Humidity, evaporation, and condensation

Correct answer: A. Evaporation is a change in which liquid water at a surface becomes water vapour and enters the atmosphere. It may occur from oceans, lakes, rivers, wet soil, and plant or animal surfaces when water molecules gain enough energy to escape. Thus, “evaporation to water vapour” correctly matches a process with its result. Option B is wrong because condensation changes water vapour into liquid droplets or, under suitable cold conditions, ice; it does not produce dry sand. Option C is wrong because transpiration is the release of water vapour from plant leaves through small openings, not the formation of rock. Option D is wrong because the dew point is a temperature at which air becomes saturated and condensation may begin; it is not an earthquake and cannot cause one by definition. The question tests whether the student knows both the process and the resulting state or material. In a phase-change question, ask: what is the starting substance, and what does it become? Liquid water becomes vapour during evaporation. Memory cue: evaporation sends water upward as vapour; condensation brings vapour back into droplets.

Correct answer: A. Moist air contains invisible water vapour. When this air touches a cold glass surface, the air immediately next to the glass is cooled. Cooling lowers the amount of vapour that the air can hold. If the air reaches its dew point, it becomes saturated and excess vapour changes into visible liquid droplets on the glass. This change from gas to liquid is condensation. B is wrong because glass does not burn vapour; no burning is involved. C is wrong because the vapour has not simply vanished—the visible droplets are the condensed water. D is wrong because pressure is not the substance into which air is changed. Memory cue: cold surface plus moist air produces condensation, like droplets on a chilled glass.

Correct answer: B, evaporation from oceans. Atmospheric moisture means water vapour present in the air. Oceans cover a very large part of Earth and provide an enormous exposed water surface. Solar energy causes some ocean water to evaporate, adding water vapour to the atmosphere. This vapour is later transported, cooled, condensed and returned through precipitation. A is wrong because the Earth’s interior is not the main exposed source of atmospheric water vapour. C is wrong because mountain erosion produces sediment and does not supply the main moisture. D is wrong because volcanic ash is solid material, not the principal source of water vapour. Land, lakes, rivers, vegetation and ice can also contribute moisture, but oceans are the dominant natural source. Memory cue: in the water cycle, oceans are the great source and precipitation is the return.

Correct answer: A, when fog forms. Fog is made of very small water droplets or, in some cold conditions, ice particles suspended near the Earth’s surface. These particles scatter and partly block light, making distant objects difficult to see. Fog is therefore a form of condensation that directly reduces horizontal visibility near the ground. B is wrong because a completely clear sky normally provides good visibility, although other factors can affect it. C and D are wrong because dry air or zero moisture supplies no droplets for fog formation. A cloud is also condensed water, but it is generally above the surface; fog is the near-surface form that creates the visibility problem described here. Memory cue: cloud at the ground is fog, and fog means reduced visibility.

Correct answer: A, continuous evaporation from the sea. The sea is a vast and generally available source of liquid water. Energy from the Sun and the atmosphere enables water molecules at the sea surface to escape into the air as water vapour. Winds can then transport this moist air to nearby coastal areas. The amount of moisture also depends on temperature, wind, and other conditions, but continuous evaporation is the simplest reason offered by this question. B is the opposite of what happens. C is false because seawater is not always solid. D is false because air is present above the sea and is the medium that receives the vapour. Memory cue: large water surface plus evaporation usually means moist air.

Correct answer: C, condensation. Water vapour is a gas and is usually invisible. When moist air cools to the dew point, it becomes saturated; further cooling or suitable surfaces allow the vapour to change into tiny liquid water droplets. These droplets scatter light and become visible as cloud, fog, mist, or a similar collection of moisture. A is the reverse process, changing liquid water into invisible vapour. B describes water moving over the land surface after rain or melting, not a change from gas to liquid. D is a general word for movement and does not name this change of state. The visibility comes from many small droplets, not from the water vapour itself. Memory cue: invisible vapour becoming visible droplets is condensation.

Direct answer: C. The amount of water vapour changes several important atmospheric processes. When moist air cools to its dew point, water vapour can condense into droplets and form clouds; further growth of droplets may contribute to precipitation. Water also absorbs and releases latent heat: evaporation takes heat from the surroundings, while condensation releases that stored energy. This affects air temperature, convection, cloud development and weather conditions. A is incorrect because water vapour does not only form rocks; rock formation is not its main weather role. B is incorrect because normal changes in atmospheric water vapour do not change Earth’s overall shape. D is incorrect because water vapour does not stop or block the sea. Memory cue: more vapour means more moisture available for clouds, rain and energy exchange, although vapour alone does not guarantee immediate rain.

Direct answer: B, condensation. Saturated air already contains the maximum water vapour it can hold at its present temperature and pressure. If additional vapour is supplied, the air cannot retain all of it as an invisible gas. The extra vapour therefore changes into tiny liquid droplets or ice particles; this change is condensation. Cooling the air can produce the same result because cooler air has a lower capacity to hold water vapour. These droplets may appear as dew, fog, cloud or, under suitable conditions, contribute to precipitation. A is incorrect because soil erosion is mainly associated with moving water, wind, ice and human activity, not the immediate addition of vapour to saturated air. C and D are unrelated physical processes. Memory cue: saturated means ‘full’; extra vapour must condense, especially if cooling occurs.

Direct answer: C. Condensation is the change of water vapour from a gas into liquid water. Air near a cold glass is cooled by the glass. If that air reaches its dew point, the water vapour can no longer remain entirely as an invisible gas, so it forms liquid droplets on the outside surface. The water on the glass does not normally pass through the glass; it comes from water vapour in the surrounding air. A describes movement of dust by wind, not a change of water vapour. B results from drying and shrinkage of soil, so it is not condensation. D describes heating and may encourage evaporation, which is the opposite change. The visible droplets make this a clear everyday example. Memory cue: cold surface plus moist air often produces condensation.

Direct answer: D, fast. Evaporation is the change of liquid water into water vapour. Very dry air contains relatively little water vapour, so it has a greater capacity to receive additional vapour from the wet surface of the skin. Sweat therefore evaporates more readily. Evaporation requires energy, which is taken from the skin and produces a cooling effect. A is incorrect because slow evaporation is more typical when the air is already humid, although wind, temperature and exposed surface area also matter. B and C are incorrect because dry air does not stop evaporation; it generally encourages it. The body may feel cooler even though the sweat disappears quickly. Memory cue: dry air accepts vapour; humid air resists further evaporation. This describes the usual effect when other conditions are comparable.

Direct answer: C, evaporation. Evaporation is the process in which liquid water at the surface changes into water vapour and enters the atmosphere. Solar heating gives water molecules energy; some energetic molecules escape from the surface into the air. This occurs from oceans, lakes, rivers, wet soil, and other moist surfaces. A is wrong because condensation changes water vapour into liquid droplets or ice, so it removes vapour from the air rather than sending surface water upward. B is wrong because precipitation is water falling from clouds to Earth as rain, snow, sleet, or hail. D is wrong because leaching is the downward movement of dissolved substances through soil, not the movement of water into the atmosphere. In the water cycle, evaporation is followed by condensation and may later be followed by precipitation. Memory cue: evaporation goes up; precipitation comes down.

Direct answer: A, wind removes moist air above the surface. When water evaporates, the air immediately above the water becomes richer in water vapour. If this moist layer remains in place, the difference between the vapour content at the surface and in the surrounding air becomes smaller, so evaporation slows. Wind moves that moist air away and replaces it with air that is usually less humid. The new air can then receive more water vapour, increasing the rate of evaporation. B is wrong because wind does not turn liquid water into stone. C is wrong because wind does not block solar energy; sunlight and temperature are separate factors. D is wrong because wind transports water vapour rather than destroying it. The exact effect depends on humidity, temperature, and wind strength. Memory cue: wind refreshes the air above a wet surface.

The correct answer is A. Absolute humidity is the mass of water vapour in a given volume of air, commonly expressed in grams per cubic metre (g/m³). A change in the vapour mass per unit volume therefore directly indicates a change in absolute humidity.

The correct answer is C. During evaporation, some molecules escape as vapour from the exposed surface of a liquid. Unlike boiling, evaporation can occur below the boiling point and does not take place throughout the liquid at once.

Correct answer: A. When humidity is high, the air already contains substantial water vapour, so sweat evaporates more slowly from the skin. Because evaporation removes heat from the body, this can make humid weather feel uncomfortable. The effect also depends on temperature and airflow.

Correct answer: C. Water vapour is variable because its amount in the air changes with location and time. Evaporation, temperature, available moisture, and weather conditions all affect how much vapour the air contains. Oceans are an important source, but water vapour is present in the atmosphere, not only in oceans.

Direct answer: B. Relative humidity is calculated by comparing the actual water vapour in air with the maximum vapour the air could hold at that temperature, then expressing the ratio as a percentage. A value of 80% means the air is holding about four-fifths of its temperature-dependent saturation capacity. It does not directly state oxygen content, temperature, or a guaranteed rainfall probability. A is wrong because oxygen percentage is a different atmospheric measurement. C is wrong because relative humidity is not a rain forecast; rain also depends on uplift, cloud development, and other conditions. D is wrong because humidity percentage is not a temperature reading. B is correct. Warning: humidity percentage is a capacity comparison, not a weather certainty.

Direct answer: A. Evaporation is a change from liquid to vapour, and it requires energy called latent heat. When the liquid takes this energy from nearby skin, soil, water, or air, that surrounding material loses some thermal energy and may cool. This is why drying sweat can make the body feel cooler. A is correct, though the amount of cooling depends on the energy transfer and conditions. B is wrong because evaporation has no necessary connection with earthquakes. C is wrong because removing heat does not make pressure zero. D is wrong because evaporation adds water vapour to the air rather than making it disappear. Memory cue: evaporation takes heat; condensation gives heat back.

Correct answer: A, 100%. Relative humidity compares the actual amount of water vapour in the air with the maximum amount the air can hold at the same temperature. In vapour-pressure form, relative humidity equals actual vapour pressure divided by saturation vapour pressure, multiplied by 100: RH = (e / es) × 100. Here e and es are equal, so e / es = 1. Therefore RH = 1 × 100 = 100%. This means the air is saturated: it is holding as much water vapour as it can hold at that temperature. It does not necessarily mean that rain is falling; condensation may begin if suitable cooling or condensation nuclei are present. B would result if actual vapour pressure were half the saturation value. C would mean no actual water vapour, while D would mean actual vapour pressure was only one-tenth of the saturation value. Thus A is the only mathematically and physically correct choice. Memory cue: actual equals possible maximum means 100% relative humidity.

Correct answer: A, saturated. Dew point is the temperature to which air must be cooled, at approximately constant moisture content, for it to become saturated. If the present air temperature is already equal to the dew-point temperature, the air has reached that saturation condition. The relative humidity is therefore approximately 100%, although the question asks for the condition rather than a numerical value. Further cooling can cause condensation, producing dew on suitable surfaces or other forms of condensation depending on the situation. B and C are incorrect because saturated air contains a high amount of water vapour relative to its capacity; it is not dry and does not lack vapour. D is incorrect because 12°C by itself does not describe extremely hot air, and temperature relative to the dew point is the important clue. The equality of air temperature and dew point indicates a very small vapour-pressure deficit. Memory cue: air temperature equals dew point means saturation is reached.

Direct answer: D, a variable component. Atmospheric composition contains gases whose proportions remain comparatively stable and components whose amounts change from place to place and time to time. Water vapour belongs to the second group: its quantity depends on evaporation, temperature, condensation, rainfall, vegetation and available water surfaces. Therefore it is called a variable atmospheric component. A is wrong because water vapour is not treated as a permanent, nearly constant component in the atmosphere. B is wrong because ‘inert’ describes chemical reactivity, not whether the amount changes; water vapour is also involved in evaporation and condensation. C is wrong because water vapour is not a metallic substance. D is correct because its concentration varies greatly with conditions. A useful caution is that ‘variable’ does not mean unimportant: water vapour strongly influences humidity, clouds, precipitation and weather. Memory cue: changing amount means variable component.

Direct answer: C, gas to liquid. Water vapour is water in the gaseous state. During condensation, the vapour loses energy as air cools or as conditions change, and molecules come closer together to form liquid water droplets. Clouds, fog and dew commonly involve this gas-to-liquid change. A is wrong because solid to gas is sublimation, such as ice changing directly into vapour. B describes deposition when vapour changes directly into a solid, as in the formation of frost under suitable conditions; it is not the usual change producing tiny liquid droplets. C is correct because gaseous water vapour becomes liquid water. D is the reverse process, evaporation, in which liquid water changes into vapour. The key exam step is to identify the starting substance as vapour and the ending substance as droplets: vapour is gas and droplets are liquid. Memory cue: condensation condenses gas into liquid; evaporation goes liquid to gas.

Direct answer: C. Saturated air already contains approximately the maximum water vapour it can hold at its current temperature. When it cools further, that capacity decreases. The extra vapour cannot remain entirely in the gaseous state, so it may change into liquid droplets through condensation or, under sufficiently cold conditions, into ice particles. A is wrong because water vapour does not chemically turn into nitrogen merely by cooling. B is wrong because the water is not destroyed; it changes phase and remains as droplets or ice. C is correct because it describes the normal response to excess vapour after cooling. D is wrong because condensation does not make atmospheric pressure zero. This process can produce dew, fog, clouds or precipitation depending on where the droplets form and how they grow. Memory cue: saturated air cooled further has less capacity, so surplus vapour condenses.

Correct answer: A. Relative humidity is the actual water-vapour pressure divided by the saturation vapour pressure at the same temperature, expressed as a percentage. So 80% means the actual value is 80% of the saturation value. It does not directly state the probability of rain or the fraction of ocean water evaporated.

Answer: A. Before or during a rain-producing situation, the air may contain a large amount of water vapour and may be close to saturation. High relative humidity reduces the rate at which sweat and other surface water evaporate, so the body’s normal cooling process becomes less effective. This can make the air feel heavy, damp, or uncomfortable. A is correct because high humidity and near-saturation explain the sensation in physical terms, although a feeling alone cannot measure humidity exactly. B is wrong because dry air usually allows faster evaporation and does not normally produce a humid, heavy feeling. C is wrong because zero water vapour would mean no atmospheric moisture. D is wrong because polar ice is not the general cause of pre-rain heaviness. Memory cue: “heavy air” usually points to moisture and weak evaporation, not greater air weight.