Hydrogen fluoride boils at about 19.51 degrees Celsius (67 degrees Fahrenheit). As a result, it will be a liquid on a chilly spring morning but will become a gas before the thermometer registers 21 degrees Celsius (70 degrees Fahrenheit). It becomes a solid only if subjected to very cold temperatures. It freezes at -83.1 degrees Celsius (-118 degrees Fahrenheit).
Hydrogen fluoride gas is colorless, but it has an unpleasant odor. It forms an azeotropic mixture with water. This means that when hydrogen fluoride dissolves in water, the solution will boil at a temperature that differs from the boiling point of either component and the resultant gas will contain the same relative amounts of hydrogen fluoride and water that the boiling solution contained. The solution will boil at 120 degrees Celsius. Hydrogen fluoride is also soluble in many organic solvents, such as alcohol and benzene.
Hydrogen plus any halogen forms an acid. For this reason, hydrogen fluoride is also called hydrofluoric acid. The acidity of hydrogen fluoride is relatively weak in comparison to such acids as hydrochloric acid and sulfuric acid.
The hydrogen fluoride molecule consists of one hydrogen atom united with one fluorine atom, so that its chemical formula is HF. However, neighboring HF molecules tend to form hydrogen bonds with one another. These hydrogen bonds are weak unions in which the hydrogen of one molecule forms a loose liaison with the fluorine of another molecule.
Hydrogen bonding explains other properties of hydrogen fluoride (HF). For example, acidity is proportional to the number of hydrogen ions that are available for reaction. Since much HF hydrogen is already occupied with hydrogen bonds, there are fewer free hydrogen ions that can take part in acidic reactions. Moreover, hydrogen bonds result in bulky HF complexes. This makes it more difficult for HF to escape from the liquid to the gaseous state. As a result, HF boils at a higher temperature than hydrogen chloride, even though the latter has a heavier molecule.
Hydrogen fluoride is a dangerous chemical. It damages body tissues on contact. If inhaled, it damages the lungs. Its corrosive action makes it hard to store. It even attacks glass, forming silicon tetrafluoride in the process. Lead and wax are safe materials for HF storage, according to the United States Department of Labor. Polyethylene plastics may also be used.