Class - 6, Science, Chapter:- 15 (Air Around Us ) Notes, NCERT Solutions & Frequently Asked Questions

Science

Class:-6, Chapter:- 15

(Air Around Us )

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐ Air:- The invisible gaseous medium around us, mainly constituted by oxygen and nitrogen is known as air. 

OR

Air is the mixture of different gases, dust particles & water vapours.

• It is through this air that we are able to carry out the process of respiration. 

⭐ Properties of Air:- 

• Air is colourless & transparent.

• It has mass & occupies space.

• It is present everywhere including water g soil.

• It exerts pressure. 

⭐ Atmosphere:- The thin blanket of air surrounding the surface of the Earth is called the atmosphere.

• Air gets scant (become thinner) as we move up in the atmosphere.

• The atmosphere is divided into five distinct layers on the basis of variations in temperature that changes due to increasing altitude. These are as follows:-

Figure 1.- Layers of atmosphere 

a) Troposphere:- It is the first layer of the atmosphere & nearest to the surface of Earth & is responsible for weather conditions. 

- The troposphere itself is said to contain about more than 75% of the atmosphere. 

b) Stratosphere:- It is the layer just above the troposphere which contains the ozone layer & where the aeroplanes fly & is also home to most of the clouds. 

c) Mesosphere:- It is the third & the coldest layer of our atmosphere & extends to 80 km above the surface of the Earth.

d) Thermosphere:- It is the fourth & the hottest layers of Earth where temperatures go to 1500༠C.

-  This is where the space shuttles go to study Earth from space.

- The air in this layer is very thin & about 99.9% of the atmosphere is said to lie below this particular layer.

e) Exosphere:- It is the outermost layer of the atmosphere where molecules & atoms escape into space.

- Beginning at 480 km above the Earth, this layer then extends into the space.

⭐ Weathercock:- It shows the direction in which the air is moving at that place.

Figure 2.- Weathercock 

⭐ Constituents of Air:- Air is a mixture of a number of gases and some other particles such as:-

a) Water Vapour:- Air contains water vapour which helps to maintain the water cycle.

- When air comes in contact with cold surfaces, the vapours  condenses into droplets of water.

- The amount of water vapour in the air varies from place to place and time to time. 

- At a normal temperature of 30°C air contains upto 4% of water vapour.

b) Oxygen:- It is the oxygen in the air that helps humans and animals carry out the respiration process. 

- Oxygen is also required for fire to keep burning. 

- If we were to keep an inverted tumbler covering a burning candle, the candle will go off in a few seconds because of the lack of oxygen-containing air due to the tumbler.

Figure 3.- Experiment to show that oxygen supports burning 

- Dry air is said contain about 21% of oxygen.

c) Nitrogen:- Dry air is said to contain about 78% of nitrogen. 

- This component of air helps plants in their growth process.

- It doesn't support burning. 

d) Carbon dioxide:- Carbon dioxide is a very small (only 0.04%) component of air and is a byproduct of respiration by humans and animals. 

- Fire also produces carbon dioxide & few other gases. This is why we feel suffocated if there is something burning inside a room. This happens due to an excess of carbon dioxide as the fire continues to burn in the room, choking out oxygen in the air.

e) Dust and Smoke:- Smoke is another component of air produced on burning.

- It is very harmful & adds fine dust particles & few other gases to the air. This is why industries use long chimneys in order to release this smoke in the air. 

- Air also contains very fine dust particles which can be seen flying when a beam of light enters a dark room. 

- It is hence advised to breathe only through nose (not through mouth) so that fine hair & mucus in nose can filter out these dust particles & we don’t inhale them & harm ourselves.

⭐ Composition of the components of air:- Oxygen (21%) and nitrogen (78%) together make up 99% of air while the remaining 1% is made up of carbon dioxide, water vapours & several other gases. 

Figure 4.- The composition of the components of air

⭐ Balance of Oxygen in the Atmosphere:- Humans/animals can’t survive without plants because they produce oxygen via photosynthesis & take up the carbon dioxide produced by animals/humans. 

Figure  5.- Respiration in Humans 

Figure 6.- Respiration in Plants 

• The balance of oxygen in the environment is thus maintained through the respiratory processes of plants and animals.

⭐ Availability of Oxygen in Water and Soil:- Both soil & water have air dissolved in them.

• When we heat or boil water, we often notice that bubbles start to form. These bubbles are in fact, an indication that air molecules are present in the water. 

Figure 7.- Air bubbles can be seen when water is heated

• To see the presence of water in the soil, we take a small lump of it in a beaker and add water to it. We see bubbles coming out of it which as we discussed, is proof of the existence of air molecules in the soil.

Figure 8.- Air particles present in soil

⭐ Availability of Oxygen for living beings:- 

a) Plants:- They use carbon dioxide of air to make their own food by the process of photosynthesis.

b) Aquatic Animals:- Most aquatic animals have gills which helps them to breathe the dissolved oxygen in water.

c) Birds:- They have a pair of lungs with air sacs which is open all time as the birds need high level of oxygen during flight.

d)  Mammals :- They breathe through lungs & take in oxygen g release carbon dioxide. 

⭐ The importance of Air:- Air has a number of uses:-

• The air which is in motion is known as wind. The wind is important for the rotation of windmills which help in drawing water from tube wells.

• They also help in running flour mills.

• Windmills are also used to produce electricity.

• Insects and birds are only able to fly because of the presence of air. 

• Boats, yachts, aeroplanes and parachutes also need air to sail and glide. 

• Air has a very important role to play in the water cycle as well.

Figure 9.- A windmill 

• It also helps in distributing the pollen and seeds from flowers of various plants.

-- NCERT Solutions -- 

Exercise Questions

Question 1.- What is the composition of air?

Solution:- Air comprises water vapour, Oxygen, Nitrogen, Carbon dioxide, dust and smoke.

Question 2.- Which gas in the atmosphere is essential for respiration?

Solution:- Oxygen in the atmosphere is essential for respiration.

Question 3.- How will you prove that air supports burning?

Solution:- Place two candles of the same length on a table. Light both the candles. Cover one of the candles with an inverted glass tumbler. We observe that the candle covered with a glass tumbler got extinguished after some time, whereas the other candle continued burning. The candle gets extinguished because the component inside of the glass tumbler, which supports burning, is limited. Most of the component is used up by the burning candle. However, the other candle is getting continued supply of air. This component of air, which supports burning, is known as oxygen.

Question 4.- How will you show that air is dissolved in water?

Solution:- Take some water in a container. Heat it slowly on a tripod stand. Before water begins to boil, look at the inner surface of the container. We observe tiny bubbles inside. 

These bubbles come from the air dissolved in water. When you heat the water, to begin with, the air dissolved in it escapes. This experiment concludes air is present in the water.

Question 5.- Why does a lump of cotton wool shrink in water?

Solution:- The lump of cotton wool shrink in water because the air inside the cotton lumps are replaced by water which makes the layer stick together.

Question 6.- The layer of air around the earth is known as ___________.

Solution:- the atmosphere.

Question 7.- The component of air used by green plants to make their food, is ___________.

Solution:- carbon dioxide.

Question 8.- List five activities that are possible due to the presence of air.

Solution:- Activities that are possible due to air are:- 

• Photosynthesis

• Cloud formation

• Respiration

• Transpiration

• Winnowing

Question 9.- How do plants and animals help each other in the exchange of gases in the atmosphere?

Solution:- During the process of respiration, animals and plants consume oxygen from the air and release carbon dioxide gas in the air. But green plants also release oxygen gas by utilizing carbon dioxide during the process of photosynthesis. Hence, in this way, plants and animals help each other in the exchange of gases in the atmosphere.

-- Frequently Asked Questions --

Question 1.- What are the properties of air?

Solution:-

• Air occupies space.

• Air is present everywhere around us.

• Air has no colour and one can see through it.

• It is transparent.

Question 2.- What is atmosphere?

Solution:- Our earth is surrounded by a thin layer of air. This layer is called atmosphere. It extends up to many kilometres above the surface of the earth.

Question 3.- What do you mean by relative humidity?

Solution:- The content of water vapour in the air is expressed in terms of humidity or relative humidity.

Question 4.- The mountaineers carry oxygen cylinder with them while climbing high mountains.Why?

Answer:- The availability of air decreases gradually as we go up in the atmosphere.

Question 5.- Describe the composition of air.

Solution:- Air contains mostly nitrogen (78.03%) and oxygen (20.99%). The remaining 1% (by volume) is shared by argon (0.34%), carbon dioxide (0.33%), other noble gases, oxides of nitrogen and sulphur etc., in the decreasing order.

-- By Durgesh Pandey

(Eklavya Coaching Institute)