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Physical and Chemical Properties: |
Hydrofluoric acid is an aqueous solution of hydrogen fluoride gas, appearing as a colorless to pale yellow fuming liquid. It has a pungent odor. Relative density is 1.15–1.18. Boiling point is 112.2°C (at 38.2% by weight). Commercial concentration is typically around 47%. It is a weak acid. The aqueous solution of hydrogen fluoride can have a solute mass fraction of up to 35.35%. It is a colorless, toxic solution. At its highest concentration, the density is 1.14 g/cm³, and the boiling point is 393.15 K (120°C). It emits fumes. Although weakly acidic, its degree of ionization is higher at high concentrations than at low concentrations, which distinguishes it from typical weak electrolytes. It is highly corrosive and particularly damaging to teeth and bones. It strongly corrodes silicon compounds. It should be stored in sealed plastic bottles. It is produced by dissolving HF in water. Used for etching glass, cleaning residual sand from castings, controlling fermentation, electro-polishing, and cleaning and etching semiconductor silicon wafers (in a mixed acid with HNO₃). Due to the relatively strong bond between hydrogen and fluorine atoms, hydrofluoric acid does not fully dissociate in water.
Hydrofluoric acid can dissolve glass (silicon dioxide), which many other acids cannot, as shown in the reaction equation:
SiO₂(s) + 6 HF(aq) → H₂SiF₆(aq) + 2 H₂O(l)
Because of this, it must be stored in plastic containers (theoretically, containers made of polytetrafluoroethylene are even better). Hydrofluoric acid is also a reducing agent. For long-term storage, not only is a sealed container required, but the air inside should be evacuated as much as possible.
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