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Its composition is not fixed; it varies at different places and times (e.g., more water vapour in coastal areas).It cannot be represented by a single, fixed chemical formula.The individual components of air (like oxygen and nitrogen) retain their original properties.Its components can be separated by simple physical methods.
(Any three reasons are acceptable)It requires a very high temperature, which can crack the glass apparatus.The heated mixture is highly unstable and can explode if any combustible impurities are accidentally present in the mixture.
Nitrogen Fixation: Free atmospheric nitrogen is converted into soluble nitrates. This is done by nitrogen-fixing bacteria like Rhizobium in leguminous plant roots, or by lightning at high temperatures (3000°C) where nitrogen and oxygen combine to form nitric oxide, which eventually forms nitric acid and reacts with metal carbonates in the soil to form metal nitrates.Nitrate Assimilation: Plants absorb these soluble nitrates from the soil through their roots and convert them into plant proteins. Animals then eat these plants and assimilate these proteins into animal proteins.Ammonification: When plants and animals die, or when animals excrete nitrogenous waste (like urea), putrefying bacteria and fungi in the soil decompose these organic proteins into ammonium compounds.Nitrification: Ammonium salts in the soil are converted first into nitrites and then into nitrates by nitrifying bacteria. These nitrates can once again be absorbed by plants.Denitrification: Certain bacteria (such as Pseudomonas) convert a portion of the soil nitrates back into nitrogen gas, which escapes into the atmosphere, keeping the atmospheric nitrogen level constant.Filtration and Drying: Atmospheric air is filtered and dried to remove dust particles, impurities, and water vapour.Liquefaction: High pressure is applied to the clean air, and it is cooled. During this cooling process, carbon dioxide gas separates out first. The remaining air (mainly nitrogen and oxygen) is allowed to expand rapidly, which cools it further until it liquefies into liquid air.Fractional Distillation: Liquid air is slowly warmed in a fractional distillation column.Nitrogen Separation: Since nitrogen has a lower boiling point of -195.5°C, it boils off first and transitions back into gaseous form, separating from the liquid.Oxygen Collection: Liquid oxygen remains behind and boils off later at -183°C, where it is collected separately as gas.
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Answer key
Chapter: 07. Air And Atmosphere
SECTION A: MULTIPLE CHOICE QUESTIONS (MCQs)
Q1. (c) Antoine Lavoisier
Q2. (b) -195.5°C
Q3. (c) Minamata
Q4. (d) 4:1
Q5. (b) Brown
SECTION B: FILL IN THE BLANKS
Q1. oxygen
Q2. Rhizobium
Q3. dry ice
Q4. nitrification
Q5. manganese dioxide
SECTION C: TRUE OR FALSE
Q1. False (Oxygen is a supporter of combustion, but it is non-combustible and does not burn itself).
Q2. True
Q3. False (Rusting is a slow oxidation process that releases a very small amount of heat and no light energy).
Q4. False (Nitrogen is slightly soluble in water, and it is less soluble than oxygen).
Q5. True
SECTION D: EXPLAIN THE TERMS
Q1. Nitrogen Fixation: The process of converting free atmospheric nitrogen gas into soluble nitrogen compounds (like nitrates) that can be easily absorbed by plants from the soil. This is done by bacteria like Rhizobium or through lightning.
Q2. Humidity: The amount of water vapour present in the air, which varies from place to place and season to season.
Q3. Precipitation: The process by which water vapour in the air condenses continuously and falls to the earth’s surface in the form of rain, hail, snow, dew, or mist.
Q4. Ignition Temperature: The minimum temperature to which a combustible substance must be heated in order to catch fire and begin burning.
SECTION E: COMPLETE THE SERIES
Q1. 65% oxygen
Q2. Rusting (or Respiration)
Q3. CaSO₄•2H₂O
SECTION F: SHORT ANSWER QUESTIONS
Q1. Air is classified as a mixture because:
Q2. Magnesium gains weight upon burning because it chemically combines with oxygen from the surrounding air to form a new solid product, magnesium oxide (MgO). The weight of the oxygen combined with the magnesium adds to the final mass of the product.
Q3. Catalytic converters are fitted in automobiles to convert toxic and harmful gases produced during fuel combustion (such as carbon monoxide and nitrogen oxides) into harmless gases before they are released into the atmosphere.
Q4. Preparing oxygen from potassium chlorate should be avoided in school laboratories because:
Q5. A mixture of oxygen and nitrous oxide is used as a local anaesthetic in dentistry.
SECTION G: DIAGRAM-BASED QUESTION
(a) Carbon dioxide (CO₂).
(b) The limewater turns milky because carbon dioxide gas reacts chemically with the calcium hydroxide (limewater) to form calcium carbonate, which is an insoluble white solid precipitate.
(c) Ca(OH)₂ + CO₂ → CaCO₃(↓) + H₂O
SECTION H: LONG ANSWER & DIFFERENTIATING QUESTIONS
Q1. The five steps of the Nitrogen Cycle are:
Q2. Comparison Tables:
(a) Combustion vs. Respiration
Parameter
Combustion
Respiration
Nature of reaction
Fast, artificial process that needs to be regulated.
Slow, natural, and self-regulated process.
Release of energy
Releases a large amount of energy as heat and light.
Releases a small amount of energy as heat and chemical energy.
Temperature requirement
Occurs at high temperatures.
Occurs at normal body temperature (37°C).
(b) Rusting vs. Burning
Parameter
Rusting
Burning
Necessary conditions
Requires both air (oxygen) and moisture (water).
Requires only air (oxygen) and heating to its ignition temperature.
Nature of oxidation
It is a slow oxidation process. No light is released.
It is a fast oxidation process. Both heat and light are released.
Q3. Large-Scale Preparation of Oxygen from Air:
SECTION I: OBSERVATION & APPLICATION-BASED QUESTIONS
Q1. Case Study: Damage to Monuments
(a) Acid Rain.
(b) Sulphur dioxide (SO₂) released from industries reacts with water vapour and oxygen in the air to produce sulphuric acid (H₂SO₄). This acid mixes with rain water, making it highly acidic.
(c) CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O
Q2. Laboratory Practice
(a) The gas is collected using the downward displacement of water. This method is suitable because oxygen gas is only slightly soluble in water, allowing it to displace the water without dissolving into it.
(b) Oxygen cannot be collected over air because it is heavier than air. If we try to collect it in an open jar over air, it will mix with the atmospheric air already inside the jar, and we will not obtain a pure sample of oxygen.
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