For making good quality mirrors, plates of float glass are use(d) These are obtained by floating mol

Q: Why do we need to float the glass on a metal in the first place?

Floating molten glass on a liquid metal bath allows gravity and surface tension to naturally create a perfectly flat, parallel surface. This eliminates the need for expensive and time-consuming mechanical polishing.

Q: Why can't we just use water as the liquid bath?

Molten glass is poured at extreme temperatures exceeding $1000^\circ\mathrm{C}$. Water would instantly flash-boil into steam, causing a violent explosion and completely ruining the glass.

Q: Doesn't mercury boil at high temperatures?

Yes, mercury boils at around $356^\circ\mathrm{C}$, which is why modern industry actually uses molten tin. However, this specific question focuses purely on the solidification logic, where mercury's exceptionally low freezing point makes it the theoretical answer.

Q: What exactly happens if the metal solidifies before the glass?

If the metal freezes first, it will form a crystalline solid surface filled with microscopic bumps and imperfections. The still-soft glass would mold to these bumps, destroying its optical clarity.

Q: Is mercury safe to use in a real factory?

Absolutely not. Mercury vapors are highly toxic and lethal. This severe health hazard, combined with its low boiling point, is the major reason why the modern float glass industry uses tin instead of mercury.

Question

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

Options:

(A) mercury

(B) tin

(C) sodium

(D) magnesium

Sigma Percentile · Accepted Answer

### The Quest for the Perfect Mirror

Have you ever stood in front of a high-quality mirror and marveled at how flawlessly flat the glass is?

It is something we take for granted today, but achieving that level of optical perfection was once one of the greatest engineering challenges in human history.

In the past, manufacturing flat glass was an incredibly brutal and labor-intensive process.

Workers had to pour molten glass into large molds to create thick plates.

Once the glass cooled, the surfaces were rough, wavy, and completely useless for mirrors or clear windows.

To fix this, the glass plates had to be painstakingly ground down and mechanically polished using massive abrasive wheels.

This process was not only dangerous but also astronomically expensive, making large mirrors a luxury only the wealthiest could afford.

### The Eureka Moment: Let Gravity Do the Work

The glass industry desperately needed a revolution.

That revolution came in the 1950s, courtesy of an engineer named Sir Alastair Pilkington.

Legend has it that Pilkington was washing dishes when he noticed how a layer of oil floated perfectly flat on the surface of the water.

He had a brilliant "eureka" moment: what if we could float molten glass on top of another liquid?

Because liquids naturally seek a level surface due to the relentless pull of gravity, the interface between the two liquids would be perfectly flat.

Furthermore, the surface tension of the molten glass would ensure that its top surface was also flawlessly smooth.

This brilliant concept birthed the **float glass process**, forever changing how the world manufactures glass.

### The Crucial Temperature Constraint

While the idea was elegant, executing it required solving a massive physics and chemistry puzzle.

What liquid could possibly support molten glass, which is poured at blazing temperatures exceeding $1000^\circ\mathrm{C}$?

Water would instantly flash-boil into steam, causing a catastrophic explosion.

The supporting liquid had to be a metal, specifically a metal with a high density so the glass would float on top.

But more importantly, there was a strict temperature constraint regarding solidification.

As the molten glass travels along the bath, it slowly cools down until it transitions from a soft, viscous liquid into a hard, solid plate.

**The critical rule is this:** The metal bath underneath must remain in a liquid state until the glass has completely solidified.

If the metal were to freeze and crystallize before the glass, it would form a rigid, bumpy surface.

The still-soft glass would immediately mold to these microscopic bumps, completely destroying the optical clarity we worked so hard to achieve.

Mathematically, our constraint is:
$$T_{\mathrm{solidify}}(\mathrm{metal}) < T_{\mathrm{solidify}}(\mathrm{glass})$$

### Analyzing the Elemental Candidates

Let's evaluate the options provided in the question: mercury, tin, sodium, and magnesium.

We are looking for a metal that refuses to freeze easily, maintaining its liquid state even as the temperature drops significantly.

**Sodium** and **Magnesium** are highly reactive metals. Exposing them to the extreme heat of molten glass and atmospheric oxygen would result in violent chemical reactions or fires.

This leaves us to consider the freezing points of the remaining candidates.

**Mercury (Hg)** is a legendary element in the periodic table because it is the only metal that remains a liquid at standard room temperature.

Its freezing point is a frigid $-38.8^\circ\mathrm{C}$.

This extraordinary property guarantees that long after the molten glass has cooled and hardened into a solid sheet, the mercury underneath will still be a perfect, undisturbed liquid.

It provides the ultimate smooth cushion for the solidifying glass.

### The Real-World Industrial Twist

Based strictly on the logic of solidification provided in the problem statement, **mercury** is the definitive answer.

It perfectly satisfies the condition of solidifying after the glass.

However, science is full of fascinating real-world twists!

If you were to actually build a float glass plant using mercury, you would face a deadly problem.

While mercury has a wonderfully low freezing point, it also has a relatively low boiling point of just $356^\circ\mathrm{C}$.

When you pour $1000^\circ\mathrm{C}$ molten glass onto it, the mercury would instantly boil, creating massive bubbles that would ruin the glass, not to mention releasing highly toxic, lethal vapors into the factory!

In reality, Sir Alastair Pilkington chose **molten tin (Sn)** for the industrial float glass process.

Tin melts at $232^\circ\mathrm{C}$ (which is safely below the temperature where glass solidifies) and it doesn't boil until a massive $2602^\circ\mathrm{C}$!

Tin survives the extreme heat without boiling, making it the true hero of modern glassmaking.

### The Final Verdict

So, why does the question point to mercury?

In the context of this specific academic problem, the examiners are testing your understanding of a single, isolated variable: the **solidification point**.

They want you to identify the metal that most obviously remains liquid at lower temperatures.

By focusing purely on the freezing point logic, mercury is the intended theoretical answer.

Understanding both the theoretical logic required for the exam and the practical engineering used in reality is what makes studying chemistry so incredibly rewarding!

Animated Solution for Chemistry - d and f-Block Elements: For making good quality mirrors, plates of float glass are use(d) These are obtained by floating molten glass over a liquid metal which does not solidify before glass. The metal used can be

Select Answer:

Visualized Solution

Visualizing the Process

The Solidification Constraint

Analyzing the Metals

Properties of Mercury

Final Conclusion

Real-World Application

The Sigma Insight: d-block Elements

The Quest for the Perfect Mirror

The Eureka Moment

The Crucial Temperature Constraint

Analyzing the Elemental Candidates

The Real-World Industrial Twist

The Final Verdict

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