Question

Why is HF acid so reactive, and why does sand dissolve in it rather than in any other solution?

Answer 1

`HF` is nasty stuff, but as acids go it is relatively weak.

Explanation:

...i.e. the following equilibrium lies to the left...

`HF(aq) + H_2O(l) rightleftharpoonsF^(-) + H_3O^+`

`pK_a=3.14`

Nevertheless, the `Si-F` bond is one of the strongest bonds known....and thus `SiO_2`, i.e. sand to a first approx., will readily dissolve in it...

`6HF(aq) + SiO_2(s) rarr SiF_6^(2-) + 2H_3O^+`

You will have to look up the thermodynamic parameters yourself.

Answer 2

Because Florine has the highest electronegativity of any element higher than the Oxygen in Silicon Oxide (sand) so Florine can replace the Oxygen in the molecule dissolving the sand.

Explanation:

Florine has an of electronegativity of 3.99 or 4.0. Hydrogen an electronegativity of 2.2. This difference in electronegativity is the greatest of any diatomic acid. The difference in electronegativity means that the Hydrogen atom has little of the electron density and is easily removed or separated from the Florine creating a high concentration of free Hydrogen ions `H^+1` This makes Hydroflouric acids extremely reactive.

Sand Silicon Oxide `(SiO_2)` is strongly bonded. Oxygen has an electronegativity of 3.5. Only Florine has a higher electronegativity.
This means that only Florine is strong enough to displace the Oxygen in the Silicon Oxide bond. So Hydro Floric acid is able to dissolve sand, glass and other substances made of Silicon Oxide.