(a) Akshay and his friends went deep sea diving where the level of oxygen is reduced. Due to lack of sufficient oxygen supply, problems in breathing occur.
(b) By carrying oxygen cylinders, this situation can be overcome.
(c) Two more situations include (i) Mountain climbing. (ii) Patients with breathing problems.
(d) Plants and animals living in the water use dissolved oxygen, i.e. oxygen gas dissolved in water for respiration through special organs or pores.
(e) Akshay is responsible and adventurous. He is cautious and observant of his surroundings.

(a) There will be no life on the earth. Oxygen is vital for life. It helps in burning, respiration, photosynthesis, etc. Oxygen is basic need of all living beings.
(b) Nitrogen does not support burning. If nitrogen gas disappear from the atmosphere, things will burn very fast.
(c) Carbon dioxide is needed for photosynthesis which occurs in green plants. In its absence, plants will not be able to prepare their food and hence, there will be no life on the earth.

(a) Motion of firki is based on the pressure/force applied by air on the blades of the firki. A firki does not rotate in a closed area because of lack of air movement.
(b) The arrow of weather cock points towards a particular direction as it shows the direction in which the air is moving at that place at a particular moment.
(c) An empty glass in fact is not empty. It is filled with air. Air is present everywhere.
(d) Breathing through mouth may harm us because we may inhale dust if present in air which is harmful.

Humans and animals use oxygen present in the environment to carry out respiration and release CO₂ as a by- product of this process. Plants perform photosynthesis by using this CO₂ present in the atmosphere accumulated due to activities of humans and animals. They release oxygen as their reaction by product which also gets out in the atmosphere. Plants depend on humans and the activities performed by them like burning of fuels, etc for CO₂ and humans depend on plants for a continuous oxygen supply. Thus, these two processes maintain a unique balance between O₂ and CO₂ concentration in our environment.

The presence of small amount of carbon dioxide gas in the atmosphere is essential for the growth and existence of living beings. The desirable proportion of carbon dioxide in the atmosphere is 0.03%. The importance of carbon dioxide in atmosphere is as follows
(i) It causes greenhouse effect, leading to the heating of the earth and its atmosphere which is essential for our survival.
(ii) It takes part in the photosynthesis process carried out by green plants to prepare food for plants and animals.

(a) Ravi observed the presence of dust particles in air. These particles are present at all times in air.
(b) Our nostrils contain small hairs and mucous lining covers the entire respiratory tract. The hairs and mucous trap these dust particles when they enter during inhalation and push them out through sneezing.These dust particles are produced due to regular activities like cleaning and dusting of house, construction of buildings, vehicles driving, etc.
(c) No, breathing through mouth allows the entry of harmful things in our body like dust particles, microorganisms, etc as mouth has no ways to prevent their entry.
(d) Ravi is an intelligent and curious boy

(a) Air is dissolved in water in the form of oxygen. Tiny bubbles are formed, when air present in the water escape on boiling it. So, tiny bubbles appeared due to the evolution of air dissolved in water.
(b) No, tiny bubbles will not appear as there is no dissolved air in this water.

Air is a mixture of gases. The major composition of air are oxygen, nitrogen, carbon dioxide, etc. The percentage composition of These dust particles are produced due to constituents of air are given below:

Nitrogen - 78%, oxygen - 20.9%, argon - 0.9%, carbon dioxide - 0.035 - 0.0,45%, water vapour - 0.03%, dust, etc.

On a sunny day, go to a room which is a little bit dark. In this room, choose a thin opening through which light can enter the room. If, there is no opening in the room, then make one. From this opening we will see that a sharp beam of sunlight enters the room. In this beam, we will observe that there are some tiny solid-like particles moving freely. These particles are the dust particles, and this shows the presence of dust particles in air.

Take a candle and fix it at the centre of a glass vessel. Add some quantity of water in the vessel. Now place an empty dry glass jar over it. Mark five lines at an equal distance above the water surface. Now, carefully light the candle and cover the glass jar. Wait for a few seconds and how the flame will be extinguished due to absence of oxygen to bum.
Now, we can observe that the water level raised in the glass jar is 1/5 of the volume of air present in the glass jar
This experiment shows that one-fifth of air is oxygen