Elements of the atmosphere

D is correct. The atmosphere contains major gases such as nitrogen and oxygen, along with smaller amounts of other gases. Water vapour varies with location and weather, and fine particles such as dust may remain suspended in the air. The other options incorrectly exclude constituents or overstate their abundance.

B is correct. Although nitrogen is more abundant in the atmosphere, oxygen is needed for aerobic respiration in many organisms and supports the burning of many fuels. Oxygen helps combustion but is not itself a fuel. This shows that a gas can be important because of its role, even when it is not the most abundant.

The ozone layer is called protective because ozone molecules absorb much of the Sun’s biologically harmful ultraviolet radiation, especially UV-B and nearly all UV-C. This reduces damage to skin, eyes, plants and aquatic organisms. The layer does not control tectonic plates, stop ocean currents or make clouds transparent. Its protective role is therefore specifically connected with filtering ultraviolet energy, making option A correct.

A is correct. Fine particles such as dust and smoke interact with light travelling from objects to the observer. They scatter some light and may absorb some of it. This reduces the contrast of distant objects against their background, making them harder to see and lowering visibility.

The correct answer is A. Dry desert surfaces often contain loose, fine mineral particles. Wind can lift these particles into the air, where they become atmospheric dust. The other options are not natural, wind-exposed sources of airborne mineral dust.

B is correct. Nitrogen is the largest component of dry air and oxygen is the second largest; together they make up nearly all of its volume. “Dry air” excludes water vapour, whose amount varies with place and time. Their status as major gases is based on their abundance, not their colour or season.

C is correct. Sulphur dioxide can react in the atmosphere to form acidic substances, which can return to the surface in rain or other precipitation and contribute to acid deposition. Tides, eclipses, and mountain building are not caused by SO₂ pollution.

A is correct. Oxygen is used in aerobic respiration, but its role is broader: it supports combustion and participates in oxidation reactions, such as the rusting of iron. Oxygen is a gas and a major component of the atmosphere; it is neither dust nor the colour of water vapour.

The governing idea is chemical inertness and atmospheric stability. Gases such as the noble gases react very little under ordinary atmospheric conditions, so natural processes do not rapidly convert or remove them. Their proportions therefore remain comparatively stable over time, although “permanent” does not mean absolutely unchanging. They do not form daily rain, are not the strongest greenhouse gases, and are not restricted to organisms. Hence option A is correct.

A is correct. During photosynthesis, plants take in carbon dioxide and release oxygen. In respiration, organisms generally use oxygen to release energy from food and produce carbon dioxide. These processes therefore exchange the two gases in opposite directions, although their amounts are not always equal.

A is correct. Ozone in the stratosphere absorbs harmful ultraviolet radiation from the Sun, especially UV-C and much of UV-B, reducing the amount that reaches Earth’s surface. Radio waves are a different part of the electromagnetic spectrum, while seismic and ocean waves are not ultraviolet radiation.

O₂ is a diatomic molecule containing two oxygen atoms and supports respiration and combustion. O₃ is ozone, a triatomic form containing three oxygen atoms; in the stratosphere it absorbs harmful ultraviolet radiation, although near the ground it is a pollutant. Their molecular structures and roles are therefore different. Option A is correct; B, C and D are factually wrong.

The correct answer is A, nitrogen. Nitrogen makes up about 78% by volume of dry air, compared with about 21% for oxygen; argon and carbon dioxide are present in much smaller proportions. Thus nitrogen is the most abundant permanent component by volume.

The correct answer is A, about 21%. Oxygen makes up approximately 21% by volume of dry air. The value of about 78% is for nitrogen, while argon is present at slightly less than 1%.

The correct answer is A. Argon is chemically inert under ordinary atmospheric conditions and its proportion in air is relatively stable, at slightly less than 1% by volume. These properties make it an inert permanent gas. It is not a greenhouse gas in the same important sense as gases such as carbon dioxide, nor does it support respiration like oxygen.

A is correct because oxygen acts as the oxidiser in ordinary combustion in air, reacting with fuel and helping the burning reaction continue. Oxygen supports combustion but is not itself the fuel.

The correct answer is A. Dry air means air considered without water vapour. Water vapour is described separately because its amount varies considerably with temperature, humidity, and location, unlike the relatively stable proportions of the main gases in dry air.

A is correct because human cells use oxygen in aerobic respiration to release usable energy from food. Nitrogen is more abundant in air, but humans do not use it directly in this way during ordinary respiration.

A is correct. An eruption can throw fine fragments of rock and magma into the air as ash and dust, sharply increasing these particles near the volcano and downwind. Eruptions may also release gases, but dust and ash are the matching option here.

The correct answer is A. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation, especially UV-B and UV-C. This shielding effect means that even a small amount of ozone can help protect life at Earth’s surface.

The correct answer is B. Permanent gases such as nitrogen and oxygen have relatively stable proportions in the lower atmosphere. In contrast, components such as water vapour vary considerably with location, weather, and season. “Permanent” refers to stable proportions, not to gases disappearing at a particular altitude.

The correct answer is C. Water vapour varies greatly with place, temperature, and weather. Excluding it from dry-air composition makes it easier to compare the more stable proportions of gases such as nitrogen and oxygen.

Correct answer: A. In combustion, oxygen reacts with a fuel in an oxidation reaction, usually releasing energy as heat and often light. Condensation nuclei are tiny particles, such as dust or salt, on which water vapour can condense; their formation is not a role of oxygen. Argon’s inertness is a property of argon itself.

Correct answer: B. Argon is a noble gas and is chemically very unreactive under ordinary atmospheric conditions. It is a relatively stable constituent of dry air, unlike water vapour, whose amount changes substantially with weather and humidity.

Correct answer: A. Nitrogen makes up about 78% and oxygen about 21% of dry air. Together, they account for about 99% of it, so studying them together explains most of the atmosphere’s gaseous composition.