Problem 167 - A Complete Resource Book in Chemistry for JEE Main

Question

For making good quality mirrors, plates of float glass are used. These are obtained by floating molten glass over a liquid metal which does not solidify before glass. The metal used can be (a) mercury (b) tin (c) sodium (d) magnesium

Step-by-Step Solution

Answer

The metal used is tin.

Step 1: Understanding the Problem

The problem requires identifying the metal used as a substrate for floating molten glass to make float glass used in mirrors. The metal must be one where the glass solidifies first.

Step 2: Analyzing the Requirements

The molten glass must float on the metal, implying the metal is denser than glass and remains liquid at the glass's solidification temperature.

Step 3: Evaluating the Options

Let's assess the options: (a) Mercury: Mercury is dense but highly toxic and not suitable for this application. (b) Tin: Tin is dense, has appropriate melting and solidification properties, and is non-toxic in the solid form. (c) Sodium: Sodium is less dense and reacts violently with water, making it unsuitable. (d) Magnesium: Magnesium has a higher reactivity and unsuitable density for such processes.

Step 4: Choosing the Most Suitable Metal

Upon evaluation, tin is the most suitable metal as it's dense, non-reactive with molten glass, and the process conditions match its physical properties.

Key Concepts

Mirror Manufacturing

Mirror manufacturing relies heavily on the float glass process, a revolutionary technique that produces smooth and uniform glass sheets. The process begins with melting the raw materials, including silica, soda, lime, magnesium oxide, and aluminum oxide, in a furnace. Once the materials are melted into glass, they are floated on a molten metal, usually tin, to form sheets. This floatation results in a perfectly flat surface by allowing the molten glass to spread evenly.
The significance of this flatness is pivotal in mirror manufacturing. Any irregularities in the glass surface can distort reflections, which is undesirable. Therefore, the float glass process is ideal for creating high-quality mirrors due to its ability to produce exceptionally smooth and even glass surfaces.

Metal Substrate in Glass Production

The process of choosing the right metal substrate for glass production is crucial. The substrate must fulfill certain criteria: it needs to be denser than molten glass to ensure proper floatation, remain liquid at glass solidification temperatures, and should not react with the glass.
This substrate acts as the support base for the molten glass, allowing it to form a flat sheet. Given these requirements, the metal must hold its liquid form without solidifying before the glass does. Furthermore, it must be chemically stable to avoid contamination of the glass product. Amongst various metals, only a select few, like tin, are considered suitable due to their density, melting point, and non-reactivity characteristics.

Glass Solidification Process

The glass solidification process is an important step in the float glass method. It occurs after the glass is floated on the liquid metal substrate. As the molten glass cools, it undergoes a transition from a viscous state to a solid one. Since the glass solidifies before the underlying metal, it forms a rigid shape while retaining the smoothness gained from being floated on a liquid surface.
This cooling process must be controlled to prevent stresses and imperfections. Thus, a gradual cooling mechanism called annealing is employed. The glass is slowly cooled in a temperature-controlled environment to relieve any internal stresses, ensuring the sheet does not crack or warp later.

Tin as a Substrate for Molten Glass

Tin is often the preferred choice of substrate in the float glass process. This is due to its ideal properties that perfectly comply with the process requirements. Tin is dense enough to sustain the floatation of molten glass but remains in a liquid state at the temperatures involved in glass production.

• Dense enough for floatation.
• Maintains liquid state during glass solidification.
• Non-reactive with glass, preventing any chemical contamination.
• Economically viable and easy to source.

Tin’s ability to remain non-reactive ensures the purity and clarity of the glass sheet, making it optimal for producing high-quality float glass which is then used in various applications, including mirrors, windows, and more. Its use in such productions underscores its fundamental role in modern glass manufacturing techniques.