:
a) 2 Marks
b) 2 Marks
c) 1 Mark
a)The oxygen content decreases as we move to high altitudes like mountains, so mountaineers carry oxygen cylinders with them to breathe there.
b)Burning of some material releases smoke that contains few gases and fine dust particles that is harmful, thus we feel suffocation in a closed room, where some material is burning.
c) This is done so that the harmful gases and smoke of factories are released at a height away from us.

:
Experiment : 3 Marks
Diagram : 2 Marks
Fill a glass vessel or a beaker with water. Place the beaker on the tripod over the burner. Let the water in the beaker heat slowly. Now look at the inner surface of the vessel before the water starts boiling, you will be able to see tiny bubbles on the inside. This happens because, on heating water, the air dissolved in the water escapes in the form of bubbles.

:
Observation : 1Mark
Reason : 2Mark
Whena bottle containing cold drink isopened, bubbles can be seen coming out with a fizzing sound.
Cold drinks contain dissolved CO₂ at high pressure. When we open a bottle of cold drink, the dissolved CO₂ gushes out in the form of bubbles with a fizzing sound due sudden decrease in pressure. The fizzing sound is because of sudden release of CO₂.

:
Each Point: 1Mark
1. Moisture determines the Earth’s climatic conditions.
2. The presence of water vapour in the air causes rain.
3. It controls the rate of evaporation from the bodies of plants and animals.

:
Definition : 1 Mark
Causes : 2 Marks
Air pollutionoccurs when harmful substances including particulates and chemicals are introduced into Earth's atmosphere. It may cause diseases, allergies or death of humans; it may also cause harm to other living organisms such as animals and food crops, and may damage the natural or built environment.
Volcanic eruption, crop pollination and human activities like burning of coal, wood, diesel oil, petrol, kerosene, etc. are the major causes of air pollution.

Answer: Option D. -> Wind
:
D
Moving air is known as wind. Short bursts of wind moving at high speeds are known as gusts. Depending on their strength, winds can be known as a breeze, gale, storm or hurricane.

Answer: Option C. -> Statement I is true and statement II is true.
:
C
The burning of fuel releases harmful gases in the atmosphere. The tall chimneys in factories would take away the gases from the breathing level of the living organisms and would release them high into the atmosphere.

Answer: Option A. -> cut off the oxygen supply.
:
A
In order for a fire to burn, all three elements must be present: heat, fuel and oxygen. If any of the elements are cut off the fire will be put out.
As blanket completely surrounds the person, it seals the person’s body surface around the fire and cuts off the oxygen supply to the fire, thereby putting it out. Hence, blanket acts as a fire extinguisher and stops the fire from spreading.

Answer: Option D. -> Ozone
:
D
Ozone is a colourless gas that can be found throughout all layers of our atmosphere.Most ozone is found in the stratosphere, which begins at about 10–16 kilometres above earth’s surface. The ozone layerprotectsus by absorbing potentially harmful ultraviolet (UV) radiation from the sun. The UVrays can cause sunburn and skin cancer.

Answer: Option A. ->  Air is a mixture of many gases.
:
A
Air occupies space and has mass, therefore, it is matter. Air can be compressed and it exerts pressure in all the directions. For example, when we inflate a balloon, air occupies the space in the balloon and exerts pressure on the walls of the balloon. Air around us is a mixture of gases. It contains nitrogen, oxygen, argon, other gases and also water vapour.