If a compound HA dissolves in water and it increases the hydronium concentration greater than that in pure water the compound is an acid. When hydrogen fluoride is dissolved in water the ions H+ and F- are produced. H+ combines with H2O to form the hydronium ion H3O+. This decreases the pH of the solution to a value less than 7.
If litmus paper is used to test the pH of the solution containing HF, it turns yellow/orange to red in color which indicates that the solution is acidic in nature.
In addition, as a chemical test, if some sodium carbonate powder is added to the solution, carbon dioxide is released. On adding sodium carbonate to HF solution, there is a fizzing sound as a gas is released. Further tests can then be carried out to confirm that the gas released is carbon dioxide.