Sulfuric acid (H₂SO₄) is one of the most widely produced industrial chemicals globally, used in applications ranging from fertilizer production to petroleum refining. Whether this substance evaporates depends heavily on the acid’s concentration and the surrounding temperature. At standard room temperature, concentrated sulfuric acid is generally considered non-volatile; it does not easily transition into a gas like water or alcohol. This low tendency to evaporate results from the powerful forces holding its molecules together, requiring significant energy input to overcome.
Why Sulfuric Acid Resists Evaporation
The resistance of concentrated sulfuric acid to evaporation stems from its unique physical properties, particularly its high boiling point and extremely low vapor pressure at ambient temperatures. Concentrated sulfuric acid, typically around 98% purity, has an exceptionally high boiling point of approximately 337 °C (639 °F) at standard atmospheric pressure. This value is significantly higher than that of water, which boils at 100 °C (212 °F).
This high boiling point is a direct consequence of the strong molecular forces present within the liquid acid. Sulfuric acid molecules are linked by extensive hydrogen bonding, creating a dense, viscous liquid structure. To transition from a liquid to a gas, the molecules must absorb enough energy to break these numerous, powerful intermolecular bonds.
At a typical room temperature of 20 °C (68 °F), the vapor pressure of concentrated sulfuric acid is less than 0.001 millimeters of mercury (mmHg). This vanishingly small vapor pressure indicates that only a minute number of molecules possess the energy required to escape the liquid surface and enter the gas phase. Consequently, concentrated sulfuric acid will not visibly evaporate without the application of substantial heat.
The Behavior of Dilute Solutions
The perception that sulfuric acid evaporates often arises from observations of its behavior in dilute solutions mixed with water. In industrial settings and common spills, sulfuric acid is frequently encountered in a diluted form. When a dilute solution is exposed to the open air, evaporation occurs, but the water component leaves the liquid first.
Water has a much higher vapor pressure than the acid at room temperature, allowing its molecules to escape into the atmosphere more readily. As the water evaporates, the remaining liquid becomes progressively more concentrated in sulfuric acid. This process is commonly used in industrial settings to concentrate dilute acid for reuse.
The liquid volume appears to shrink as the water departs, and the resulting, more concentrated solution becomes increasingly corrosive and hazardous over time. This water loss can transform a relatively mild dilute spill into a highly concentrated, dangerous residue.
The Danger of Sulfuric Acid Mists
While concentrated sulfuric acid resists evaporation at low temperatures, forcing it into the gas phase by heating presents a different and serious hazard: the formation of acid mists. When concentrated sulfuric acid is heated significantly above 300 °C (572 °F), it begins to thermally decompose.
This decomposition releases sulfur trioxide (SO₃) gas and water vapor into the surrounding air. The sulfur trioxide gas immediately seeks out any available moisture, including atmospheric water vapor, and rapidly reacts with it to reform sulfuric acid.
This newly formed acid instantly condenses into microscopic liquid droplets suspended in the air, creating a highly corrosive aerosol known as an acid mist. This mist is extremely hazardous because the fine droplets are easily inhaled deep into the lungs. Mist formation is the primary inhalation risk when sulfuric acid is handled at high temperatures, as the liquid itself is not turning into a breathable vapor.