Air & Atmosphere

(a) Air is an invisible mixture of gases that surrounds the Earth and whose composition varies from place to place and from time to time.

(b) Atmosphere is a giant blanket of air that wraps around the Earth and extends upto hundreds of kilometres above the earth's surface as a colourless form of matter.

Names of important scientists and their studies which led to the discovery of the components of air are listed below:

1. John Mayow — He proved that air is a mixture containing an active component and an inactive component.
2. Joseph Priestley — He discovered oxygen gas by heating mercury [II] oxide in a hard glass test tube.
3. Antoine Lavoisier — He named the components of air, active as oxygen and inactive as nitrogen.

Apparatus: The apparatus was set up as shown, with the mercury in the curved necked retort and air in the bell jar.

Procedure: The mercury in the retort was then heated for several days.

Observation:

1. A red layer was formed on the heated mercury surface in the retort.
2. The level of mercury in the trough rose by $\dfrac{1}{5}$ .

Conclusions:

1. The oxygen in the retort combined with the mercury forming mercury [II] oxide [red layer] and the level in the trough rose by $\dfrac{1}{5}$ th of the original volume, thereby occupying the space of the used oxygen in the bell jar.
2. The active part of the air removed by mercury on heating was named 'oxygen'.
3. The remaining inactive part of the air in the bell jar was then named 'nitrogen'.

The fact that 4⁄5^th of the total volume of air is nitrogen is useful for the following reasons:

1. Controlled Combustion — Nitrogen being a non-supporter of combustion, dilutes the activity of oxygen in air and controls the combustion of substances.
2. Growth of Plants — Free nitrogen of air is converted into soluble nitrogen compounds which are absorbed by plants and contribute to their growth and development.
3. Breathable Air — Nitrogen in the atmosphere dilutes the oxygen, making it safe for us to breathe. Without this dilution, pure oxygen could be harmful and even flammable.

Growth and development of plants — Free nitrogen of air is converted to soluble nitrogenous compounds in the soil. These are absorbed by plants and converted to plant proteins which play a vital role in their growth and development.

Flushing food packages — Nitrogen being inert or unreactive is filled into food packets to drive out the oxygen. Hence, bacterial growth inside the food packages is reduced and thereby the food is well preserved and fresh.

Plants manufacture food by the process of photosynthesis. Carbon dioxide in the atmosphere is absorbed by the plants in the presence of chlorophyll [the green pigment in plants] and sunlight, resulting in the formation of carbohydrates and release of oxygen.

The sun's rays consist of infrared, visible rays and ultra violet rays. Carbon dioxide in the atmosphere allows the reflected ultraviolet rays to pass through but prevents the infrared radiations from being radiated out, resulting in warming of the earth.

Due to pollution, concentration of carbon dioxide in the atmosphere is increasing. This excess carbon dioxide is warming the earth's surface to a harmful extent resulting in global warming.

(a) Utility of water vapour:

1. For controlling evaporation.
2. For determining climatic condition.
3. For growth of plant and animal.

(b) Utility of rare or inert gases:

1. In advertisement signs
2. In observation balloons
3. In electric lamps.
4. In photography

Environment effect of three different polluting gases:

1. Sulphur dioxide — cause respiratory problems
2. Nitrogen dioxide — initiates acid rainfall
3. Dust & smoke particles — causes respiratory problems and precipitates rain.

Nitrogen is not considered a polluting gas for the following reasons:

1. It is an inert or unreactive gas.
2. It is utilized by plants for their growth and development.
3. It controls combustion by diluting the activity of oxygen.
4. It is not a greenhouse gas and does not contribute to global warming.

(a) To study the presence of oxygen and nitrogen component

Apparatus : A trough , bell jar, cork and crucible, a piece of phosphorus.

Procedure :

1. The trough is filled with water and a bell jar divided into 5 equal parts, '1 to 5' is placed over it.
2. A crucible containing white phosphorus is placed on a cork which is made to float on water.
3. The level of water inside and outside are adjusted to one level.
4. The phosphorus is then ignited by means of a heated wire.

Observations :
White fumes are seen initially in the bell jar and level of water in the bell jar rises by 1⁄5.
Phosphorus + Oxygen ⟶ Phosphorus pentoxide

Conclusion :

1. Oxygen the active component of air [i.e., 1⁄5^th of air] is used up in burning.
2. Nitrogen the inactive component of air [i.e., 4⁄5^th of air] is not used up in burning.

Hence air contains oxygen which supports combustion and nitrogen which does not support combustion.

(b) To study the presence of carbon dioxide in air

Apparatus : A test tube containing lime water with two outlets - A and B.

Procedure : Air is sucked slowly from the outlet B

Observations : The lime water in the test tube turns milky.

Conclusion : Air contains carbon dioxide which turns lime water milky.

(c) To study the presence of water vapor in air

Procedure : A glass tumbler filled with ice cubes is exposed to the air.

Observations : Water droplets are seen on the outer surface of glass tumbler

Conclusion : Air contains water vapour which condenses on the cool outer surface.

Four Properties of compounds:

1. It has a fixed composition.
2. It has definite set of properties.
3. It cannot be separated by physical means.
4. Chemical Formula possible.

Four Properties of mixture:

1. It has a variable composition.
2. It has no definite properties.
3. It cannot be separated by physical means.
4. No chemical formula possible.

Air is considered a mixture and not a compound because:

1. Composition — Air has a variable composition
2. Properties — Air has no specific set of properties
3. Separation of components — Air can be separated into its components by physical means
4. Chemical formula — No chemical formula is possible for air.

Air pollution is the condition of air made unclean due to the introduction of foreign elements from natural sources and manmade sources to the air.

Sources of a few main pollutants are:

1. Natural Sources:
   1. Volcanic eruptions
   2. Lightning discharge
2. Man-made Sources:
   1. Automobile exhausts
   2. Chemical industries
   3. Burning of fuels

Acidic oxides responsible for acid rain are:

1. Oxides of Sulphur — Sulphur dioxide [SO₂]
2. Oxides of Nitrogen — Nitrogen dioxide [NO₂]

Formation of acid rain:
The acidic oxides of Sulphur and Nitrogen combine with water vapour in presence of sunlight forming the respective acids.

Sulphur dioxide + water vapour ⟶ Sulphurous acid [H₂SO₃]

Sulphur trioxide + water vapour ⟶ Sulphuric acid [H₂SO₄]

Nitric oxide + oxygen ⟶ nitrogen dioxide [NO₂ ]

Nitrogen dioxide + water ⟶ Nitrous acid [HNO₂] & Nitric acid [HNO₃]

Presence of these acids causes acid rain.

Damage to heritage buildings:
Acid rain damages heritage buildings as it corrodes metallic surfaces & weakens building materials such as statues, marble, limestone [CaCO₃] etc. It causes damage to heritage structures such as the Taj Mahal, which is made of limestone and reacts with acids present in acid rain.

(a) Labelled diagram of the laboratory preparation of oxygen from hydrogen peroxide is shown below:

(b) Manganese dioxide is used as the catalyst. It alters the rate of the reaction and increases the rate of evolution of oxygen.

(c) Hydrogen peroxide ⟶ Water + oxygen

(d) Oxygen gas is collected by the downward displacement of water because:

1. Oxygen is very slightly soluble in water
2. Oxygen is almost as heavy as air hence it cannot be collected over air.

(a) Non-Metals

1. Sulphur + Oxygen ⟶ Sulphur dioxide
2. Carbon + Oxygen ⟶ Carbon dioxide
3. Phosphorus + Oxygen ⟶ Phosphorus pentoxide

(b) Metals

1. Sodium + Oxygen ⟶ Sodium oxide
2. Calcium + Oxygen ⟶ Calcium oxide
3. Magnesium + Oxygen ⟶ Magnesium oxide

(a) Combustion and respiration

(b) Combustion & rusting

(a) Magnesium gains weight on burning due to the mass of the oxygen of the air which has combined with magnesium forming magnesium oxide.

(b) Candle gains weight on burning due to mass of oxygen of the air which has combined with carbon and hydrogen of the candle C_xH_y producing water vapour and carbon dioxide.

(a) Oxygen tents provide artificial supply of extra oxygen to help patients with lung problems breathe easier. This reduces the load on the patient's lungs helping them to work better.

(b) A mixture of oxygen with gases like hydrogen or acetylene on burning, produces high temperature flames of around 2800°C to 3000°C. Such high temperature flames are suitable for melting metals quickly. Hence, oxygen finds utility in welding & cutting of metals.

1. Acid rain damages heritage buildings because it corrodes metallic surfaces and weakens building materials like statues, marbles, limestone. Taj Mahal is predominantly made of lime stone and it reacts with acids present in acid rain.

2. In the laboratory preparation of oxygen from potassium chlorate, manganese dioxide acts as a catalyst altering the rate of the reaction. It initiates the decomposition of potassium chlorate (KClO₃) at a lower temperature.

3. As oxygen is neutral in nature so it has no effect on blue litmus paper, but carbon dioxide is slightly acidic in nature, hence it turns blue litmus paper to slightly pink.

4. The product obtained from the oxidation of sulphur is sulphur dioxide which is a non-metallic acidic oxide. Hence, it turns moist blue litmus red indicating its acidic nature.

5. Combustion and respiration show similarity because they both involve the breakdown of substances with the use of oxygen to release energy, producing carbon dioxide and water as byproducts.

1. Phosphorus pentoxide [P₂O₅]

2. Phosphoric acid [H₃PO₄]

3. Hydrated iron (III) oxide [Fe₂O₃.xH₂O ]

4. Sulphur trioxide [SO₃]

5. Oxygen [O₂]

1. Nitrogen of the air is utilized by plants for formation of proteins.

2. The gas evolved as a product of combustion as well as respiration is carbon dioxide

3. A candle burns in a jar of oxygen, but gets extinguished in nitrogen.

4. Air has variable composition and hence there is more oxygen at lower altitudes than at higher altitudes.

5. Rusting is a chemical reaction which involves slow oxidation.

1. To determine the component of air used up in burning, by using phosphorus and a bell jar.

2. To study the presence of carbon dioxide.

3. To prove that oxygen is non-combustible, but supports combustion.

4. To prepare oxygen in the laboratory from hydrogen peroxide and a catalyst.

5. To demonstrate the burning of sulphur in a jar of oxygen.