Passage: Isocyanic acid is a chemical compound with the formula HNCO, discovered in 1830 by Liebig and Wöhler.  This colourless substance is volatile and poisonous, with a boiling point of 23.5 °C.  Isocyanic acid is the simplest stable chemical compound that contains carbon, hydrogen, nitrogen, and oxygen, the four most commonly found elements in organic chemistry and biology.

Question: What is the boiling point of colourless substance ?
23.5 °C