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Symbiotic bacteria are present in nodules of pod-bearing plants such as peas, grams, etc.

Two substances from which oxygen can be obtained at a large scale : Air, water.

The process by which oxidation of food in our body takes place is respiration.

Symbiotic bacteria is not present in the nodules of rice plant.

Nitrogen is utilised by plants for their growth and development.Free nitrogen of the air is converted to nitrogen compounds which are absorbed by the plants and converted to plant proteins. In the absence of nitrogen no plants will be survived on the earth.

Nitrogen being inert or unreactive is filled into food packets to drive out the oxygen and reduce bacterial growth. The food package then stays well preserved.

Three noble gases present in the air are :

1. Helium: It is used for filling weather observation balloons.

2. Argon: It is used for filling electric bulbs at low pressure.

3. Neon: It is used for making advertising glow tubes, used in big shops, hotels etc. at night.

Sodium, carbon.

1. Joseph Priestley discovered the oxygen gas.

2. Oxygen is slightly heavier than air.

3. Manganese dioxide acts as a catalyst.

4. Oxygen is collected by the downward displacement of water.

5. Oxygen occupies about 1/5th of air by volume.

6. Oxygen is neutral to litmus testing.

Significance of water vapour in atmosphere is as follows:

1. Presence of water vapour in air determines the climatic conditions. Excessive water vapor causes rain. They also produce snow, fog, mist, hails and other phenomena, depending upon the temperature.

2. The presence of water vapor controls the rate of evaporation from the plants and animals.

3. The presence of water vapour is very essential for the growth of plants.

4. It is very essential for the health and comfort of animals.

all the above

Physical properties of oxygen gas are :

1. Oxygen is a colorless, odorless and tasteless gas.

2. Under laboratory conditions, oxygen is practically insoluble in water.

3. Oxygen is slightly heavier than air.

4. Under high pressure and low temperature, oxygen can be changed to the liquid state. Liquid oxygen is slightly bluish in color.

(a)

1. When mercuric oxide is heated, it decomposes to give mercury and oxygen.

2. Potassium nitrate on heating gets converted into molten potassium nitrite with the release of oxygen.

(b) Potassium chlorate needs heating for quite some time (to a high temperature) before it decomposes.

Oxides are binary compounds formed by the chemical combination of a substance metal or a non-metal with oxygen.

Examples :

Metal:

1. Sodium oxide (Na₂O).

2. Calcium oxide (CaO).

Non-metal:

1. Sulphur dioxide (SO₂).

2. Carbon dioxide (CO₂).

1. Provides moisture for plant and animal growth.

2. Determines climatic conditions by providing rain, snow, mist etc.

3. Prevents undue drying up of the plant and animal bodies.

Dry oxygen can be obtained if the gas so collected is passed f over a tower containing drying agents like phosphorus pentaoxide, fused calcium chloride or bubbling the gas through concentrated sulphuric acid. Then the dry gas is collected by downward displacement of mercury.

(a) Sulphur : Blue litmus turns red.

(b) Phosphorus : Blue litmus turns red.

(c) Calcium : Red litmus turns blue.

(d) Magnesium : Red litmus turns blue.

1. Potassium nitrate crystals on heating strongly yield potassium nitrite and oxygen gas.

2. Sulphur dioxide gas dissolves in water to form sulphurous acid.

3. Phosphorus pentaoxide on dissolving in water forms phosphoric acid.

4. To prevent rusting, the underside of the ships is coated with red lead oxide paint.

5. The nitric oxide gas reacts with the oxygen and water vapor to form nitric acid vapor.

(a) Uses of oxygen 

1. Oxygen is used by firemen, miners, aviators, sea divers and even by every living being. 

2. Oxygen is necessary for burning of fuels. 

3. Oxygen mixed with hydrogen as a fuel produces.a flame with a very high temperature of about 2800°C. 

4. As a fuel in spacecraft. 

(b) All living beings use atmospheric oxygen in breathing and burning of fuels and in the formation of oxides of nitrogen. Yet the amount of oxygen in the air remains more or less constant. This is because green plants return oxygen to the atmosphere by the process of photosynthesis.

Acidic oxides

1. Formed when non-metals are burnt in oxygen

2. When dissolved in water form acids, eg. carbon dioxide [CO ₂]

Basic oxide

1. Formed when metals are burnt in air

2. turn red litmus to blue.

eg. potassium oxide [K₂O]

2KNO₃ + heat → 2KNO₂ + O₂(g)

2KClO₃ + heat → 2KCl + 3O₂

2HgO + heat → 2Hg + O₂

CaO + H₂O → Ca(OH)₂

(a) When the air contains harmful or undesirable substances, generated by the activities of man or nature, such that their concentration interferes with human health or is injurious to plants or animals, it is said to be polluted air.

(b) Air gets polluted in nature by following ways :

• During the forest fire, a large number of carbon particles (smoke) and carbon monoxide pollute the air.
• When volcanoes erupt, a huge amount of ash and sulfur-dioxide is added to the air as pollutants.

(a) Pollution of air is caused by humans in the following ways :

1. Large amount of coal burnt in thermal plants throw out huge amounts of smoke and ash in the atmosphere.

2. Vehicles using petrol or diesel give out a lot of smoke and harmful gases like carbon dioxide, lead-carbon- monoxide.

3. Burning of garbage causes a lot of pollution due to the formation of carbon monoxide, sulphur dioxide and nitrogen dioxide.

4. Pesticides spread in the fields also cause air pollution.

(b) Pollution of air by humans can be minimised in the following manner :

1. In the thermal power plants, steel plants etc., electric precipitators should be used to reduce the smoke and ash entering the air.

2. Efficient internal combustion engines should be used in vehicles.

3. Improved chulha or stoves should be used to minimize the production of smoke and carbon monoxide.

4. We must grow more trees as they absorb harmful gases.

Difference between rusting and burning

A metal highly resistant to rusting Tin.

(a) When fuels bum they produce sulphur dioxide, sulphur trioxide, nitrogen dioxide, hydrogen sulphide when these gases mix with rainwater. They produce sulphuric and nitric acid. These acids mix with rain water to form acid rain.

(b) Five measures for the prevention of air pollution are:

1. By using smokeless sources of energy, like solar energy and electrical energy, in place of conventional fossil fuels.

2. By using filters for the. smoke coming out of the chimneys of factories and power plants.

3. By using internal combustion engines in vehicles for the complete and efficient burning of fuel.

4. By locating industries away from residential areas.

5. By growing more trees.

Carbon dioxide, methane, nitrous oxide, etc.

Air Pollution : “Mixing of UNWANTED and HARMFUL SUBSTANCES in air is called AIR POLLUTION.”

In the laboratory preparation of oxygen from potassium chlorate, manganese dioxide acts as a catalyst i.e. it increases the rate of production of oxygen without getting itself consumed.

(a) Uses of oxygen are :

• Spaceships: Liquid oxygen is used for burning fuel in rockets and spaceships as there is no air in the space.
• Cutting and welding: Oxygen and hydrogen are made to bum in a specially designed torch. The oxyhydrogen flame can easily melt metals and used for cutting and welding them.
• Chemical industry: Oxygen is used in large amount for preparing sulphuric acid from sulphur and nitric acid from ammonia.

(b) Tests for oxygen gas are :

• It rekindles glowing wooden splinter.
• If mixed with colorless nitric oxide gas, it forms reddish brown fumes of nitrogen dioxide.
• It dissolves in alkaline pyrogallol solution and turns it brown.

Water vapour and energy.

Nitrogen dioxide + Oxygen + Water → Nitric acid 

The nitric acid formed comes down with the rain and combines with the carbonates in the soil to form soluble nitrates.

A blanket of air around earth is called atmosphere. Without atmosphere life would not be possible as atmosphere protects us from harmful gases. We could not live without air present in atmosphere. In absence of the atmopshere, the earth would get so cold at night that we would not be able to survive. No CO₂ and N₂ for plants without atmosphere.

Four steps taken to control air pollution :

1. Tall chimneys be installed in factories and power houses so that smoke and gases rise high and get diluted. 

2. GROW more tall trees to absorb CO₂ Also plants help in bringing rains. 

3. Use unleaded petrol and CNG in all public transport vehicles 

4. Industries should be located far away from residential areas.

Rusting : Slow conversion of iron into its hydrated oxide in the presence of moisture and air is called rusting. 

Conditions for rusting: 

1. Presence of moisture (water). 

2. Presence of oxygen (air). 

Chemical name of rust is hydrated iron oxide [Fe20rxH20]

Ozone is present in the upper layer of atomosphere called stratosphere. It protects the earth from harmful ultraviolet rays of the sun. 

Some air pollutants like chlorofluorocarbon (CFC) react with ozone present in the atmosphere. These pollutants reduce the density of the air. The ultra-violet rays from the sun reach directly on the earth which affect human health, causing skin diseases and cancer and also increase the earth’s temperature.

(a) Two air pollutants that affect our health are : 

1. DUST 

2. SMOKE 

(b) Two air pollutants that cause acid rain are : 

1. SULPHUR DIOXIDE (SO₂) and 

2. NITROGEN DIOXIDE (NO₂) 

(c) Two air pollutants that cause global warming are : 

1. CARBON DIOXIDE (CO₂) 

2. METHANE (CH₄)

(a) Pollutants: Air contains substances which are harmful to plants and animals. These harmful substances are called pollutants.

(b) Acid rain: When sulphur trioxide and nitrogen oxide present in the air mix with rainwater they form sulphuric acid and nitric acid respectively. Rainwater containing these acids is called acid rain.

(c) Global warming: An increase in the percentage of carbon dioxide, methane, nitrous oxide and chlorofluorocarbon traps the heat causing the temperature of the earth and its surroundings to rise. This is known as global warming.

(d) Smog: Oxides of nitrogen form a mixture of smoke and fog known as smog which affects our eyes too.

4. nitrogen dioxide and sulphur dioxide

Air is a mixture because : 

1. Air has no formula, a mixture has no formula whereas compound has a formula. 

2. No energy changes are involved to form air from various gases. 

3. When air is formed out of its constituents no change in mass and no change in volume takes place. 

4. Properties of air vary from place to place and time to time. i.e. there is more CO₂ in towns as compared to villages where more oxygen prevails as compared to towns. 

5. Components of air can be separated by simple physical methods.

“Air is a mixture” The following are in evidence which proves that air is a mixture.

1. The composition of air varies from place to place and from time to time.

2. The components of air retain their individual properties.

3. Liquid air has no definite boiling point.

4. No energy exchange occurs when the components of air are mixed with each other.

Air is a mixture because : 

1. Air has no formula, a mixture has no formula whereas compound has a formula. 

2. No energy changes are involved to form air from various gases. 

3. When air is formed out of its constituents no change in mass nor change in volume takes place. 

4. Properties of air vary from place to place and time to time. i.e. there is more CO₂ in towns as compared to villages where more oxygen prevales as compared to towns. 

5. Components of air can be separated by simple physical methods.

golden yellow in colour

Sodium burns in oxygen with a golden yellow flame.

all the above

Phosphorus pentoxide (P₂O₅).