Ozone is an allotrope of oxygen. Ozone is a gas having 3 atoms of oxygen in its molecule, so the molecular formula of ozone is O3. Ozone is produced by the action of sunlight on oxygen in the upper layers of atmosphere :
The formation of ozone gas starts at an altitude of about 16 kilometres from the surface of earth. The amount of ozone in the atmosphere gradually increases from this height until at about 23 kilometres altitude, there is the thickest blanket (thickest layer) of ozone gas in the atmosphere all round the earth. The ozone layer in the upper atmosphere is of great importance because it absorbs the harmful ultra-violet radiations present in sunlight coming from the sun. In this way, most of the ultra-violet radiations are prevented from reaching the earth and we are saved from their dangerous effects.
We shall now describe simple tests for showing the presence of oxygen, carbon dioxide, and water vapour, in the air or atmosphere.
1. To Test the Presence of Oxygen in Air (or Atmosphere). We know that oxygen is a supporter of combustion which helps other objects in burning. Actually, combustion is a reaction in which a substance combines with the oxygen of air vigorously, with the liberation of heat. From this discussion we conclude that combustion (or burning) of la substance in air is the surest test for the presence of oxygen gas in the air (or atmosphere). So, to detect the presence of oxygen in air or atmosphere, we apply a lighted match stick to any easily combustible substance, say a piece of cloth soaked in kerosene. The cloth starts burning with the evolution of heat. Since the cloth dipped in kerosene burns in air or atmosphere, it shows that the air (or atmosphere) contains oxygen gas, which helps it in the process of burning or combustion,
2. To Test the Presence of Carbon Dioxide in Air (or Atmosphere). The presence of carbon dioxide gas in the air can be tested by using lime-water. When air is passed through lime-water for a long time, then lime-water turns milky, showing the presence of carbon dioxide in the air (or atmosphere). This is because of the fact that carbon dioxide gas present in atmosphere reacts with lime-water (calcium hydroxide solution) to produce a white precipitate of calcium carbonate. And this calcium carbonate precipitate produces milkiness in solution.
3. To Test the Presence of Water Vapour in Air (or Atmosphere). The presence of water vapour in air or atmosphere can be tested by cooling the air so as to condense the water vapour present in it to form droplets of liquid water. In order to test the presence of water in the air or atmosphere, we take a glassful of ice-cold water and keep it for sometime in a warm room. We will find that small droplets of water appear on the outside surface of this glass. The formation of these water droplets on the cold outer surface of the glass shows that the air or atmosphere contains water vapour.
This can be explained as follows : When we keep the glass filled with ice-cold water in a room, then the air surrounding the glass gets cooled. The water vapour present in this cooled air condenses on the cold surface of glass in the form of small droplets. Thus, we can say that the water vapour present in the atmosphere condenses on the surface of any cold object, so that a glass of cold water will have droplets of this condensed water vapour on its sides.