Sulphur Dioxide - SO₂

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What is Sulphur Dioxide?

Romans has used this sulphur dioxide compound in winemaking because they discovered that the empty wine glass has smelled fresh on burning sulphur candles inside the glass. The chemical formula of sulphur dioxide can be given as SO2. It also exists in fewer concentrations in the atmosphere of Earth, but it is the third most significant gas present in the atmosphere of Venus. It is also found in the case of volcanic eruptions and in the effluents, which are released from industries.

Structure of Sulphur Dioxide

This compound is explained as a bent molecule and contains a bond order of 1.5. The resonance structure is described using the valence bond theory with the s and p orbitals.

Structure of sulphur dioxide holds the chemical composition of SO2, and it is also commonly referred to as sulfurous anhydride, sulfur oxide, or sulfurous acid anhydride.

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Preparation of Sulphur Dioxide

  1. In the chemical laboratory, sulphur dioxide can be prepared using the reaction of either a metallic bisulfite or a metallic sulphite with the dilute acid. For example, a reaction between the sodium sulphite and dilute sulphuric acid will result in the production of SO2.

Na2SO3 + H2SO4 → Na2SO4 + H2O + SO2

  1. Commercially, sulphur dioxide can be obtained as a by-product, which is released by the roasting of sulphide ores. The gas that is obtained is dried, liquefied and then stored in the steel cylinders.

4FeS2 (s) + 11 O2 (g) → 2Fe2O3 (s) + 8SO2 (g)

pH of Sulfur Dioxide

In juices, which are highly acidic, there requires a less sulfur dioxide. For example, 15 mg l−1 with the free sulfur dioxide at a pH value of 3.0 has similar antimicrobial effect same as 150 mg l−1 at a pH value of 4.0 Percentage of sulfite, molecular, and bisulfite sulfur dioxide in the aqueous solution as a pH function.

Occurrence of Sulphur Dioxide

It can be found on Earth and in the atmosphere at about 1 ppm and exists in small concentrations.

Whereas, on other planets, it is found in multiple concentrations, the most significant being the atmosphere of Venus, where it is considered as the third-most significant atmospheric gas at 150 ppm. At that point, it condenses to produce clouds, and it is a vital component of chemical reactions in the atmosphere of the planet and contributes to global warming.

It has also been implicated as a primary agent in the warming of early Mars, with the estimates of the concentrations in the lower atmosphere as higher as 100 ppm, though it exists only in trace amounts. On both Mars and Venus, as on Earth, its major source is thought to be volcanic. Whereas, the atmosphere of Io, a natural satellite of Jupiter, is 90% of the sulfur dioxide and trace amounts are thought to exist in Jupiter's atmosphere.

It is thought to exist in abundance as a block of ice on the Galilean moons - as frost or subliming ice on the trailing hemisphere of Io, and in the crust and mantle of Ganymede, Europa, and Callisto, possibly also in the form of liquid and readily reacting with water.x

Properties of Sulphur Dioxide

  • Sulphur Dioxide is a colourless gas with an odour of rotten egg.

  • It liquefies easily and highly soluble in water.

  • SO2 also dissolves in water to produce sulphurous acid because of which it possesses an acidic character. The chemical reaction for this can be given as follows:

H2O + SO2 → H2SO3

  • This compound does not support combustion, and it is not combustible.

  • SO2 is also a strong oxidizing agent.

2H2S + SO2 → 3S + 2H2O

  • It acts as a reducing agent, which can be chemically shown as follows:

SO2 + 2H2O → H2SO4 + 2H

Uses of Sulphur Dioxide

Let us look at the uses of sulphur dioxide as listed below:

  • It can be used as a food preservative in the food industries.

  • To remove the excess chlorine, it acts as a bleaching agent and also as a disinfectant.

  • It acts as a refrigerant in cold storage plant

  • It can also be used as a solvent and reagent in the laboratory.

  • It is advised to follow safety measures against sulphur dioxide because inhaling this compound causes serious health problems like respiratory problems and even premature death.

  • It is also used as a preservative in many compounds other than food compounds such as for dried apricots, dried figs, dried fruits.

Solved Example


What happens when the concentrated H2SO4 is added to calcium fluoride, and So3 is passed through water?


It produces hydrogen fluoride, and the chemical reaction can be given as follows:

CaF2 + H2SO4 → CaSO4 + 2HF

It dissolves SO3 to form H2SO4, and the chemical reaction can be given as follows:

SO3 + H2O →  H2SO4

FAQ (Frequently Asked Questions)

1. How is Sulphur Dioxide Formed?

Answer: Sulfur dioxide (SO2), is defined either as a colourless gas or liquid which has a strong and odorous smell. It has derived from the burning of fossil fuels (which are coal and oil) and the smelting of sulphur-containing mineral ores (for example, aluminium, copper, zinc, iron and lead). Sulfur dioxide compounds quickly dissolve to produce sulfuric acid in water.

2. Explain if Sulfur Dioxide is a Compound?

Answer: Sulfur dioxide, (SO2) is an inorganic, toxic, thick, colourless gas substance. In the intermediate steps of forming sulfuric acid, it can be produced in huge quantities. Sulfur dioxide also has a pungent, and unpleasant odour which is quite similar to the scent of a match that has just been struck.

3. How Does the Sulphur Dioxide Form Acid Rain?

Answer: Acid rain takes place when released into the atmosphere by the compounds of sulfur dioxide (SO2) and nitrogen oxides (NOX) and carried by air and wind currents. The NOX and SO2 react to produce nitric and sulfuric acids with oxygen, water, and other chemicals. After that, they combine prior dropping to the table, with water and the other materials.

4. Is Sulphur Dioxide an Acid?

Answer: Sulfur dioxide is defined as acid gas, and this can be demonstrated in an easy manner by adding water and a few drops of universal indicator to the gas container. The resulting acid is the sulphuric acid (H2SO3), which is a weakly dibasic acid.