How do water and sulfur dioxide react

64.064 g / mol
1 ml / m3 or 2.7 mg / m3 (TRGS 900)
2.9285 g / l (gas, 0 ° C, 1013 hPa)
(Sulfur dioxide to air = 2.27)
−75.45 ° C
−10.02 ° C
Conc. At 20 ° C 112.7 g / l

Colorless, strongly pungent smelling gas

Hazard classes + category

Gases under pressure, compressed. gas
Acute toxicity inhalation 3
Skin corrosion / irritation 1B
Special notes for the school
According to the German RISU, gas bottles with hydrogen sulfide may not be stored in schools. Expectant or nursing mothers are not allowed to work with this substance. When sulfur is heated in the test tube, the sulfur can start to burn. It burns to sulfur dioxide. All work that produces sulfur dioxide is carried out in the fume cupboard. The low occupational exposure limit must be taken into account.
Effect on the human body

Sulfur dioxide is a powerful breath poison. Even low concentrations of 0.04 percent in the air can cause coughing, shortness of breath or inflammation of the airways and mucous membranes. In the event of poisoning, plenty of fresh air should be breathed and, if necessary, artificial ventilation should be provided. Solutions of sulfur dioxide in water burn the stomach walls when they are drunk.

Chemical-physical properties

Sulfur dioxide is a colorless, pungent smelling gas that is not flammable. At −10 ° C it solidifies to a colorless liquid. At room temperature (+20 ° C) around 40 liters of sulfur dioxide dissolve in one liter of water. When sulfur dioxide is dissolved in water, sulphurous acid is also formed. However, the equilibrium of this reaction is far to the left, so that only relatively little sulphurous acid is formed:

SO2 + H2O H2 SO3  

When sulfur dioxide is introduced into water, the universal indicator turns red.


The gas is not only readily soluble in water, but also in many organic solvents. It is about 2.3 times heavier than air. Organic dyes are discolored by sulfur dioxide. In contrast to bleaching with oxygen, the effect is not based on oxidation, but on the reducing effect of sulfur dioxide. It is therefore used to bleach straw or wool. Sulfur dioxide is toxic to many organisms. Microorganisms are inhibited in their growth. Conifers are also particularly sensitive. Sulfur dioxide in the air - for example from exhaust gases - partially turns into sulfur trioxide SO3 oxidized:

2 SUN2 + O2   2 SUN3

Sulfur trioxide forms sulfuric acid when it comes into contact with the air humidity in the clouds, while the sulfur dioxide partially reacts with water to form sulphurous acid. Both acids are therefore contained in acid rain. Acid rain leads to acidification of the water and it can trigger forest death. There are also natural sources of emissions for sulfur dioxide: In the vicinity of volcanoes or hot springs, the sulfur dioxide content can be increased many times.

Sulfur dioxide is not flammable, but it can be oxidized to sulfur trioxide in the presence of catalysts and is therefore an important intermediate in the production of sulfuric acid. When heated with hydrogen in glowing tubes, it is reduced to sulfur or hydrogen sulfide. It reacts with chlorine to form sulfuryl chloride SO2Cl2, an important intermediate in organic chemistry. With nitrates, sulphates and nitrogen oxides are formed when heated.
Sulfur burns with a blue flame. This creates a pungent smelling and toxic gas that irritates the airways. It is sulfur dioxide:

S + O2  SO2      ΔHR. = −297 kJ / mol

Sulfur burns with a blue flame.

Another important process for the production of sulfur dioxide is the roasting of the mineral pyrite or other sulphide ores. The pyrite is heated to over 800 ° C, whereby roasting gases are formed:

4 FeS2 + 11 O2  2 feet2O3 + 8 SUN2    
The sulfur dioxide is separated from the roasting gases by absorption with cold water and then driven out of the water again with water vapor. There are also other processes, in particular sulfur dioxide is produced as a by-product in many chemical reactions.
Sulfur dioxide is an important intermediate product in the manufacture of sulfuric acid. It is also used in the chemical industry to produce salts such as sulphides, sulphites, thiosulphates or dithionites. In liquefied form it is used as a solvent. It is used in sulfochlorination for the manufacture of detergents and for bleaching wool and straw. In the food industry, it is used to preserve fruit and vegetables and to smoke out vermin and pathogens. There are several ways to sulphurise wine: You can burn sulfur in the empty wine barrels, or you can add a substance such as potassium disulphite to the wine, which splits off sulfur dioxide.

Further information and media
Threats to the forest ecosystem
Double contact process for the production of sulfuric acid