Identification of Gases: Chemical reactions produce gases. These gases help identify the products and mechanisms involved. It is therefore vital to identify these gases through tests using their properties. These properties include colour, odour, combustibility, action on litmus paper. Ever wondered how to conduct a chemical test for the presence of colourless and odourless gases? Let us explore how these gases are identified.

Identification of Gases – Overview

Gases such as carbon dioxide, hydrogen, oxygen have many practical uses and are present in abundant quantity in our environment. To identify these gases, scientists rely on their characteristic properties. A characteristic property is a distinctive trait that can be observed. In this lesson we will learn about some of the common gases found in our environment and how their characteristic properties are used for their identification.

Hydrogen \(\left( {{{\rm{H}}_{\rm{2}}}} \right)\)

Hydrogen is a colourless, odourless gas that makes a squeaky pop when ignited in the presence of air. It burns with a pale blue flame, extinguishes burning splinters, and is neutral to the litmus test. It reacts rapidly with oxygen to form water.

Oxygen \(\left( {{{\rm{O}}_{\rm{2}}}} \right)\)

Oxygen is a colourless, odourless gas that supports combustion. Hence, it re-ignites a glowing splint. It turns colourless alkaline pyrogallol solution brown and is neutral to the litmus test. 

Carbon Dioxide \(\left( {{{\rm{CO}}_{\rm{2}}}} \right)\)

Carbon dioxide is a colourless, odourless gas and can be tested by two methods. It extinguishes a burning splinter. This is a poor test for gases as any oxygen-gas will result in this. It may lead to misidentification, mostly with hydrogen.
1) The best way of testing carbon dioxide is to bubble it through lime water. A positive test will result in the lime water turning milky. The milkiness is due to the formation of Calcium Carbonate. It is insoluble in water and thus forms a milky white precipitate. 
2) It turns moist blue litmus red and generally evolves with brisk effervescence.

Sulphur Dioxide \(\left( {{{\rm{SO}}_{\rm{2}}}} \right)\)

It is a colourless gas with a pungent smell. It extinguishes a burning splinter. It turns:
1) Acidified potassium dichromate \(\left( {{{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}} \right)\) solution from orange to transparent green.
2) Acidified potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_{\rm{4}}}} \right)\) solution from pink to clear colourless.

Hydrogen Sulphide \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{S}}} \right)\)

It is a colourless gas with a rotten egg smell. It turns- 
1) Moist lead acetate paper black.
2) Acidified solution pink to colourless.
3) Acidified solution orange to green.
4) Moist blue litmus red.

Nitrogen Dioxide \(\left( {{{\rm{NO}}_{\rm{2}}}} \right)\)

It is a reddish-brown gas with an irritating smell. It turns-
1) Acidified potassium iodide solution containing starch solution blue.
2) Moist blue litmus red.

Hydrogen Chloride (HCl)

It is a pungent-smelling colourless gas. It turns moist blue litmus red and gives-
1) Dense white fumes with ammonium hydroxide.
2) Curdy white ppt with a solution.

Ammonia \(\left( {{{\rm{NH}}_{\rm{3}}}} \right)\)

It is a colourless pungent-smelling gas that gives-
1) Dense white fumes with \({\rm{HCI}}\).
2) Pale blue ppt with solution.
3) A pale brown or a reddish-brown ppt with Nessler’s reagent (Potassium Mercuric Iodide).

Chlorine \(\left( {{{\rm{Cl}}_{\rm{2}}}} \right)\)

It is a greenish-yellow pungent-smelling gas. Chlorine is used to  disinfect water and is used everyday for treatment of sewage and industrial waste. It is used as a bleaching agent to produce paper and cloth. It turns-
1) Moist starch Iodide paper blue-black.
2)  Moist blue litmus red & then bleaches it.

Bromine \(\left( {{{\rm{Br}}_{\rm{2}}}} \right)\)

Bromine has an atomic number of \(35\) and is represented with symbol Br. It is the third-lightest halogen. Its properties are intermediate between those of chlorine and iodine. It is a reddish-brown gas with a pungent odour. The intensity of reddish gas increases on heating the reaction mixture after adding solid  to the reaction mixture.

Iodine \(\left( {{{\rm{I}}_{\rm{2}}}} \right)\)

It is an odourless violet-coloured gas. It turns starch paper blue, and a layer of violet sublimate is formed on the sides of the tube. 

Water Vapour \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right)\)

It is a colourless gas and is neutral to litmus. It turns-
1) white anhydrous copper sulphate blue.
2) blue cobalt chloride paper pink.

Summary

Gases play an essential role in the qualitative analysis of salts. But how do we identify gases if we can neither see, smell or touch them? The identification is based on the characteristic properties of gases such as colour, odour and precipitate formation. This lesson describes tests to identify the most common gases such as oxygen, hydrogen, carbon dioxide, water vapour, etc. It also explains how to identify colourless and odourless gases.

FAQs

Q.1. How can we identify oxygen and carbon dioxide?
Ans: Oxygen can be identified through the burning splinter test. If the splint re-ignites in the presence of a gas, then it is oxygen. Carbon dioxide can be identified by bubbling it through lime water. Lime water reacts with carbon dioxide to form a milky white appearance of calcium hydroxide.

Q.2. How to identify the presence of water vapour?
Ans: Water vapour is neutral to litmus. It turns white anhydrous copper sulphate blue and blue cobalt chloride paper pink.

Q.3. Which gas gives dense white fumes with HCl?
Ans: When a rod dipped in \({\rm{HCI}}\) is introduced in a jar containing ammonia gas, dense white fumes of ammonia chloride are observed.

Q.4. How can you identify Hydrogen gas?
Ans: The burning splint test can identify hydrogen. A splint is lit and held near the tube’s opening containing the gas. The gas is hydrogen if the splint is extinguished with a distinctive ‘squeaky pop’ sound.

Q.5. Which gases are collected by upward delivery?
Ans: Gases less dense than air are collected by upward delivery. The gas jar is inverted to get lighter gases collected at the top, and the air is displaced from the gas jar. This process is also known as Downward displacement of air. This process collects gases such as ammonia and hydrogen.

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