Gases properties, emissions, reactions

Different gases have different physical and chemical properties. We can identify gases using those properties. Some gases have colours and smells as physical properties. As chemical properties, some gases are combustible, dissolve in water and react with water, behave as an acid or base, react with metals or non metals. Gases are released when some solid compounds are heated.


In this tutorial we discuss about what are the gases in the world, properties of gases, how these gases are prepared, reactions of them, identifying gas.

What gases are produced in a reaction?

Following gases are formed frequently in laboratory reactions.

  • Hydrogen (H2)
  • Nitrogen (N2)
  • Oxygen (O2)
  • Carbon dioxide (CO2)
  • Carbon monoxide (CO)
  • Sulfur dioxide (SO2)
  • Sulfur trioxide (SO3)
  • Ammonia (NH3)
  • Hydrogen sulfide (CO2)
  • Nitrogen oxide (NO)
  • Nitrogen dioxide (NO2)

Physical properties of gases

We study specific characteristics of some gases.

Colourful gases

  • Nitrogen dioxide (NO2) : brown
  • Fluorine (F2 : light yellow
  • Chlorine (Cl2) : yellow green

Toxic, harmful gases

  • Ammonia (NH3)
  • Carbon monoxide (CO)
  • Sulfur dioxide (SO2)
  • Sulfur trioxide (SO3)
  • Hydrogen sulfide (H2S)
  • Nitrogen dioxide (NO2)
  • Fluorine gas (F2)
  • Chlorine (Cl2)

What are the acidic gases

Acidic gases dissolve in water and react with water to give H3O+ ions.

  • Carbon dioxide (CO2)
  • Sulfur dioxide (SO2)
  • Sulfur trioxide (SO3)
  • Hydrogen sulfide (H2S)
  • Dinitrogen trioxide (N2O3)
  • Nitrogen dioxide (NO2)
  • Dinitrogen pentoxide (N2O5)
  • Chlorine (Cl2)

Acidity of carbon dioxide

CO2 dissolves in water and give carbonic acid(H2CO3). H2CO3 is a weak acid and a dibasic acid.

Dissociation of carbonic acid

Dissociation of carbonic acid

Second dissociation is more weaker than first dissociation.


What are the basic gasses

Ammonia behave as a basic gas. NH3 gas dissolve very well in water and give ammonium hydroxide (NH4OH). NH4OH is a weak base.

Dissociation of ammonium hydroxide

Dissociation of ammonium hydroxide

Combustible gases

Some gases readily react with oxygen. Therefore identifying those gases is secure to prevent accidents.

  • Hydrogen (H2)
  • Ethene (C2H2)
  • Nitrous oxide(N2O)

Anions which emit gases when adding diluted acid

  • HCO3-
  • CO32-
  • NO2-
  • SO32-
  • S2-
  • S2O32-

NaHCO3 and HCl reaction

sodium bicarbonate and-HCl reaction

CaCO3 and HCl reaction

calcium carbonate and HCl reaction.jpg

NaNO2 and HCl with heating reaction

sodium nitrite and HCl reaction

Na2S and HCl reaction

sodium sulfide and HCl reaction.jpg

Na2SO3 and HCl reaction

sodium sulphite and HCl reaction.jpg

Na2S2O3 reaction

sodium thiosulfate and HCl reaction

Anions which emit gases when adding concentrated acid

  • NO3-
  • Br-
  • I-

Sodium nitrate and concentrated sulfuric acid with heating

sodium nitrate and concentrated sulfuric acid

Potassium bromide and concentrated sulfuric acid with heating

Potassium bromide and concentrated sulfuric acid

Potassium iodide and concentrated sulfuric acid

Potassium iodide and concentrated sulfuric acid

Effects of gases to the environmental pollution

Relationship of acid rains and gases

Nitrogen dioxide(NO2) and sulfur dioxide(SO2) gases are the main gases to cause acid rains. Acid rain can contain nitric acid (HNO3.) or and sulfuric acid(H2SO4) or both.


Effect of nitrogen dioxide to acid rain

NO2 reacts with water and forms HNO3 acid which comes down as the acid rain.

Effect of nitrogen dioxide to acid rain

Effect of sulfur dioxide to acid rain

SO2 reacts with O2 in the air under the influence of sunlight producing SO3 which can react with rain water to produce H2SO4 acid contributing to acid rain.

Effect of sulfur dioxide to acid rain

Photochemical smog and gases

Nitrogen dioxide(NO2), unburnt hydrocarbons contain in the exhaust gases of vehicle engines. These vehicle exhaust cause to form harmful substances such as ozone(O3), aldehydes and peroxyacyl nitrates(PAN). NO2 dissociates as a result of the UV radiation from the sunlight forming free radicals resulting number of chain reactions.

nitrogen dioxide dissociation


What are the brown colour gases

  • Nitrogen dioxide (NO2) : brown

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