Know About Some Common Gases

1.1 Discovery, Atmospheric Abundance & Physical Traits

Oxygen ($\text{O}_2$) constitutes approximately $20.95\%$ by volume and $23\%$ by mass of dry atmospheric air, and almost $89\%$ by mass of water ($\text{H}_2\text{O}$). Discovered independently by Carl Wilhelm Scheele (1772) and Joseph Priestley (1774), it was named "Oxygen" (meaning acid producer) by Antoine Lavoisier in 1777.

Physical Properties: Colorless, odorless, tasteless gas, slightly heavier than air (vapor density $16$ vs air's $14.4$). It is slightly soluble in water ($3\text{ cm}^3$ per $100\text{ cm}^3$ of water at $20^\circ\text{C}$), which is critically sufficient to sustain aquatic life. It liquefies at $-183^\circ\text{C}$ into a pale blue liquid and solidifies at $-218.8^\circ\text{C}$.

1.2 Laboratory Preparation: Thermal Decomposition of Potassium Chlorate

In the laboratory, oxygen is primarily prepared by heating a mixture of powdered Potassium Chlorate ($\text{KClO}_3$) and Manganese Dioxide ($\text{MnO}_2$) in a $4:1$ ratio by weight in a hard glass test tube.

Crucial Chemical Dynamics: Pure $\text{KClO}_3$ decomposes only when heated above $650^\circ\text{C}$, which can melt normal glass and risk explosive thermal decomposition. The addition of $\text{MnO}_2$ lowers the required decomposition temperature drastically to $200^\circ\text{C} - 240^\circ\text{C}$ and accelerates oxygen liberation without being consumed.

1.3 Catalysis & Alternative Preparation at Room Temperature

Definition of Catalyst (অনুঘটক): A substance that alters (increases or decreases) the speed of a chemical reaction without itself undergoing any permanent chemical change or change in mass is called a catalyst. $\text{MnO}_2$ functions as a positive catalyst.

Cold / Room Temperature Preparation: Oxygen can also be prepared without any heating by adding Manganese Dioxide to Hydrogen Peroxide ($\text{H}_2\text{O}_2$):

Gas Collection Method: Oxygen is collected by the downward displacement of water (জল অপসারণ দ্বারা) in a pneumatic trough because oxygen is only sparingly soluble in water and does not react with water. It is not collected by displacement of air because its density is very close to air.

1.4 Chemical Properties & Combustion of Elements

Oxygen is non-combustible (does not burn itself) but is a vigorous supporter of combustion. A glowing wooden splint rekindles into a bright flame when inserted into a jar of oxygen.

• Carbon (C): Burns with a glowing orange spark producing acidic Carbon Dioxide: $\text{C} + \text{O}_2 \\to \text{CO}_2$.
• Sulfur (S): Burns with a brilliant blue flame forming pungent Sulfur Dioxide: $\text{S} + \text{O}_2 \\to \text{SO}_2$.
• Magnesium (Mg): Burns with a dazzling white light forming white basic Magnesium Oxide: $2\text{Mg} + \text{O}_2 \\to 2\text{MgO}$.
• Sodium (Na): Burns vigorously with a bright golden-yellow flame forming Sodium Peroxide and basic Sodium Oxide: $4\text{Na} + \text{O}_2 \\to 2\text{Na}_2\text{O}$.
• Iron (Fe): Heated iron wire tipped with sulfur burns with brilliant scintillating sparks in oxygen forming black ferroso-ferric oxide: $3\text{Fe} + 2\text{O}_2 \\to \text{Fe}_3\text{O}_4$.

1.5 Oxides Classification & The Mechanism of Rusting

Oxides formed by combination with oxygen are classified into four major chemical groups:

The Chemistry of Rusting (লোহায় মরিচা পড়া): Rusting of iron is an electrochemical slow oxidation process requiring both Oxygen and Liquid Water / Moisture simultaneously. In dry oxygen or boiled air-free water, iron never rusts.

Rust Prevention Methods: (1) Barrier protection (painting, greasing, oiling), (2) Galvanization (গ্যালভানাইজেশন) — coating iron sheets with a thin protective layer of molten Zinc ($\text{Zn}$), and (3) Electroplating with Chromium or Nickel.

2.1 Occurrence, Lightest Gas & Unique Nature

Hydrogen ($\text{H}_2$) is the lightest known chemical substance and the most abundant element in the entire universe (constituting over $75\%$ of all normal baryonic matter, fueling nuclear fusion in stars). Discovered by Henry Cavendish in 1766 (who termed it inflammable air), it was named Hydrogen (meaning water generator) by Lavoisier.

Physical Properties: Colorless, tasteless, odorless gas with a density of merely $0.0899\text{ g/L}$ (approximately $\frac{1}{14.4}$ times the density of air). It is virtually insoluble in water ($2\text{ cm}^3$ in $100\text{ cm}^3$ water at $20^\circ\text{C}$) and diffuses through porous barriers faster than any other gas (Graham's Law).

2.2 Laboratory Preparation from Granulated Zinc & Dilute Sulfuric Acid

In the laboratory, hydrogen gas is prepared at room temperature by reacting Granulated Zinc (দানাযুক্ত দস্তা) with Dilute Sulfuric Acid ($\text{H}_2\text{SO}_4$) in a Woulfe's bottle fitted with a thistle funnel and a delivery tube.

Why Granulated Zinc is Used instead of Pure Zinc:

• Catalytic Impurities: Granulated zinc contains trace impurities of copper ($\text{Cu}$). These impurities set up minute electrochemical couples that act as positive autocatalysts, substantially speeding up the rate of hydrogen evolution. Pure zinc reacts extremely sluggishly.
• Porous High Surface Area: Granulated zinc is prepared by pouring molten zinc into cold water, creating uneven, spongy globules with immense exposed surface area for rapid acid contact.

Safety Warning: Nitric acid ($\text{HNO}_3$) is never used because it is a powerful oxidizing agent that oxidizes the nascent hydrogen directly into water ($\text{H}_2\text{O}$), releasing nitrogen oxides ($\text{NO}_2, \text{NO}$) instead of $\text{H}_2$.

2.3 Collection Method & Precautions

Hydrogen is collected over a pneumatic trough by the downward displacement of water. Although it is lighter than air, collecting it by upward displacement / air displacement is strictly avoided in open school labs because mixtures of hydrogen and atmospheric air form dangerously violent explosive mixtures.

Critical Lab Precaution: The lower stem of the thistle funnel must be immersed deeply below the surface of the dilute acid in the Woulfe's bottle; otherwise, hydrogen gas will escape backwards through the funnel into the room, creating an acute explosion hazard.

2.4 Chemical Properties, The 'Pop' Test & Combustion

Hydrogen is a combustible gas that burns vigorously in air or oxygen with an almost invisible, pale blue flame, producing pure water vapor and generating immense thermal energy ($286\text{ kJ/mol}$):

Non-supporter of Combustion: If a burning candle is pushed inside an inverted gas jar filled with pure hydrogen, the candle is immediately extinguished inside the jar, while the hydrogen burns quietly at the mouth where it meets atmospheric oxygen.

The Definitive 'Pop' Sound Test: When a small test tube containing hydrogen mixed with a little air is brought near a flame, it ignites with a characteristic sharp "pop" sound. This test is universally used to verify the presence of hydrogen gas.

2.5 Reducing Action & Industrial Significance

Hydrogen is a powerful reducing agent (বিজারক). When dry hydrogen gas is passed over heated black Copper(II) Oxide ($\text{CuO}$) in a combustion tube, it strips oxygen from the oxide, reducing it to reddish-brown metallic copper, while hydrogen itself gets oxidized to water:

Major Industrial Uses:

• Haber Process for Ammonia: $\text{N}_2 + 3\text{H}_2 \\xrightleftharpoons[\text{Fe / Mo}]{450^\circ\text{C}, 200\text{ atm}} 2\text{NH}_3$, essential for chemical fertilizers (urea, ammonium sulfate).
• Hydrogenation of Oils: Hardening liquid vegetable oils into solid fats (vanaspati ghee) using finely divided Nickel ($\text{Ni}$) catalyst at $200^\circ\text{C}$.
• Zero-Emission Clean Fuel: Liquid hydrogen is used as cryogenic fuel in space rockets. When burned, its only combustion product is water ($\text{H}_2\text{O}$), producing zero carbon emissions.

3.1 Atmospheric Abundance, Natural Role & Properties

Carbon Dioxide ($\text{CO}_2$) is present in atmospheric air in small concentrations (approximately $0.04\%$ by volume, or $\sim 420\text{ ppm}$). Despite its low concentration, it is the fundamental carbon building block for all earthly life via plant photosynthesis.

Physical Properties: Colorless gas with a faint, pleasantly acidic smell and sharp taste. It is $1.53$ times heavier than air (vapor density $22$ vs air's $14.4$). It dissolves moderately in water ($1$ volume of water dissolves about $1$ volume of $\text{CO}_2$ at $20^\circ\text{C}$), forming weak carbonic acid ($\text{H}_2\text{CO}_3$).

3.2 Laboratory Preparation: Marble Chips & Dilute Hydrochloric Acid

In the laboratory, carbon dioxide is prepared at room temperature without any heating by reacting Marble Chips (Calcium Carbonate, $\text{CaCO}_3$) with Dilute Hydrochloric Acid ($\text{HCl}$) in a Woulfe's bottle or Kipp's Apparatus.

Critical Examiner Point — Why Dilute $\text{H}_2\text{SO}_4$ Cannot Be Used:

If dilute Sulfuric Acid is added to marble chips, an initial burst of gas occurs, but the reaction stops almost immediately. This occurs because the byproduct Calcium Sulfate ($\text{CaSO}_4$) is practically insoluble in water. It forms a dense, impervious crystalline crust over the surface of the marble chips, preventing any further acid from reaching the underlying $\text{CaCO}_3$:

Collection Method: Since $\text{CO}_2$ is $1.5$ times denser than air and fairly soluble in water, it is collected by the upward displacement of air (বাতাসের ঊর্ধ্বমুখী অপসারণ) into an upright gas jar.

3.3 The Limewater Confirmatory Test (Two-Stage Reaction)

The definitive test for identifying $\text{CO}_2$ is bubbling the gas through freshly prepared, clear Limewater [Calcium Hydroxide solution, $\text{Ca(OH)}_2$]:

Stage 1 (Initial bubbling): The clear limewater turns milky / turbid due to the precipitation of insoluble white Calcium Carbonate particles:

Stage 2 (Prolonged excess bubbling): When excess $\text{CO}_2$ is continuously bubbled through the milky solution, the milkiness completely disappears, and the liquid turns sparkling clear again because insoluble $\text{CaCO}_3$ converts into highly soluble Calcium Bicarbonate:

Note: Boiling this clear solution decomposes $\text{Ca(HCO}_3)_2$, reforming insoluble $\text{CaCO}_3$ and restoring milkiness.

3.4 Fire Extinguishing Dynamics & Burning Magnesium Ribbon

Carbon dioxide does not burn, nor does it support ordinary combustion. Because its density is $1.5$ times greater than air, it can be poured from one vessel into another like water, blanketing the burning material, displacing atmospheric oxygen, and suffocating the flames.

Burning Magnesium Exception: Although a burning candle or wooden splinter is instantly snuffed out in $\text{CO}_2$, a burning Magnesium ribbon continues to burn brilliantly inside a jar of carbon dioxide with white smoke and black specks:

Magnesium has an extraordinary chemical affinity for oxygen; its intense burning temperature ($>2000^\circ\text{C}$) decomposes $\text{CO}_2$ molecules to capture oxygen, liberating elemental carbon.

3.5 Dry Ice (Solid $\text{CO}_2$) & Modern Applications

When gaseous carbon dioxide is subjected to high pressure ($>50\text{ atm}$) and cooled, it liquefies. When liquid $\text{CO}_2$ is allowed to expand rapidly through a small nozzle into atmospheric pressure, the dramatic temperature drop freezes it instantly into a snow-like solid called Dry Ice (শুষ্ক বরফ).

• Sublimation at $-78.5^\circ\text{C}$ ($194.65\text{ K}$): Dry ice does not melt into a liquid; under standard atmospheric pressure, it sublimes directly from solid to vapor without wetting surfaces, hence named "dry" ice.
• Uses: Preserving perishable food and vaccines during transport, creating dense fog effects on theatre stages, carbonating soft drinks (soda water under pressure), and filling portable fire extinguishers.

4.1 Atmospheric Abundance & The Chemical Inertness of the Triple Bond

Nitrogen ($\text{N}_2$) makes up the overwhelming majority of Earth's atmosphere — $78.08\%$ by volume. Discovered by Daniel Rutherford in 1772, it is a colorless, odorless, tasteless gas with a vapor density of $14$, slightly lighter than air ($14.4$).

Why Nitrogen is Chemically Inert at Ambient Temperatures:

In a nitrogen molecule ($\text{N}_2$), the two nitrogen atoms share three pairs of electrons, forming an exceptionally strong covalent triple bond ($\mathbf{N \equiv N}$). The bond dissociation energy required to break this triple bond is immense ($945.3\text{ kJ/mol}$). Consequently, at ordinary room temperatures, nitrogen shows almost complete chemical inactivity and acts as an inert diluent in air that moderates the fiery reactivity of oxygen.

4.2 High-Temperature Reactions & Lightning Fixation

Under extreme thermal energy ($>3000^\circ\text{C}$) or during atmospheric lightning discharges, nitrogen overcomes its triple bond barrier and combines directly with oxygen:

When this nitric acid reaches the soil, it reacts with soil minerals (such as calcium carbonate) to form soluble nitrates ($\text{Ca(NO}_3)_2$), providing natural nitrogenous nutrition for plants.

4.3 Biological Nitrogen Fixation & The Nitrogen Cycle

Living organisms require nitrogen for synthesizing amino acids, proteins, and nucleic acids (DNA & RNA), but plants cannot absorb elemental gaseous $\text{N}_2$ directly from air. They rely on:

• Symbiotic Bacteria: Rhizobium bacteria residing inside root nodules of leguminous plants (peas, gram, lentils) convert atmospheric $\text{N}_2$ directly into assimilable ammonium compounds.
• Free-Living Microorganisms: Soil bacteria like Azotobacter and Clostridium, and cyanobacteria (blue-green algae) like Anabaena and Nostoc.
• Denitrifying Bacteria: Microorganisms like Pseudomonas break down soil nitrates back into gaseous $\text{N}_2$, maintaining atmospheric equilibrium in the Nitrogen Cycle.

4.4 Systematic Gas Identification Matrix for School Laboratories

Students frequently encounter unknown gas jars in practical exams. Use this rigorous flowchart and matrix to differentiate the four common gases:

5.1 The Greenhouse Effect & Global Warming

Natural Greenhouse Effect: Solar shortwave radiation passes unhindered through Earth's atmosphere and warms the planetary surface. The warmed earth radiates this energy back towards space as longwave infrared (thermal) radiation. Greenhouse gases in the troposphere absorb a substantial fraction of this outgoing infrared radiation and re-emit it in all directions, acting like a planetary thermal blanket that maintains Earth's average temperature at a hospitable $+15^\circ\text{C}$ (without it, Earth would freeze at $-18^\circ\text{C}$).

Major Greenhouse Gases: Carbon Dioxide ($\text{CO}_2$ — responsible for $\sim 60\%$ of human-induced warming), Methane ($\text{CH}_4$), Nitrous Oxide ($\text{N}_2\text{O}$), Chlorofluorocarbons ($\text{CFCs}$), and Water Vapor ($\text{H}_2\text{O}$).

Enhanced Greenhouse Effect & Global Warming: Excessive combustion of fossil fuels (coal, petroleum), rapid industrialization, and massive deforestation have driven atmospheric $\text{CO}_2$ from pre-industrial levels of $280\text{ ppm}$ to over $420\text{ ppm}$, trapping excess thermal energy. Consequences include polar ice-cap melting, rising sea levels threatening coastal cities (such as Kolkata and the Sundarbans), erratic monsoons, and extreme climatic disasters.

5.2 Acid Rain (অ্যাসিড বৃষ্টি) — Causes & Chemical Mechanism

Normal unpolluted rainwater is slightly acidic ($\text{pH} \approx 5.6$) due to dissolved atmospheric $\text{CO}_2$ forming weak carbonic acid ($\text{H}_2\text{CO}_3$). When the $\text{pH}$ of precipitation falls below $5.6$, it is termed Acid Rain.

Pollutant Sources & Reactions:

• Sulfur Dioxide ($\text{SO}_2$): Released by thermal power plants, petroleum refineries, and metallurgical smelters: $$2\text{SO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \\longrightarrow 2\mathbf{\text{H}_2\text{SO}_4}\text{ (Sulfuric Acid)}$$
• Nitrogen Oxides ($\text{NO}_x$): Emitted by high-temperature internal combustion engines of automobiles and jet aircraft: $$4\text{NO}_2 + 2\text{H}_2\text{O} + \text{O}_2 \\longrightarrow 4\mathbf{\text{HNO}_3}\text{ (Nitric Acid)}$$

Devastating Consequences:

• Stone Leprosy (পাথরের কুষ্ঠ): Acid rain dissolves calcium carbonate in marble historical monuments like the Taj Mahal and Victoria Memorial, converting shiny marble into brittle, crumbling calcium sulfate: $$\text{CaCO}_3 + \text{H}_2\text{SO}_4 \\longrightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2\\uparrow$$
• Destruction of Aquatic Ecosystems: Low $\text{pH}$ releases toxic aluminum ions ($\text{Al}^{3+}$) from soil into lakes, suffocating fish gills and destroying aquatic biodiversity.
• Defoliation & Soil Leaching: Damages leaf stomata, washes away essential nutrients (calcium, magnesium, potassium) from agricultural soil.

5.3 The Stratospheric Ozone Layer ($\text{O}_3$) & UV Shielding

In the stratosphere (at an altitude of $15 - 35\text{ km}$ above sea level), ozone molecules form a delicate protective canopy called the Ozone Layer (ওজোন স্তর), which filters out $>99\%$ of the Sun's lethal, high-energy Ultraviolet-B and UV-C radiation ($200 - 315\text{ nm}$).

Photochemical Formation & Dissociation (Chapman Cycle):

5.4 Ozone Depletion by Chlorofluorocarbons (CFCs)

Synthetic coolant compounds known as Chlorofluorocarbons (CFCs / Freons), widely used in domestic refrigerators, air conditioners, aerosol sprays, and foam insulation, drift unchanged into the stratosphere due to their chemical inertness. Once exposed to intense stratospheric UV radiation, they photolyze to release reactive free Chlorine radicals ($\text{Cl}^\bullet$):

The Catalytic Destruction Loop: Notice that the atomic chlorine radical ($\text{Cl}^\bullet$) is regenerated at the end of the second step! A single chlorine atom can catalytically destroy over $100,000$ ozone molecules before being removed, causing massive seasonal ozone holes over Antarctica.

Global Treaty: The historic Montreal Protocol (1987) successfully mandated the global phase-out of CFCs in favor of ozone-friendly hydrofluorocarbons (HFCs) and hydrocarbons.