (The False Claim)
The statement asserts that crystallinity develops on cooling of the molten mass of the glass. This statement is not true. The very definition of glass formation involves cooling a molten material (like molten silica and other components) so rapidly that the atoms or molecules do not have enough time to arrange themselves into an ordered, crystalline structure. Instead, they become locked in a disordered, random arrangement, forming an amorphous solid. If the molten mass were cooled slowly, crystals *would* have time to form, resulting in a crystalline solid, not glass. The characteristic property of glass is its lack of crystallinity upon cooling from the melt.
Statement 4: Coloring Glass Explained
This statement indicates that coloured glasses can be made by adding different colouring materials. This is definitively true. The colour of glass is typically achieved by adding specific metal oxides or metal salts to the molten glass mixture. For example:
- Cobalt oxide produces a deep blue colour.
- Iron compounds can create green or brown colours.
- Copper compounds can yield red, blue, or green colours depending on the oxidation state and formulation.
- Manganese dioxide can give a purple or violet tint.
These additives impart colour by selectively absorbing certain wavelengths of light.
Therefore, the statement that is not true for glass is the one describing the development of crystallinity during the cooling process of molten glass.
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