Question

Glass is corroded by

• A. fluorine
• B. sulphuric acid
• C. phosphoric acid
• D. NaOH

Hint:

Silicate glass is resistant to almost all acids except those containing fluorine (such as \(\text{F}_2\) and HF).
The reaction forms gaseous \(\text{SiF}_4\), which is why hydrofluoric acid is used for glass etching and must be stored in plastic bottles.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the chemical substance from the given options that is capable of corroding or etching silicate glass.
Silicate glass is renowned for its excellent chemical inertness against almost all strong mineral acids and organic chemicals, which is why it is used for laboratory glassware.

Step 2: Key Formula or Approach:
The structural network of common glass is composed primarily of silicon dioxide (\(\text{SiO}_2\)).
Silica is highly stable, but it can be chemically attacked by fluorine-containing species (such as elemental fluorine gas, \(\text{F}_2\), or hydrofluoric acid, HF) because the silicon-fluorine (\(\text{Si}-\text{F}\)) bond is thermodynamically much stronger than the silicon-oxygen (\(\text{Si}-\text{O}\)) bond.

Step 3: Detailed Explanation:
Let us analyze the chemical reactivity of the substances with glass:
- Sulfuric acid (Option B) and phosphoric acid (Option C) do not react with silica at room temperature. Concentrated sulfuric acid is commonly stored in glass containers.
- Sodium hydroxide (Option D) can dissolve glass very slowly at high concentrations and elevated temperatures, but it does not cause rapid corrosion under normal conditions.
- Fluorine (Option A) and its compounds like hydrofluoric acid (HF) react vigorously with silicon dioxide. The reaction of fluorine gas with silica is highly exothermic and proceeds as follows:
\[ \text{SiO}_2(s) + 2\text{F}_2(g) \to \text{SiF}_4(g) + \text{O}_2(g) \] - This reaction converts the solid silica network into volatile silicon tetrafluoride gas (\(\text{SiF}_4\)), resulting in rapid, severe corrosion and dissolution of the glass surface.
- Because of this unique reactivity, fluorine gas and hydrofluoric acid must never be stored in glass bottles; instead, containers made of specialized plastics (like Teflon or polyethylene) or certain metals (like Monel) are used.

Step 4: Final Answer
Therefore, glass is corroded by fluorine, which corresponds to option (A).