Hydrogen chloride preparation. Solid sodium chloride is in the round-bottomed flask at left. When sulphuric acid is dropped onto the sodium chloride from the burette above it, hydrogen chloride gas is formed. The gas travels through the glass tube to the conical flask at centre, which contains water, which moistens the hydrogen chloride gas. The moist gas then travels to the glass column at right, where, as it is an acid, it turns the blue litmus paper red. Dry hydrogen chloride gas would not cause the colour change.