Glass is soluble in:

The correct answer is: HF (Hydrofluoric Acid)

Glass is primarily composed of silicon dioxide (SiO₂), also known as silica. To understand why glass dissolves in HF, we need to look at the chemistry of how HF reacts with SiO₂.

Step 1: Composition of Glass

Common glass is made mainly of silicon dioxide (SiO₂). The silicon atom in SiO₂ forms a strong, extended network of Si−O bonds, which makes glass extremely resistant to most acids.

Step 2: Why most acids cannot dissolve glass

Strong acids like H₂SO₄ (sulfuric acid), HClO₄ (perchloric acid), and even aqua-regia (a mixture of HCl and HNO₃ in a 3:1 ratio) cannot break the robust Si−O network in glass. This is precisely why glass containers are used to store these acids safely.

Step 3: The unique reactivity of HF with glass

Hydrofluoric acid (HF) is the exception. Fluorine is the most electronegative element and has a very small atomic size. This allows fluoride ions (F⁻) to attack the silicon atom in SiO₂ very effectively. The Si−F bond that forms is one of the strongest single bonds in chemistry, which is the driving force behind this reaction.

Step 4: The Chemical Reaction

When HF reacts with SiO₂, it produces silicon tetrafluoride (SiF₄), a gas, and water:

${\mathrm{SiO}}_2+4\mathrm{HF}\to {\mathrm{SiF}}_4\uparrow +2H_{2}O$

If excess HF is present, SiF₄ further reacts to form the soluble fluorosilicic acid:

${\mathrm{SiF}}_4+2\mathrm{HF}\to H_2{\mathrm{SiF}}_6$

Step 5: Practical Implication

Because HF dissolves glass, it cannot be stored in glass containers. Instead, HF is stored in containers made of plastic (such as polyethylene or Teflon) or wax-coated vessels. This property of HF is also used industrially for etching and frosting glass surfaces.

Key Takeaway: Among HF, H₂SO₄, HClO₄, and aqua-regia, only HF can dissolve glass because fluoride ions form extremely strong Si−F bonds, breaking apart the SiO₂ network that constitutes glass.