Prepare Sulphur Dioxide Gas and Study its Physical and Chemical Properties

Our Objective

Preparation of sulphur dioxide and analyse of its physical and chemical characteristics

The Theory

Sulphur dioxide is produced as a waste gas when sulphur-containing materials, like metal, are heated, as when melting metal or burning coal or oil. It is a significant contaminant that the environment is currently considered to be at risk. 

Structure of sulphur dioxide 

The twisted structure of sulphur dioxide results from the polar covalent bonds that the sulphur atom forms with two oxygen atoms. Sulphur dioxide has the chemical formula SO2. Sulphur that has been sp2 hybridised is found in sulphur dioxide. The O-S-O bond angle is 119°, whereas the ideal bond angle is 120°. This is brought on by the sulphur atom's single electron pair. The lone pair-bond pair repulsion causes the bond angle to decrease from 120° to 119°. 

 Preparation of Sulphur Dioxide

Sulphur dioxide and traces of sulphur trioxide (6–8%) are generated when sulphur is burned in the air. 

S(s)+O2(g) → SO2(g)

Laboratory Preparation of Sulphur Dioxide- Sulphur dioxide is made in the lab by reacting sulphites with dilute sulphuric acid. This process also produces sodium sulphate and water as by-products. 

Na2SO3(s)+H2SO4(aq) → SO2(g)+Na2SO4(aq)+H2O(l) 

Copper turnings are gently heated in a flask with strong sulfuric acid to produce sulphur dioxide. 


Industrial Preparation of Sulphur Dioxide- Sulphur dioxide is generated commercially as a by-product of roasting sulphide ores like Iron Pyrites or Zinc blend. 


Physical Properties 

  • Sulphur dioxide is a colourless, lethal gas with an awful, suffocating smell. 
  • It is easily soluble in water and has a lower density than air. 
  • When the pressure is 2 atm, and the temperature is normal, it is easily liquefied. 
  • Its freezing point is 197.8 K, while its boiling point is 263 K. 
  • It is a non-flammable gas that acts as a solvent for substances like phosphorus, sulphur, and iodine. 

Chemical Properties 

Sulphur dioxide is acidic because it combines with water to produce sulphurous acid. The blue litmus test becomes crimson in its acidic aqueous solution. SO2 is therefore categorised as a sulphurous acid anhydride. 


Sodium hydroxide solution and sulphur dioxide combine quickly to form sodium sulphite, which then reacts with more sulphur dioxide to become sodium hydrogen sulphite. 



The solution looks milky when sulphur dioxide gas is bubbled through lime water. Calcium bisulphite develops when you pass too much sulphur dioxide gas, eliminating milkiness. 



When there is moisture present, SO2 acts as a bleaching agent. Among other things, it can bleach coloured wool, silk, flowers, and hair. Sulphur dioxide's bleaching action is explained by the production of nascent hydrogen, which transforms the colouring agent into a colourless reduced outcome. 

SO2+2H2O → H2SO4+2[H] 

The bleaching impact of SO2 is relatively transient. The chemical will eventually regain its original colour when standing in the air due to oxidation by air. 

Sulphur dioxide decolourises acidified potassium permanganate (KMnO4) solution. Acidified potassium dichromate solution (K2Cr2O7) is also turned green by SO2. The reactions with KMnO4 and K2Cr2O7 are due to the reducing property of SO2 and oxidising nature of acidified KMnO4 and K2Cr2O7. 

2KMnO4   + 5SO2   + 2H2O → K2SO4   + 2MnSO4   + 2H2SO4 . 

K 2 Cr2 O7 + 4 H2 SO4 → K 2 SO4 + Cr2 (SO4 )3 + 4H2 O + 3[O] 

[SO2 + H2 O + [O] → H2 SO 4 ]× 3 

K 2 Cr2 O7 + H2 SO4 +   3SO2   → K 2 SO4 + Cr2 (SO4 )3 + H2 O . 

Uses of Sulphur dioxide 

  • Sulphuric acid, one of the most often used compounds on the globe, is mostly composed of sulphur dioxide. 
  • It is used to create many sulphites, such as calcium hydrogen sulphite and sodium hydrogen sulphite. These sulphites are used to preserve the freshness of jams, pickles, and jellies. 
  • Common insecticide sulphur dioxide may also be used to sterilise materials like wood or straw. 
  • It is employed to refine sugar, petroleum, and delicate bleach materials like wool. 

Learning Outcomes

  1. The experiment helps students know about the preparation of sulphur dioxide and methods of preparation. 
  2. It will give the information or the idea about its physical and chemical properties. 
  3. Students will be aware and learn the skill to use concentrated acids. 
  4. The students will acquire necessary laboratory skills and be more familiar with the apparatus and the chemicals.