Guide: Collection, Drying and Identification of Gases

Guide: Collection, Drying and Identification of Gases 

updated 1 Jan 2013
You will deal with several gases in your chemistry questions. Therefore you need to be familiar with these gases and some of their properties. You need to know how to identify these gases and collect these gases in a dry condition. Most of the time these gases are products of a reaction but they are moist in nature. 
Gas Chemical Formula
Hydrogen H2
Oxygen O2
Nitrogen N2
Ammonia NH3
Carbon Dioxide CO2
Sulphur Dioxide SO2
Chlorine Cl2
Hydrogen Chloride HCl
Gas Colour Odour Tests
H2 colourless odourless extinguishes a lighted splinter with a pop sound
O2 colourless odourless rekindles a glowing splint
CO2 colourless odourless turns limewater chalking upon bubbling (formation of white precipitate)
NH3 colourless pungent smell turns moist red litmus paper blue
Cl2 greenish-Yellow choking smell turns moist blue litmus paper red and then bleaches it
SO2 colourless choking smell turns moist blue litmus paper redturns acidified potassium dichromate solution from orange to green
HCl white misty fumes unpleasant, acrid odour turns moist blue litmus paper red
There are 4 methods of gas collection.
(a) Downward delivery
(b) Upward delivery
(c) Water displacement
(d) Gas Syringe
Limitations and advantages
Each of these methods have their limitations and advantages.
We use downward delivery to collect gases that are heavier than air. (e.g. CO2 , NO2 , SO2 , HCl, Cl2, Br2)
We used upward delivery to collect gases that are lighter than air (e.g. H2, NH3)
We use water displacement when the gases are not easily soluble in water. (H2, N2, O2, CO2)
The gas syringe is useful for us to determine the actual volume collected.
Summary Table for Gas Solubility in Water and Density 
Gas Solubility in Water Density in comparison with air
ammonia soluble less dense
chlorine fairly soluble denser
hydrogen insoluble less dense
nitrogen insoluble less dense
oxygen insoluble denser
carbon dioxide insoluble denser
hydrogen chloride soluble denser
sulfur dioxide soluble denser


The gaseous products may often by moist. There are methods to pass these gases through certain substances (drying agents) to remove the moisture so as to obtain dry gases.
These are some possible drying agents.
1. Concentrated sulfuric acid H2SO4
2. Quick lime (CaO: Calcium oxide)
3. Fused CaCl2(s)
However, the drying agents are not suitable for drying all types of gases. We have to be careful when dealing with gases with acidic nature (HCl) and gases with alkaline nature (NH3)
Summary Table for Drying Agents (Gas)
Gas CaCl2 H2SO4 CaO Reason
CO2 Yes Yes No Acidic gas, reacts with CaO (base)
O2 Yes Yes Yes  
Cl2 Yes Yes Yes  
NH3 No No Yes It reacts with CaCl2, alkaline gas, reacts with H2SO4
H2 Yes Yes Yes  
SO2 Yes Yes No Acidic gas, reacts with CaO (base)



One comment

  1. Rida

    Very informative and helpful. Thanks a lot for helping me out. I want to suggest that you should expand the website and also attach information regarding other subjects too.

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