Elements of the atmosphere

The correct answer is A, argon. In dry air, nitrogen makes up about 78% by volume, oxygen about 21%, and argon about 0.93%, making argon the third most abundant gas. Carbon dioxide, ozone and methane are present in much smaller amounts.

The governing concept is the percentage composition of Earth’s atmosphere. Nitrogen, at about 78%, and oxygen, at about 21%, together form nearly all of dry air, so option A is correct. Ozone and methane are trace gases, helium and hydrogen are present only in very small amounts, and dust and ash are suspended particles rather than the dominant gaseous pair.

A is correct: nitrogen is a permanent gas whose proportion in dry air is broadly stable; it makes up about 78% of dry air. Water vapour varies considerably, smoke is a mixture of particles and gases, and a cloud droplet is liquid water rather than a gas.

The governing concept is atmospheric variability. Water vapour is a variable element because its concentration changes substantially with temperature, place, season, evaporation, and condensation; therefore option B is correct. Nitrogen and oxygen are major gases whose proportions are comparatively stable, while argon is also relatively constant. Water vapour may range widely, unlike these permanent or nearly permanent gases.

C is correct. An aerosol consists of tiny solid particles or liquid droplets dispersed in air. Volcanic ash is made of fine solid particles released by eruptions, so it is a natural atmospheric aerosol. Argon, oxygen, and nitrogen are gases, not aerosols.

D is correct: oxygen supports ordinary combustion by reacting with fuel and helping release energy as heat and light. Oxygen is not itself the fuel. Nitrogen and argon do not support ordinary burning, while carbon dioxide generally does not support combustion and can help smother flames.

The correct answer is A. Nitrogen makes up about 78% of dry air and is relatively unreactive under ordinary combustion conditions. By diluting the oxygen in air, it generally makes combustion less intense than it would be in oxygen-rich conditions. This does not mean nitrogen actively extinguishes every fire.

Direct answer: D, water vapour. Fog is a cloud that forms near the ground. The basic process is condensation: moist air cools to, or near, its dew point, and water vapour changes into countless tiny liquid droplets. These droplets remain suspended near the surface and scatter light, making distant objects difficult to see. Water vapour therefore supplies the water that becomes fog. Argon and helium are noble gases and do not normally condense into fog droplets under ordinary atmospheric conditions. Nitrogen is abundant, but it is not the material that changes into the droplets. Thus D is correct. A helpful distinction is that water vapour is invisible, while fog consists of visible suspended droplets formed after condensation; the vapour is the starting material, not the final droplet itself.

The correct answer is B, a noble or inert gas. Argon is a noble gas and is chemically unreactive under ordinary conditions. It is a minor, relatively stable component of dry air, making up about 0.93% by volume. Oxygen, not argon, is the main gas used in human respiration; water vapour is an example of a variable gas.

The correct answer is C: nitrogen, oxygen, argon. In dry air, nitrogen makes up about 78% by volume, oxygen about 21%, and argon about 0.93%. Arranging these from greatest to least gives nitrogen, oxygen, then argon.

The correct answer is C, an aerosol. An aerosol is made up of tiny solid particles or liquid droplets suspended in a gas. Smoke contains fine particles dispersed in air, so it is an aerosol. It is not itself a permanent gas, water vapour, or ozone.

The correct answer is D. An ozone molecule contains three oxygen atoms, so its chemical formula is O₃. Ordinary oxygen is O₂. Ozone in the stratosphere absorbs much harmful ultraviolet radiation, while ozone near the ground can be an air pollutant.

Option A is correct because the atmosphere is the envelope of air surrounding Earth and contains a mixture of gases, variable water vapour, and suspended solid or liquid particles such as dust and aerosols. Rocks, lava, rivers, glaciers, mountains, plateaus, and plains are features of the lithosphere or hydrosphere, not the basic constituents of the atmosphere.

The correct answer is B, ozone. Most atmospheric ozone is in the stratosphere, where it absorbs much of the Sun’s harmful ultraviolet radiation and reduces the amount reaching Earth’s surface. Its concentration is small compared with nitrogen and oxygen, but its absorption has an important protective effect.

The correct answer is C, dust and smoke. Their suspended particles scatter and may absorb visible light, making distant objects harder to see and reducing visibility. Water droplets can also reduce visibility when they form fog, but dust and smoke are the best answer for particle haze.

Option B is correct because the atmosphere is a mixture rather than a single substance. Its main gases include nitrogen and oxygen, and it also contains variable water vapour plus suspended particles such as dust, smoke, and aerosols. The atmosphere is therefore not made only of oxygen or stones, and the statement that it contains no gases directly contradicts its definition as Earth’s gaseous envelope.

Option C is correct: an ozone molecule has the formula O₃ and contains three oxygen atoms. Ordinary molecular oxygen is O₂, with two oxygen atoms per molecule. They are different molecular forms of the same element. They do not occur in equal amounts, and oxygen is not an inert gas or a form of water.

The correct answer is B. Water vapour is variable: it enters the atmosphere through evaporation and can condense into clouds as moist air cools. It is therefore involved in humidity, cloud formation, and precipitation. Evaporation and condensation also absorb or release latent heat, influencing temperature. Its amount does change, so C is incorrect.

The correct answer is D. Argon is a noble gas and is chemically inert under ordinary atmospheric conditions. It is a regular, relatively stable constituent of dry air, even though it is much less abundant than nitrogen and oxygen. It is a gas, not water vapour or a dust particle, and it does not form ozone.

The correct answer is B: oxygen supports combustion. In ordinary burning, a fuel reacts with oxygen and releases energy as heat and often light. Oxygen is therefore needed for the combustion reaction to continue; it is not itself a fuel.

The correct answer is B: nitrogen, oxygen, argon. In approximately dry air, these gases make up about 78%, 21%, and 0.93% by volume, respectively. Decreasing order goes from the greatest proportion to the smallest, so nitrogen comes first, followed by oxygen and then argon.

The correct answer is C: aerosols. Aerosols are tiny solid particles or liquid droplets suspended in a gas. Ash consists of fine solid particles, and smoke contains fine particles dispersed in air, so both are classified as atmospheric aerosols.

The correct answer is B, N₂. A nitrogen molecule contains two nitrogen atoms, shown by the subscript 2. O₃ is ozone, while CO₂ is carbon dioxide and Ar is argon.

The correct answer is A. Water vapour varies from place to place and over time. Evaporation adds it to the air, while condensation and precipitation remove it. Its amount is therefore affected by conditions such as temperature and weather; it is not constant like the main permanent gases.

Correct answer: C. Argon is a noble gas and is very unreactive under ordinary atmospheric conditions. Its proportion in dry air is small but nearly constant, so it is considered a permanent gas. Nitrogen is the most abundant gas, oxygen is used in aerobic respiration, and cloud droplets are liquid water.