Carbon dioxide
Carbon dioxide is a colourless and odourless gas. It has a slightly sour taste. Carbon dioxide is moderately soluble in water. Carbon dioxide gas neither burns nor supports burning. Carbon dioxide extinguishes a burning fire. Plants and animals consume oxygen from air for respiration and produce carbon dioxide. Plant and animal matter burns by consuming oxygen from air and produces mainly carbon dioxide and a few other gases. If some material is burning in a closed room, we feel suffocated. This is due to the accumulation of excess of carbon dioxide (and some carbon monoxide) in the air of the room which is produced by the burning material.
Air contains only about 0.03 per cent of carbon dioxide gas. Though the amount of carbon dioxide in air is comparatively small but it is a very important component of air. The carbon dioxide gas present in air is important to green plants. Green plants need carbon dioxide gas to make food. In sunlight, green plants use carbon dioxide from air and water from soil to make food by the process of photosynthesis. So, all the food which animals (including human beings) eat is made from carbon dioxide gas of the air. Without carbon dioxide in air there would be no plants or animals on the earth. The plants which live in water use. the carbon dioxide dissolved in water for their needs.
WATER VAPOUR
Air also contains water vapour. We cannot see 'water vapour' present in air because it is "water in the form of a gas. We performed an activity in the previous chapter to show the presence of water vapour in air. When air comes in contact with a cool surface (as that of a glass containing ice-cold water), the water vapour present in it condenses to form tiny drops of liquid water on the cold surface. Water vapour comes into the air from the evaporation of water present on the earth (in ponds, lakes, rivers and oceans, etc.) and from transpiration. The presence of water vapour in air is important for the working of water cycle in nature. This is because it is water vapour present in air which rises high in the sky alongwith hot air, gets cooled, condenses to form clouds and then brings rain on the earth.