Elements of the atmosphere

The governing concept is the definition of a factor and the classification of sets by cardinality. A factor of 1 must divide 1 exactly, and among natural numbers only 1 does so. Therefore U = {1}, which contains exactly one element. A set with one member is called a singleton set, so option A is correct; it is neither empty, infinite, nor a two-element set.

Nitrogen is the most abundant constituent of dry air, forming approximately 78% of the atmosphere by volume. Oxygen follows at about 21%, while argon and carbon dioxide occur in much smaller proportions. Therefore, option A is correct. The question asks for the largest amount, not the gas that is most important for respiration or climate.

The correct answer is B: oxygen supports respiration and combustion. Many organisms use oxygen in aerobic respiration to release usable energy from food. Oxygen also supports combustion when a fuel reacts with it and releases heat, often with light; oxygen itself is not the fuel. Clouds form from condensed water vapour, and dust mainly comes from soil, rocks and other particles.

The correct answer is B: ozone in the stratosphere absorbs much of the Sun’s harmful ultraviolet radiation, reducing the amount that reaches Earth’s surface. This protection helps limit damage to living things. Wind speed is mainly influenced by pressure differences and other atmospheric conditions, while ocean waves are produced mainly by wind and other ocean processes.

The correct answer is C: argon is an inert, or noble, gas. It does not readily react with other substances under ordinary conditions. Argon makes up about 0.93% of dry air, far less than nitrogen or oxygen. Water vapour is H₂O, while argon is not the main greenhouse gas.

Dust particles are tiny solid aerosols suspended in the atmosphere. They may come from soil, deserts, roads, fires or other surfaces, and they can provide nuclei on which water vapour condenses during cloud formation. Therefore option B is correct. Oxygen and nitrogen are gases, while ozone is a gaseous form of oxygen rather than a solid atmospheric particle.

Molecular oxygen normally exists as a diatomic molecule, meaning that two oxygen atoms are chemically bonded together. Its formula is therefore O₂, so option D is correct. CO₂ is carbon dioxide, N₂ is nitrogen, and O₃ is ozone, a different triatomic form of oxygen. The subscript 2 specifically indicates two oxygen atoms in one oxygen molecule.

Nitrogen gas occurs mainly as a diatomic molecule, so two nitrogen atoms combine to form one N₂ molecule. Consequently, option A is correct. O₂ represents molecular oxygen, O₃ represents ozone, and CO₂ represents carbon dioxide. The subscript 2 is not a charge; it shows that each molecule contains two atoms of nitrogen.

The governing atmospheric concept is the protective role of stratospheric ozone. Ozone absorbs a significant part of the Sun’s harmful ultraviolet radiation before it reaches Earth’s surface, so option A is correct. This protection helps reduce biological damage. Ozone does not increase river water, form soil, or control lava; those alternatives are unrelated to its atmospheric function.

Water is unusual among the major atmospheric constituents because it can exist in all three common states. It is present as invisible water vapour, as liquid droplets in clouds and rain, and as solid ice crystals or snow particles. Thus option A is correct. Nitrogen, argon, and helium are gases under ordinary atmospheric conditions and are not normally present as cloud droplets or ice particles.

The correct answer is A: visibility may decrease. Suspended dust particles scatter and may absorb light, making the air hazy. As a result, less light from distant objects reaches our eyes clearly, so those objects may look blurred.

The correct answer is B: water vapour. Humidity refers to the moisture in the air, which is present as water vapour. Relative humidity compares the amount of water vapour in the air with the maximum amount it can hold at that temperature.

The correct answer is A: trace gases. “Trace” means present in a very small amount, so gases found in tiny concentrations compared with major atmospheric gases such as nitrogen and oxygen are called trace gases. Some, such as ozone and carbon dioxide, have important roles despite their small amounts.

The correct answer is B, argon. In dry air, argon makes up about 0.93% by volume. Nitrogen and oxygen are much more abundant, at about 78% and 21%, respectively; ozone occurs in only trace amounts.

Nitrogen is the most abundant gas in dry atmospheric air, making up roughly 78% by volume. As a diatomic molecule, N₂ is relatively unreactive because its triple bond is strong, although nitrogen can react under suitable conditions. Therefore option A is correct. It does not only form clouds, remain solid, or create ultraviolet radiation.

The correct answer is C, ozone. Ozone in the stratosphere absorbs much of the Sun’s harmful ultraviolet radiation, helping protect living organisms at Earth’s surface. This protective region is known as the ozone layer.

A variable gas is one whose concentration changes noticeably with location, season, weather or other atmospheric conditions. Water vapour varies greatly because evaporation, condensation, temperature and rainfall continuously change its amount. Thus, option D is correct. Nitrogen, oxygen and argon are comparatively stable permanent gases in the atmosphere, although their proportions can show small local variations.

The atmosphere is commonly described through three broad components: permanent and variable gases, water vapour, and suspended solid or liquid particles such as dust, smoke and salt aerosols. Therefore, option A is correct. Rocks, soil and lava belong mainly to the lithosphere, water bodies belong to the hydrosphere, and mountains, plateaus and plains are landforms rather than atmospheric elements.

The governing concept is condensation. Clouds form when moist air rises, cools, and reaches a condition in which water vapour changes into tiny liquid droplets or ice crystals around microscopic particles. Thus, water vapour is the atmospheric component that directly supplies the material for clouds. Argon, helium, and nitrogen are gases that do not normally condense to form clouds. Cloud formation is therefore linked with humidity, cooling, and condensation.

The correct answer is D, O₂. Most oxygen in the atmosphere is present as molecules made of two bonded oxygen atoms. O₃ is ozone, another form of oxygen, but it occurs in much smaller amounts. CO₂ and N₂ are different compounds or elements, not the usual form of atmospheric oxygen.

A is correct. Argon is a noble gas with a complete outer electron shell, so it is very unreactive under ordinary conditions and is commonly called inert. It is present in air but does not support respiration as oxygen does.

The correct answer is C, aerosols. Aerosols are tiny solid particles or liquid droplets suspended in air. Dust consists of fine solid particles, so it is an atmospheric aerosol. Water vapour is gaseous water, while permanent and greenhouse gases are gases rather than dust particles.

The correct answer is D. Nitrogen makes up most of the atmosphere and is relatively unreactive under ordinary conditions. By diluting oxygen in the air, it helps prevent combustion from being as rapid as it would be in oxygen-rich conditions. It does not remove all water vapour, destroy ozone as a general role, or form the oceans.

B, oxygen, is correct. Oxygen makes up about 20.95% of dry air, commonly rounded to 21%. Nitrogen makes up about 78%, while argon and carbon dioxide occur in much smaller proportions.

D, O₃, is correct. Each ozone molecule contains three oxygen atoms, as indicated by the subscript 3. Ordinary molecular oxygen is O₂, with two oxygen atoms per molecule.