Chapter 16: Gases

Electron dot structure of carbon dioxide (CO2), showing double bonds between carbon and each oxygen atom.

Labelled apparatus for the laboratory preparation of carbon dioxide gas: Woulfe's bottle with limestone and dilute HCl, delivery tube, and gas jar.

1. 1Collect the apparatus and chemicals required for the preparation of gas.
2. 2Keep some pieces of limestone or marble pieces or calcium carbonate powder or egg shells into the Woulfe's bottle.
3. 3Arrange the thistle funnel in one opening of the Woulfe's bottle and the delivery tube in another, making them airtight with rubber corks.
4. 4Pour dilute hydrochloric acid through the thistle funnel until it covers the limestone, keeping the funnel's opening inside the acid layer.
5. 5Allow the gas to pass through the delivery tube and collect it in a gas jar kept straight upright.
6. 6Observe the reaction between calcium carbonate and acid.
7. 7Carbon dioxide is formed and passes to the gas jar through the delivery tube. Being heavier than air, it is collected by the upward displacement of air.

• The end of the delivery tube should not be dipped into the acid.
• The end of the thistle funnel should be dipped into the acid.

• A burning matchstick brought near the mouth of the gas jar gets extinguished, because carbon dioxide is neither combustible nor a supporter of combustion.
• Moist blue litmus paper does NOT turn red with carbon dioxide (it is only weakly acidic); phenolphthalein remains colourless/unchanged, which helps confirm the gas.
• When carbon dioxide is passed through lime water for some time, the lime water turns milky white due to the formation of insoluble calcium carbonate. Passing the gas for a long time makes the milky colour disappear again due to the formation of soluble calcium bicarbonate.

Passing carbon dioxide through clear limewater turns it milky white — a standard test for CO2.

• It is an acidic (non-metallic) oxide which is colourless and odourless.
• It dissolves in water to form carbonic acid, so it is sour in taste.
• It is slightly soluble in water.
• It is 1.5 times heavier than air.
• It shows acidic properties and changes moist blue litmus paper red.
• It is non-toxic, but organisms can die by suffocation in an atmosphere of carbon dioxide due to lack of oxygen.
• It can be changed to liquid at high pressure and low temperature.
• When cooled below -78°C, it changes to solid form known as dry ice.
• This gas is neither combustible nor does it support combustion.

1. 1Carbon dioxide is neither combustible nor a supporter of combustion. It helps extinguish fire. However, burning magnesium continues to burn brightly inside a jar of CO2, producing white magnesium oxide and black carbon powder — this proves CO2 contains carbon. 2Mg(s) + CO2(g) → 2MgO(s) + C(s)
2. 2Carbon dioxide dissolves in water to form carbonic acid. This gas is mixed into soft drinks at high pressure to give them a sour taste. CO2(g) + H2O(l) ⇌ H2CO3(aq)
3. 3When passed into lime water for some time, it forms insoluble calcium carbonate (milky white). Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l). On passing for a long time, soluble calcium bicarbonate forms and the milky colour disappears: CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)
4. 4When a few drops of KOH are added inside a test tube filled with CO2 and inverted in water, the water level rises inside the tube, because KOH absorbs the CO2, lowering the pressure and drawing water in. 2KOH + CO2 → K2CO3 + H2O
5. 5Green plants use carbon dioxide (absorbed from air) and water (absorbed by roots) in the presence of sunlight trapped by chlorophyll to prepare food (photosynthesis). 6CO2 + 6H2O --Solar energy/Chlorophyll--> C6H12O6 + 6O2
6. 6Carbon dioxide reacts with red hot coke at 900°C to form carbon monoxide. CO2 + C → 2CO

• Dissolved in soft drinks at high pressure.
• Used by plants as a raw material for photosynthesis (food preparation).
• Used to extinguish fire, inside fire extinguishers.
• Used to prepare dry ice, which preserves fruits, vegetables, and meat at low temperature.
• Liquid carbon dioxide is used in the purification of sugar by the carbonation process in sugar mills.
• Used to prepare urea (NH2CONH2), washing soda (Na2CO3), and baking soda (NaHCO3).
• Used to make carbogen — a mixture of 95% oxygen and 5% carbon dioxide, used for artificial respiration in patients suffering from pneumonia.
• Used in bakery products.

A CO2/DCP fire extinguisher: sulphuric acid mixes with sodium bicarbonate when the plunger is jerked, producing CO2 gas that smothers the fire.

Electron dot structure of ammonia (NH3), showing single bonds between nitrogen and three hydrogen atoms, with one lone pair on nitrogen.

Labelled apparatus for the laboratory preparation of ammonia gas by heating ammonium chloride and calcium hydroxide, passed through a lime tower for drying.

1. 1Collect the apparatus and chemicals required.
2. 2Mix ammonium chloride and calcium hydroxide in the ratio of 2:1 and put the mixture into the hard glass test tube.
3. 3Fix a delivery tube in the mouth of the test tube with a rubber cork to make it airtight. Arrange the test tube in a slightly inclined position using a stand. Connect the other end of the delivery tube to a lime tower to obtain pure and dry ammonia gas (without the lime tower the ammonia obtained is impure).
4. 4Heat the mixture gently and observe the formation of ammonia gas.
5. 5The lime tower filled with calcium oxide (CaO) absorbs moisture and gives dry, pure ammonia. Since the gas is highly soluble in water, it cannot be collected by displacement of water — it is collected by the downward displacement of air, as it is lighter than air.

• The mouth of the hard glass test tube should be slightly inclined downward so that water vapour produced passes to the lime tower and does not crack the hot glass tube.
• The mouth of the hard glass test tube should be airtight with the help of a cork.
• To obtain dry ammonia gas, it should be passed through a lime tower; since it is highly soluble in water, it should not be collected by the displacement of water.

• Ammonia is basic, so it changes moist red litmus paper to blue.
• When a glass rod dipped in concentrated hydrochloric acid is brought near the mouth of a gas jar containing ammonia, dense white fumes of ammonium chloride are formed.

Moist red litmus paper turns blue when held near ammonia gas, confirming its basic nature.

• It is a colourless gas with a strong, pungent odour.
• It is lighter than air.
• This gas is highly soluble in water.
• It is a basic gas, so it changes moist red litmus paper to blue.
• Ammonia liquefies at -33.4°C and solidifies at -78°C.

1. 1It is soluble in water and forms ammonium hydroxide when dissolved. NH3(g) + H2O(l) → NH4OH(aq)
2. 2Ammonia reacts with acids to form ammonium salts. NH3(g) + HCl(aq) → NH4Cl(aq) [Ammonium chloride]; 2NH3(g) + H2SO4(aq) → (NH4)2SO4(aq) [Ammonium sulphate]
3. 3Ammonia solution (NH4OH) reacts with acid to form salt and water. 2NH4OH(aq) + H2SO4(aq) → (NH4)2SO4(aq) + 2H2O(l); NH4OH(aq) + HCl(aq) → NH4Cl(aq) + H2O(l)
4. 4Ammonia and carbon dioxide react at a high temperature of 1500°C and high pressure to form urea, an important chemical fertilizer. NH3(g) + CO2(g) --1500°C, pressure--> NH2-CO-NH2(s) + H2O(l)
5. 5When ammonia burns in an atmosphere of oxygen, it produces nitrogen gas and water. 4NH3(g) + 3O2(g) → 6H2O(l) + 2N2(g)
6. 6When a mixture of ammonia and oxygen is passed over platinum gauze at about 800°C, it produces nitric oxide. 4NH3(g) + 5O2(g) --800°C, Pt--> 6H2O(l) + 4NO(g)
7. 7When ammonia is passed through molten sodium, it forms sodamide and hydrogen gas. 2NH3(g) + 2Na(s) → 2NaNH2(s) + H2(g)

• Used to make fertilizers like ammonium sulphate, ammonium nitrate, urea, and ammonium phosphate.
• Used in the manufacture of nitric acid and plastics.
• Used to make washing soda.
• Used to make medicines of ammonium salts.
• Used to make blue prints of maps.
• Used as a cooling agent (refrigerant) in refrigerators.
• Used as a cleansing agent to remove stains of oil and grease.

The ammonia fountain experiment: water rises into a flask of dry ammonia gas and turns pink with phenolphthalein, proving ammonia's high solubility.

The natural greenhouse effect: greenhouse gases in the Earth's atmosphere trap outgoing heat and warm the surface.

• Increase in temperature (global warming).
• Change in the water cycle.
• Negative impacts on human health.
• Decrease in agricultural productivity.
• Melting of snow in the Himalayas and glaciers.
• Increase in sea level, causing sinking and flooding of coastal regions.
• Degradation in biodiversity.
• Desertification.
• Imbalance in the ecosystem.

An artificial greenhouse traps short-wave sunlight, which converts to long-wave heat radiation that cannot escape, warming the inside.

• Plants of any season can be grown inside it throughout the year.
• It helps food crops grow in very cold places.
• Useful to produce flowers, green leafy vegetables, fruits, and different types of plants.
• Plants inside greenhouses can help control environmental pollution.
• Summer plants can be grown in the winter season, and vice versa.

1. 1Fully ban the production and use of chlorofluorocarbons (CFCs).
2. 2Decrease the use of petroleum products and coal; increase the use of renewable energy sources.
3. 3Promote alternate sources of energy like hydroelectricity, wind energy, and solar energy.
4. 4Plant more trees.
5. 5Reduce the production of carbon dioxide.

• 2SO2 + O2 → 2SO3
• SO3 + H2O → H2SO4
• CO2 + H2O → H2CO3

1. 1Fades monuments like temples, buildings, and statues made of marble: CaCO3(s) + H2SO4(aq) ⇌ CaSO4(s) + CO2(g) + H2O(l)
2. 2Increases the acidity of soil, decreasing agricultural productivity.
3. 3Gets mixed into water sources and affects aquatic animals.
4. 4Causes skin diseases in humans.
5. 5Causes other negative effects on human health.

Acid rain gradually fades and damages marble monuments by reacting with calcium carbonate.

• Reduce the production of oxides of nitrogen and sulphur.
• Use renewable sources of energy in place of fossil fuel.
• Raise awareness about the causes and effects of acid rain.

• Carbon Dioxide: A colourless, odourless acidic gas (CO2) formed by burning carbon-containing substances, respiration, and decay.
• Dry Ice: Solid carbon dioxide, formed when CO2 is cooled below -78°C.
• Ammonia: A colourless, pungent, basic gas (NH3) that is highly soluble in water.
• Lime Tower: A tower filled with calcium oxide (CaO) used to dry and purify ammonia gas.
• Greenhouse Effect: The process of trapping solar energy inside a greenhouse (or the Earth's atmosphere), increasing its temperature.
• Acid Rain: Rain water made more acidic (pH 3-5) by gases like SO2, CO2, and NOx dissolving in atmospheric water vapour.
• Carbogen: A mixture of 95% oxygen and 5% carbon dioxide used for artificial respiration.

• Do not confuse the collection methods: CO2 (heavier than air) is collected by upward displacement of air; NH3 (lighter than air) is collected by downward displacement of air.
• Remember NH3 cannot be collected over water because it is highly soluble in water.
• Do not confuse the litmus test: CO2 turns moist blue litmus red (acidic); NH3 turns moist red litmus blue (basic).
• Always mention the correct ratio (2:1) of NH4Cl to Ca(OH)2 in ammonia preparation.
• Remember the thistle funnel's end must stay dipped in acid, while the delivery tube's end must NOT dip into the acid, in CO2 preparation.

• CO2: acidic, colourless, odourless, heavier than air, turns limewater milky, does not support combustion.
• CO2 lab preparation: CaCO3 + 2HCl → CaCl2 + H2O + CO2 (Woulfe's bottle, upward displacement of air).
• NH3: basic, colourless, pungent smell, lighter than air, highly soluble in water.
• NH3 lab preparation: 2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3 (ratio 2:1, lime tower, downward displacement of air).
• Greenhouse effect = trapping of solar heat by greenhouse gases (CO2, CH4, N2O, CFCs, O3).
• Acid rain = rain water made acidic (pH 3-5) by SO2, CO2, NOx dissolving in atmospheric moisture.
• Dry ice = solid CO2 below -78°C.
• Carbogen = 95% O2 + 5% CO2, used for artificial respiration.