Answer: Option D. -> Hydrogen < Oxygen < Nitrogen
:
D
The correct ascending order of percentages of the gases as per the composition of air is: hydrogen and other gases - 1%, oxygen - 21% and nitrogen - 78%.

Answer: Option B. -> Carbon dioxide
:
B
Air over industrial cities is polluted. This is due to the emission of carbon dioxide and other harmful gases from the factories and industries in their surroundings. Hence, the percentage of carbon dioxide is more near industrial cities compared to its normal percentage in the atmosphere.

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Scuba divers carry cylinders containing oxygen because they cannot utilise the oxygen dissolved in water for respiration.

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Correct Answer : 1Mark
Use: 1 Mark
Oxygen and carbon dioxide.
Plants use carbon dioxide for photosynthesis and oxygen for respiration.

Answer: Option D. -> The flame was put off and water rose in the jar.
:
D
The flame of the candle will burntill there is oxygen available in the jar. After sometime, when all the oxygen is consumed, candle flame will gooff. The water from the container will rise in the jar to occupy the space of the consumed oxygen gas.

Answer: Option C. -> It contains water vapour.
:
C
Air is a mixture of invisible gases, that surrounds the earth. It mainly consists of gases namely nitrogen (78%), oxygen (21%), argon (0.9%), carbon-dioxide (0.03%) and water vapour. Composition of air is as follows:

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Each property :1Mark
The properties of air are as follows:
1) It is transparent.
2) It supports all life forms on Earth.
3) It is colourless.
4) It occupies space.
5) It is present everywhere - on land, in water and in soil.

:
a) 1 Mark
b) 1 Mark
c) 1 Mark
a)There is a large amount of moisture in the air on a cloudy day. So, clothes do not dry quickly on a cloudy day. The rate of evaporation is higher i.e. clothes dry quicker when the air is less humid.
b)Hydrogen is a combustible gas and forms an explosive mixture with air, so it is not advisable to fill hydrogen gas in big air balloons.
c) Since air is much heavier than helium, the air pushes the light helium balloon towards higher altitudes where the air is also lighter.

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Experiment : 3 Marks
Diagram : 2 Marks
Take a candle and fix it at the center of a glass container. Add some quantity of water in the glass container. Now place an empty glass jar over it. Above the water surface, mark five lines at equal distances as depicted in the diagram below.
Carefully light the candle and wait for a few seconds. The flame will be extinguished due to the absence of oxygen. We can observe that the water level in the glass jar is 1/5 of the volume of air present in the glass jar. The above experiment proves that one part of the air present in the glass jar supports the combustion process, i.e., oxygen. Hence, 1/5^th of the volume of air is oxygen. Thus, we can conclude that air contains nitrogen and oxygen in the ratio 4:1.

Answer: Option B. -> oxygen
:
B
Aquatic animals use the oxygendissolvedin water for respiration. A majority of aquatic animals like fish and prawns breathe through special organs called gills. Gillsare projections of the skin that help in using oxygen dissolved in water.