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Thermochemistry explores the heat changes that occur during chemical reactions, making it an integral part of Chemistry Topics.
Is air present everywhere around us?
Air is a mixture of several gases. We cannot see or smell air but air is everywhere around us. All living things require air. Air is inside the human body as well as in the bodies of all other animals and birds. This is because air is necessary for the breathing of all the living things. No living things can survive without air. We can live without food and water for a few days but we cannot live for more than a few minutes without air. Air is present on the ground, on the top of trees and buildings as well as high up in the sky. Air is present in the soil. Some air is present even in the water of ponds, lakes, rivers and oceans in the dissolved form.
Though we cannot see air but we can feel its presence when it starts moving (or blowing). Moving air is called wind. Moving air makes the clothes hanging on a clothes line sway. And the air blowing in a garden stirs the leaves and branches of trees. When a fan is switched on, the air moved by the blades of the fan makes the pages of an open book flutter back and forth. It is the moving air which makes it possible for us to fly a kite in the sky (by pushing against it). And it is also the moving air which pushes the sails of yacht (sailing boat) and makes it move in water. In agriculture, moving air helps us separate husk from wheat grains during winnowing. Fligh speed winds can even uproot the trees and blow off the tin roofs of houses and factories.
We have seen many little children playing with “phirki” (wind-vane). When a child holds the stick of the phirki in his hand, stands in an open area and adjusts the direction of phirki blade, the phirki starts rotating (see Figure). It is the moving air (or blowing air) which makes a phirki rotate. Another device which works by utilising the moving air is weathercock. In a weathercock, a broad blade in the shape of a cock is fixed on an arrow which is attached to a rotating axis (see Figure). When the moving air pushes the cock blade, then the cock and its arrow rotate and align in the direction of moving air. Thus, weathercock is a device which shows the direction in which the air is moving (or blowing) at a place.
Air is transparent, so we can see through it. Air fills all the space available to it. Since air is a gaseous substance, it can be compressed easily. When we fill a football bladder or a bicycle tube with air by using a pump, we actually compress air. The important properties of air are as follows :
- Air is colourless, tasteless and odourless gaseous substance.
- Air has mass.
- Air occupies space.
- Air dissolves in water.
- Air can be compressed.
Air is Present Everywhere Around Us
Air is present everywhere around us (though it cannot be seen by us). Even those containers and vessels which appear to be empty to us are filled with air. For example, an empty bottle is not really empty, it contains air in it. Even when we turn an empty bottle upside down, the air remains in it. We can show by performing a simple activity that air occupies space and that even an empty bottle is full of air.
Activity
We take an empty glass bottle and hold it in the inverted position (with its open mouth facing downwards). The inverted glass bottle is put in water kept in a vessel [as shown in Figure (a)]. We will find that water does not enter into the inverted glass bottle. The water does not enter into the glass bottle because the bottle is filled with air. Actually, all the space in the bottle is occupied by air. The air present in the empty glass bottle prevents the water from entering it. From this activity we conclude that air occupies space.
If we tilt the bottle held in water, we will find that the air present in the bottle goes out in the form of air bubbles [see Figure (b)]. As the air from the bottle escapes, water starts entering the glass bottle. Actually, the space vacated by the air leaving the bottle is now occupied by water. The above activity shows that a glass bottle which appears to be empty to us is actually filled with air.
Atmosphere
We live on the earth. Our earth is surrounded by a layer of air (or envelope of air). The envelope of air that surrounds the earth is called atmosphere (see Figure). The atmosphere (or layer of air) extends up to many kilometres above the surface of earth. We live within the atmosphere of the earth. Atmosphere is essential for life on earth. This is because the air of atmosphere provides oxygen gas for breathing by all the living organisms including us. As we go higher up in the atmosphere, the amount of air becomes less and less.
The air at the top of very high mountains is so thin that it does not have enough oxygen for the people to breathe properly. So, the people who climb high mountains (called mountaineers) carry cylinders containing oxygen gas with them. They breathe in oxygen from these cylinders (so as to survive on high mountains).