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.
LIST OF GASES
IDENTIFICATION OF GASES
|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|
COLLECTION OF GASES
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|
DRYING OF GASES
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)
|CO2||Yes||Yes||No||Acidic gas, reacts with CaO (base)|
|NH3||No||No||Yes||It reacts with CaCl2, alkaline gas, reacts with H2SO4
|SO2||Yes||Yes||No||Acidic gas, reacts with CaO (base)|