A colourless liquid, HF; r.d. 0.99; m.p. –83°C; b.p. 19.5°C. It can be made by the action of sulphuric acid on calcium fluoride. The compound is an extremely corrosive fluorinating agent, which attacks glass. It is unlike the other hydrogen halides in being a liquid (a result of hydrogen bond formation). It is also a weaker acid than the others because the small size of the fluorine atom means that the H-F bond is shorter and stronger. Solutions of hydrogen fluoride in water are known as hydrofluoric acid.