The study of coordination compounds is an important aspect of Chemistry Topics, as it examines the bonding and reactivity of metal complexes.
Composition of Air: Definition, Properties, Diagram
We will now describe what air is made up of. This is called composition of air. We know that air is a gas. For a long time, people thought that air was just a single gas. It has now been proved by experiments that air is not a single gas. Air is a mixture of many gases. The major component of air is nitrogen gas. Almost four-fifths of air is nitrogen gas. The second major component of air is oxygen gas.
About one-fifth of air is oxygen gas. In addition to nitrogen and oxygen gases, air also contains small amounts of carbon dioxide gas, water vapour and some other gases (such as argon and helium, etc.) The air may also contain some dust particles. We can now describe the composition of air as follows : Air is mainly a mixture of nitrogen and oxygen gases with small amounts of carbon dioxide gas, water vapour and other gases. Some dust particles may also be present in air. The composition of air in terms of percentage of its various components can be written as follows :
1. Nitrogen : 78 per cent
2. Oxygen : 21 per cent
3. Carbon dioxide : 1 percent
Other gases, and
Thus, air contains mainly nitrogen and oxygen gases. In fact, nitrogen and oxygen gases, taken together, make up about 99 per cent of air (78 percent + 21 percent = 99 per cent). The remaining 1 per cent of air consists of carbon dioxide gas, water vapour, a few other gases and some dust particles.
The composition of air can also be represented with the help of a diagram as shown in Figure. The composition of air is not always exactly the same. The composition of air changes slightly from place to place and season to season. For example, the air over industrial cities usually has a higher amount of carbon dioxide in it than the air over open spaces. The air in coastal areas may have more water vapour than inland areas. The air also contains more water vapour in rainy season. Similarly, the amount of dust in the air is more in windy places than other areas.
Activity To Show That Air Is A Mixture Of Gases
We take a trough (a kind of vessel) and place a gas jar stand in it. Fix a candle on the gas jar stand [see Figure (a)]. Fill half the trough with water. Dissolve some caustic soda (sodium hydroxide) in this water to form caustic soda solution. Also add a few drops of ink in water to colour it.
(i) To start the activity, we light the candle with a matchstick. Cover the burning candle by placing an inverted gas jar over it [see Figure (a)].
(ii) After a short time, the candle stops burning and water rises up in the gas jar to a certain level [see Figure (b)]. The candle stops burning because all the oxygen of air present in the gas jar is used up by the burning candle. Oxygen is a supporter of combustion (or burning). So, when oxygen present in air contained in the gas jar is all used up, the burning candle gets extinguished.
(iii) When the candle burns, then oxygen of air in the gas jar is used up and carbon dioxide gas is formed. This carbon dioxide gas is absorbed (or dissolved) by the caustic soda solution in the trough. The absorption of carbon dioxide gas by caustic soda solution creates a vacuum (low pressure) in the gas jar. Water rises up in the gas jar to fill this vacuum.
(iv) Water in the gas jar rises to about one-fifth part of the volume of air initially present in the gas jar [see Figure (b)]. The volume of water risen in the gas jar is equal to the volume of oxygen present in the air in the gas jar (which was consumed during the burning of candle). From this we conclude that about one-fifth of air is oxygen. The major part of air which is not used up by a burning candle and remains behind in the gas jar is nitrogen. Since water does not rise in the remaining four-fifth part of the gas jar in this activity, it shows that about four-fifths part of air is nitrogen.
We will now discuss all the components of air in detail.
Nitrogen is a colourless, tasteless and odourless gas. Nitrogen gas is slightly soluble in water. Nitrogen gas is not essential for breathing (or respiration). Nitrogen gas neither bums itself nor supports the process of burning. That is, nitrogen does not support combustion. Nitrogen is a very unreactive gas. In this respect, nitrogen is just the opposite of oxygen (which is a very reactive gas).
Nitrogen gas present in air is important for living things. All the living things need nitrogen compounds for their growth. The plants use nitrogen gas of the air to make proteins. These proteins are used by the plants for their own growth as well as for the growth of human beings and other animals. Thus, nitrogen is needed by plants to make proteins.
Oxygen is a colourless, tasteless and odourless gas. Oxygen gas is slightly soluble in water. Oxygen gas is essential for breathing (or respiration). We can live for several days without food and water but we will die very quickly without oxygen. So, the oxygen present in air is important for the living things. All the living things need oxygen of air for breathing.
Oxygen gas does not burn itself but it allows other things to burn in it. So, oxygen is a supporter of burning (or supporter of combustion). Thus, oxygen (or air) is also necessary for the process of burning (or combustion) of fuels such as wood, coal, kerosene, petrol and LPG. Thus, oxygen supports life and combustion. Let us discuss this in somewhat detail.
1. Oxygen is Necessary for Living Things
All the living organisms breathe air to remain alive. So, air is necessary to sustain the life of all the living things: men, animals and plants. When a person breathes in air, it is actually the oxygen present in air which is utilized in the process of respiration. So, we can also say that oxygen is necessary for breathing. Thus, whether we say that air is necessary for breathing or oxygen is necessary for breathing, it means the same thing.
All the living things (animals and plants) use the oxygen of air for respiration. During respiration, oxygen breaks down food to give carbon dioxide, water and energy. This energy sustains the life of all the animals and plants. The animals and plants living on land take the oxygen required for breathing from the air around them. But the animals and plants living in water take the oxygen required for breathing from the air dissolved in water. We will now describe a simple activity to show that air (or oxygen) is necessary for the survival of living things.
We take a small glass jar and put some food and water in it. A few living insects are caught and put in this glass jar. The mouth of glass jar is closed tightly by tying a polythene sheet around it. We will find that the insects gradually become inactive and die after a few days. This can be explained as follows: The closed glass jar had air in it which contained some oxygen. This oxygen was used by the insects for breathing. After a few days all the oxygen of air in the closed glass jar was used up by the insects in breathing. Since no oxygen was left in the closed glass jar, the insects could not breathe any more and hence died. This activity shows that air (or oxygen) is necessary for breathing and maintaining life.
Under ordinary conditions, we use the air around us for breathing purposes. But under special situations, oxygen gas cylinders are used for breathing purposes. Three such special situations are: a patient having breathing difficulties, a mountaineer climbing a high mountain and a diver going into deep sea. Let us discuss all this in somewhat detail.
The component of air which is utilised in breathing (or respiration) is oxygen. The patients suffering from diseases like asthma, etc., have sometimes difficulty in breathing properly in air because only one- fifth part of air is oxygen. Similarly, a patient undergoing a major surgical operation also has difficulty in breathing properly in the surrounding air. So, oxygen gas is given to patients in hospitals who have difficulty in breathing properly. All the hospitals have cylinders filled with oxygen gas for such patients. Figure (a) shows a patient being given oxygen gas from a cylinder in a hospital.
The persons who climb high mountains are called mountaineers. The mountaineers carry oxygen gas cylinders with them while climbing high mountains [see Figure (b)]. At high altitude of mountains, the air is very thin. So, the air at high altitudes contains very little oxygen due to which it becomes difficult to breathe properly. The mountaineers use the oxygen gas contained in cylinders for breathing properly under conditions of high altitudes.
When the divers go deep into sea-water, they carry oxygen gas cylinders with them [see Figure (c)], This oxygen gas is used by divers for breathing while they are under water. This is because there is no free oxygen in the sea-water which can be used by the divers for breathing. Please note that the little dissolved oxygen present in sea-water cannot be used for breathing by human beings such as a diver. It can be used only by animals like fish which live in water.
2. Oxygen is Necessary for Burning (or Combustion)
The process of burning of a substance is called combustion. Air is necessary for the burning of a substance (or combustion of a substance). In fact, air is known as a supporter of combustion. A substance cannot burn without air. Now, when a substance burns in air, it is actually the oxygen present in air which is used up in the process of burning. So, we can also say that oxygen is necessary for burning (or combustion). Thus, whether we say that air is necessary for burning or oxygen is necessary for burning, it means the same thing. The substances which are burning need a supply of fresh air all the time. This is because fresh air is rich in oxygen. We will now describe a simple activity to show that air (or oxygen) is necessary for the burning of things (or combustion of things).
We take a candle and fix it on a table. The candle is lighted by using a burning matchstick. The candle will continue to burn in this case (see Figure a)I. This is because fresh air is continuously available to the candle for its burning process. The fresh air supplies sufficient oxygen necessary for the continuous burning of the candle.
We now cover the burning candle by putting an inverted gas jar over it. After a short time, the candle stops burning, it gets extinguished [see Figure (b)). This can be explained as follows: When the burning candle is covered with a gas jar, then the candle takes the oxygen necessary for burning from the air enclosed in the gas jar. Since only a small amount of air is present inside the gas jar. al its oxygen is used up in a short time. And when all the oxygen of air inside the gas jar is used up. then the burning candle gets extinguished.
Thus, when a burning candle is covered with a gas jar, then the fresh supply of air to the candle is cut oil and hence it stops burning. From this activity we conclude that air is necessary for burning (or combustion) of things. In fact, the process of burning cannot continue without a continuous supply of air (or oxygen). In other words, the process of burning can occur only in the presence of oxygen (of air).
When we burn a candle, paper, kerosene, coal, wood or cooking gas (LPG), oxygen is needed. The oxygen needed for the burning of candle, paper, kerosene, coal, wood and cooking gas comes from the air around us. Thus, for burning a substance continuously so as to make fire, a continuous supply of fresh air is needed (to provide sufficient oxygen for the burning process). If, however, we cut off the supply of fresh air to a burning substance, then the burning substance will not get oxygen necessary for burning to continue and hence the substance will stop burning (or the fire will get extinguished). This will become more clear from the following examples.
If burning coal is covered with a vessel, it stops burning after some time. This is because when we cover the burning coal with a vessel, then the supply of oxygen of fresh air to the burning coal is cut off.
And in the absence of oxygen of air, the coal stops burning. The coal fire gets extinguished. Similarly, when a burning candle is covered with a gas jar, the supply of oxygen of fresh air is cut off and it stops burning. It is a common practice that if the clothes of a person catch fire (while working in the kitchen, etc.), a woollen blanket is put around the burning clothes to extinguish the fire. Actually, when the burning clothes of a person are covered with a woollen blanket, then the supply of fresh air (or oxygen) to the burning clothes is cut off and hence the clothes stop burning (or the fire gets extinguished). In all the above cases when the supply of air (or oxygen) to a burning substance is cut off, then the process of burning (or combustion) also stops. Thus, the presence of oxygen (of air) is necessary for combustion to take place.