2 air and its constituents - testlabz.com

1Class-VII Chemistry Question Bank

2 AIR AND ITS CONSTITUENTSQuestion-Answers.Q. 1. Why is oxygen called active air?Ans. Oxygen supports life on earth. All living beings need oxygen for

respiration. It is essential for combustion. So, oxygen is called activeair.

Q. 2. Carbon dioxide plays no role in respiration but all life wouldcome to an end if there were no carbon dioxide in air. Support thisstatement with relevant facts.

Ans. Carbon dioxide is not used during respiration but given out during theprocess. It is used by plants during the process of photosynthesis whenoxygen is given out. So carbon dioxide helps in maintaining the oxygenbalance in the atmosphere. Life would come to an end if there were nocarbon dioxide in air.

Q. 3. Explain the role of moisture in modifying the earth’s climate.Ans. Moisture determines the earth’s climatic conditions. The presence of

water vapour in the air causes rain. It controls the rate of evaporationfrom the bodies of plants and animals.

Q. 4. What is air pollution?Ans. When air contains substances that are harmful to plants and animals, it

is said to be polluted.Q. 5. What are the causes of air pollution?Ans. Volcanic eruption, crop pollination and human activities like burning of

coal, wood, diesel oil, petrol, kerosene, etc. are the major causes of airpollution.

Q. 6. Suggest five measures to prevent air pollution.Ans. Air pollution can be prevented as follows:

(i) By using smokeless sources of energy like solar energy andelectrical energy.

(ii) By using filters for the smoke coming out of the chimneys offactories and power plants.

(iii) By using internal combustion engines in vehicles for complete andefficient burning of fuel.

(iv) By locating industries away from factories.

2


(v) By growing more trees.Q. 7. What is nitrogen fixation?Ans. The process of conversion of free atmospheric nitrogen into its

compounds is called nitrogen fixation.Q. 8. What are the ways in which nitrogen fixation occurs.Ans. (i) Nitrogen fixation occurs biologically with the help of symbiotic

bacteria living in the root nodules of leguminous plants.(ii) Nitrogen fixation occurs non-biologically during lightning when

nitrogen and oxygen present in air combine to form nitric oxide,which further reacts with oxygen to form nitrogen dioxide.Nitrogen dioxide then reacts with water to form nitric acid andnitrous acid. Nitric acid reacts with metals carbonates to formmetal nitrates.

Q. 9. Explain the conversion of nitrogen into nitrates when there islightning.

Ans. During lightning, nitrogen and oxygen present in air combine to formnitric oxide, which further reacts with oxygen to form nitrogen dioxide.

N2 + O2 electricdischarge

⎯⎯⎯⎯→ 2NO2NO + O2 ⎯→ 2NO22NO2 + H2O ⎯→ HNO2 + HNO3

Nitric acid reacts with metal carbonates to form metal nitrates whichhelp in the growth and development of plants.CaCO3 + 2HNO3 ⎯→ Ca(NO3)2 + H2O + CO2

Q. 10. What are the harmful effects of sulphur dioxide, nitrogen dioxideand hydrogen sulphide present in the air?

Ans. Sulphur dioxide, nitrogen dioxide and hydrogen sulphide cause manyserious respiratory problems. They destroy the ozone layer, whichprotects us from ultraviolet radiations of the sun. They also cause acidrain.

Q. 11. What happens when (a) mercuric oxide and (b) potassium nitrateare heated?

Ans. Both mercuric oxide and potassium nitrate give out oxygen on heating.

2 HgO Heating⎯⎯⎯⎯→ 2 Hg + O2

2 KNO3 Heating⎯⎯⎯⎯→ 2 KNO2 + O2


Q. 12. Why is hydrogen peroxide preferred in the preparation of oxygengas?

Ans. Hydrogen peroxide is preferred for laboratory preparation of oxygenbecause:(i) no heating is required

(ii) the rate of evolution of O2 is moderate and therefore under control(iii) H2O2 is a safe chemical.

Q. 13. Why is oxygen gas collected by the downward displacement of water?Ans. Oxygen gas is collected by the downward displacement of water

because:(i) Oxygen is only slightly soluble in water. So, it can be collected

over water without fear of excessive dilution.(ii) Oxygen is slightly heavier than air, so it cannot be collected over

air.Q. 14. What happens when a glowing splinter is introduced in a jar

containing oxygen?Ans. Oxygen is a supporter of burning. So a glowing splinter rekindles when

introduced in a jar containing oxygen.Q. 15. Give four uses of oxygen. What is carbogen?Ans. (i) Oxygen is used during respiration.

(ii) Oxygen is used for burning or combustion.(iii) Liquid oxygen acts as a propellent.(iv) Oxyhydrogen flame provides a temperature around 2800°C which

is used for welding and cutting of metals.Carbogen is a mixture of oxygen (95%) and carbon dioxide

(5%) which is given to patients to stimulate breathing.Q. 16. How is oxygen renewed in air?Ans. Oxygen is renewed in air by the process of photosynthesis.Q. 17. What is rust? What are the two conditions of rusting?Ans. Rusting is the process in which iron slowly reacts with oxygen in the

air in the presence of moisture and produces a flaky brown substancecalled rust. The two most important conditions of rusting are:(i) Presence of oxygen (air) (ii) Presence of moisture.

Q. 18. State at least four ways of preventing rusting?Ans. Rusting can be prevented by :

(i) Galvanisation (ii) Oil painting(iii) Alloy formation (iv) Electroplating.

4


Q. 19. What is green house effect?Ans. The trapping of the earth’s radiated energy by carbon dioxide present in

the air, so as to keep the earth warm, is called green house effect.Q. 20. What evidence can you give to support the idea that air is a

mixture?Ans. The following evidences show that air is a mixture and not a compound:

(i) The composition of air varies from place to place and from timeto time.

(ii) The components of air retain their individual properties.(iii) No exchange of energy occurs when the components of air are

mixed with each other.(iv) Liquid air has no definite boiling point.

Q. 21. Why are rare gases inactive compared to other elements?Ans. Inert gases have eight electrons in their outermost orbit. So, they do not

react with any other substances. Hence, the rare gases are inactivecompared to other elements.

Q. 22. Why is sulphuric acid not used for the preparation of carbondioxide in the laboratory?

Ans. Dilute sulphuric acid is not used in the laboratory preparation of carbondioxide because calcium sulphate formed during the reaction isinsoluble in water. It covers the marble chips and stops the reaction.CaCO3 + H2SO4 (dil) ⎯→ CaSO4 + H2O + CO2

Exercises

A. Complete and balance the given equations.1. 2Mg + O2 ⎯→⎯→⎯→⎯→⎯→ _______2. _______ + O2 ⎯→⎯→⎯→⎯→⎯→ Fe2O33. CaCO3 + _______ ⎯→⎯→⎯→⎯→⎯→ CaCl2 + H2O + CO24. _______ + H2O ⎯→⎯→⎯→⎯→⎯→ H2CO35. Ca(OH)2 + CO2 ⎯→⎯→⎯→⎯→⎯→ CaCO3 + _______6. CO2 + C ⎯→⎯→⎯→⎯→⎯→ _______7. 6CO2 + 12H2O ⎯→⎯→⎯→⎯→⎯→ C6H12O6 + 6H2O + _______

8. S + O2 Heat⎯⎯⎯→


9. KClO3 Heat⎯ →⎯⎯ _______ + _______

10. Na2O + H2O ⎯→⎯→⎯→⎯→⎯→

11. CH4 + O2 ⎯→⎯→⎯→⎯→⎯→ _______ + _______

12. SO2 + H2O ⎯→⎯→⎯→⎯→⎯→ _______

13. MgO + H2O ⎯→⎯→⎯→⎯→⎯→ _______

14. 2H2O2 Heat⎯ →⎯⎯ _______ + _______

15. CaCO3 Heat⎯ →⎯⎯ _______ + _______Ans. 1. 2Mg + O2 ⎯→⎯→⎯→⎯→⎯→ 2MgO

2. 4Fe + 3O2 ⎯→⎯→⎯→⎯→⎯→ 2Fe2O33. CaCO3 + 2HCl ⎯→⎯→⎯→⎯→⎯→ CaCl2 + H2O + CO24. CO2 + H2O ⎯→⎯→⎯→⎯→⎯→ H2CO35. Ca(OH)2 + CO2 ⎯→⎯→⎯→⎯→⎯→ CaCO3 + H2O6. CO2 + C ⎯→⎯→⎯→⎯→⎯→ 2CO7. 6CO2 + 12H2O ⎯→⎯→⎯→⎯→⎯→ C6H12O6 + 6H2O + 6O2

8. S + O2 Heat⎯ →⎯⎯ SO2

9. 2KClO3 Heat⎯ →⎯⎯ 2KCl + 3O2

10. Na2O + H2O ⎯→⎯→⎯→⎯→⎯→ 2NaOH11. CH4 + 2O2 ⎯→⎯→⎯→⎯→⎯→ CO2 + 2H2O12. SO2 + H2O ⎯→⎯→⎯→⎯→⎯→ H2SO313. MgO + H2O ⎯→⎯→⎯→⎯→⎯→ Mg(OH)2

14. 2H2O2 Heat⎯ →⎯⎯ 2H2O + O2

15. CaCO3 Heat⎯ →⎯⎯ CaO + CO2

B. Give reasons for the following.Q. 1. Photosynthesis is a natural process for reducing air pollution.Ans. Carbon dioxide given out during respiration by plants and animals, is

used by green plants for photosynthesis. So, the process ofphotosynthesis is a natural process for reducing air pollution.

6


Q. 2. A glowing splinter rekindles when introduced in a jar of oxygenand a moist blue litmus paper shows no change when placed in atest tube containing oxygen gas.

Ans. A glowing splinter rekindles when introduced in a jar of oxygen becauseoxygen is a supporter of combustion. There is no change in moist bluelitmus paper when introduced in a test tube containing oxygen gasbecause oxygen is neutral in nature.

Q. 3. An oxy-hydrogen flame is used for welding and cutting metals.Ans. Hydrogen is a combustible gas and oxygen is a supporter of

combustion. Thus, oxy-hydrogen flame produces a temperature of about2800°C at which the metal melts, enabling it to be cut and welded.

Q. 4. Patients suffering from respiratory problems are kept in anoxygen tent.

Ans. Patients suffering from respiratory problems are kept in an oxygen tentto get more and constant supply of oxygen.

Q. 5. A product formed during the burning of a candle turns whiteanhydrous copper sulphate blue.

Ans. Water vapour is formed as one of the products during the burning of acandle which turns white anhydrous copper sulphate blue.

Q. 6. Baking powder is used in the preparation of cakes and bread.Ans. Carbon dioxide is liberated by the ingredients of baking powder when

it is added to the dough. This causes the dough to rise during thepreparation of cakes and breads.

Q. 7. Rusting of iron nails does not occur when kept in a test tubecontaining boiled water, but occur when kept in ordinary water.

Ans. There is no oxygen in boiled water. Both oxygen and moisture arenecessary for rusting. So rusting of iron nails does not occur when keptin a test tube containing boiled water, but occurs when kept in ordinarywater.

Q. 8. The product of oxidation of calcium dissolves in water to give asubstance whose solution turns moist red litmus blue.

Ans. Calcium dissolves in water to form calcium hydroxide which is alkalinein nature and turns moist red litmus blue.

Q. 9. Carbon dioxide is used in fire extinguishers.Ans. Carbon dioxide is neither combustible nor supports combustion. It is

heavier than air and cuts off the supply of oxygen. So, it is used in fireextinguishers.


Q. 10. Oxygen is collected by the downward displacement of water.Ans. Oxygen is slightly soluble in water and slightly heavier than air. So, it

is collected by the downward displacement of water.Q. 11. Carbon dioxide is collected by the upward displacement of air.Ans. Carbon dioxide is heavier than air and fairly soluble in water, so it is

collected by the upward displacement of air.Q. 12. Lime water turns milky on passing carbon dioxide through it.Ans. When CO2 gas is passed through lime water, insoluble calcium

carbonate is formed. Due to the formation of insoluble calciumcarbonate, it appears milky.

Q. 13. It is not advisable to fill hydrogen gas in big air balloons.Ans. Hydrogen is a combustible gas and forms an explosive mixture with air,

so it is not advisable to fill hydrogen gas in big air balloons.Q. 14. Clothes do not dry on a cloudy day.Ans. There is a large amount of moisture in the air on a cloudy day. So,

clothes do not dry on a cloudy day.C. Differentiate between the following:

1. Burning and respiration.Ans. Respiration is a slow oxidation reaction while burning is a fast

oxidation reaction.2. Collection of gas by downward displacement of water and upward

displacement of air.Ans. The gases which are lighter than air and insoluble in water are

collected by the downward displacement of water. The gases whichare heavier than air and soluble in water are collected by the upwarddisplacement of air.

3. Galvanising and tinplating.Ans. Galvanising is the process of coating iron sheets with a layer of zinc,

while tinplating is the process of coating iron sheets with a layer oftin.

4. Baking powder and baking soda.Ans. Baking soda is sodium hydrogen carbonate (NaHCO3). Baking powder

is prepared by the reaction between baking soda and potassiumhydrogen tartrate.

5. Carbogen and anesthesia.Ans. Carbogen is a mixture of oxygen (95%) and carbon dioxide (5%),

which is used for artificial respiration.

8


Anaesthesia is a mixture of oxygen and nitrous oxide, which isgiven to dental patients for rendering the concerned area ‘numb’.

D. Tick ( ) the most appropriate answer.1. The catalyst used in preparation of oxygen from hydrogen peroxide is

(a) magnesium oxide (b) manganese oxide(c) manganese dioxide (d) manganese chloride

2. Sodium burns in oxygen with a(a) golden yellow flame (b) brick red flame(c) green flame (d) blue flame

3. The gas evolved by the action of hydrochloric acid on marble chips is(a) nitrogen (b) carbon dioxide(c) oxygen (d) sulphur dioxide

4. Dry ice is(a) solid carbon dioxide (b) super cooled ice(c) crushed ice (d) solid nitrogen

5. When carbon dioxide is passed through lime water, it turns(a) light blue (b) light yellow(c) light green (d) milky

6. Galvanising is the process of coating iron with layer of molten(a) zinc (b) tin (c) copper (d) silver

7. Carbon dioxide is an oxide of(a) carbon (b) sulphur (c) carbonates (d) hydrocarbon

8. A gas which neither burns nor supports burning is(a) oxygen (b) nitrogen(c) hydrogen (d) carbon monoxide

9. Harmful substances in the air are called(a) microbes (b) pollutants (c) inert gases (d) none of the above

10. A gas that can be prepared by decomposing hydrogen peroxide is(a) carbon dioxide (b) nitrogen(c) ozone (d) oxygen

11. Oxides are produced when elements react with(a) hydrogen (b) water (c) carbon (d) oxygen

12. Ignition temperature is the(a) lowest temperature at which a material begins to burn.


(b) highest temperature at which a material begins to burn.(c) optimum temperature at which a material burns.(d) none of the above

13. Decay of dead plants and animals is an example of(a) slow oxidation (b) rapid oxidation(c) spontaneous oxidation (d) a reduction process

14. A glowing splinter lowered in a jar full of oxygen will(a) stop glowing (b) burst into flames at once(c) give out smoke (d) none of these

15. Carbon dioxide gas is:(a) heavier than air (b) lighter than air(c) as heavy as air (d) none of these

Ans. 1. (c) 2. (a) 3. (b) 4. (a) 5. (d) 6. (a) 7. (a) 8. (b) 9. (b) 10. (d)11. (d) 12. (a) 13. (a) 14. (b) 15. (a).

E. Complete the following sentences with appropriate words:1. ........................ gas can cause acid rain.2. Every nine parts of water by weight contain ...................... parts by

weight of oxygen.3. Dust particles help in the ........................ of water vapour present in the

air.4. ........................ gas is used in filling weather observation balloons.5. The solution of carbon dioxide in water is called ................... acid.6. Solid carbon dioxide is called ........................ .7. Burning of a candle is an example of ........................ oxidation.8. Sodium catches fire when kept open in the air. This is an example of

........................ oxidation.9. The constant exchange of carbon with the environment is termed as

........................ .10. ........................ gas is evolved when carbonates are treated with acid.11. Oxides of ..................... are formed under very high energy conditions.12. Nitrogen and oxygen can be separated from the air by ...................... .13. ........................ gas is used to extinguish a fire.14. Space vehicles need ........................ to burn their fuel in space.15. Common explosives are compounds of ........................ .16. The amount of water vapour in the air is called ................... .17. ........................, an important constituent of air, is converted into

10


compounds by the symbiotic bacteria and then absorbed by plants forthe manufacture of proteins.

18. Rainwater which contains dissolved ........................ pollutants resultsin acid rain.

19. Sulphur burns in oxygen to give sulphur dioxide which dissolves inwater to give ........................ acid.

20. ................... and ................... are two conditions required for rusting.21. The salt formed when calcium carbonate reacts with dilute

hydrochloric acid is ........................ .22. In the laboratory preparation of oxygen from hydrogen peroxide,

........................ acts as a catalytic agent.23. ........................ decomposes on heating to form sodium carbonate,

water and carbon dioxide.24. The molecules of carbon dioxide present in the atmosphere reflect

........................ radiations given out by the earth.25. Oxygen was discovered by ........................ .26. Liquid oxygen has a boiling point of ........................ .27. If some limewater is left in air, a white crust is formed on the surface

due to the presence of ........................ in air.28. The products of burning of a candle (or any hydrocarbon) are

........................ .29. The percentage of oxygen is approximately ....................... in the

atmosphere and ........................ in the earth’s crust.30. ........................ is used as a catalyst in the preparation of oxygen from

potassium chlorate.Ans. 1. Sulphur dioxide 2. eight 3. precipitation 4. Helium 5. carbonic

6. dry ice 7. slow 8. spontaneous 9. carbon cycle 10. Carbon dioxide11. nitrogen 12. fractional distillation 13. Carbon dioxide 14. oxygen15. nitrogen 16. humidity 17. Nitrogen 18. oxides of sulphur andnitrogen 19. sulphurous 20. Oxygen, moisture 21. calcium chloride22. manganese dioxide 23. Sodium bicarbonate 24. infrared 25. Priestley26. –183°C 27. carbon dioxide 28. CO2 and H2O 29. 21%, 50%30. Manganese dioxide

F. Write true or false in front of the following statements:1. The chemical formula of rust is Fe2O3.xH2O.


2. Atmospheric nitrogen can be directly absorbed by plants to form plantproteins.

3. Oxygen gas is used for making soft drinks.4. LOX is short name for liquid oxygen.5. Nitrogen is a gaseous non-metal used for artificial respiration.6. Dry ice is used in the refrigeration of sea food.7. Atmospheric carbon dioxide acts as food for plants.8. Sodium burns in oxygen with a brick red flame.9. Carbon dioxide turns moist red litmus paper blue.

10. Neon gas is used in making glow tubes for advertisement purposes.11. Air is a compound.12. Carbon dioxide is given out during photosynthesis.13. Respiration is a chemical change.14. A basic oxide turns moist blue litmus red.15. Oxygen is slightly heavier than air.16. Burning of cooking gas is an exothermic reaction.17. Magnesium oxide is an acidic oxide.18. Milk becoming curd is a process of slow oxidation.

Ans. 1. T 2. F 3. F 4. T 5. F 6. T 7. T 8. F 9. F 10. T 11. F 12. F 13.T 14. F 15. T 16. T 17. F 18. T

G. Statements given below are incorrect. Write the correct statements.1. Carbon dioxide allows the earth to radiate out heat rapidly at night.

Ans. Carbon dioxide does not allow the earth to radiate out heat rapidly atnight.

2. Helium gas at high pressure is filled in electric bulbs.Ans. Argon gas is filled in electric bulbs at low pressure.

3. Oxygen is prepared in the laboratory by treating hydrogen withmanganese dioxide.

Ans. Oxygen gas is prepared in the laboratory by treating hydrogenperoxide with manganese dioxide.

4. Calcium burns in oxygen with a golden yellow flame.Ans. Sodium burns in oxygen with a golden yellow flame.

5. Oxygen gas turns moist blue litmus paper red.Ans. Carbon dioxide gas turns moist blue litmus paper red.

6. The chemical name of magnesite is magnesium sulphate.

12


Ans. The chemical name of magnesite is magnesium carbonate.7. Moist oxygen causes rusting of copper.

Ans. Moist oxygen causes rusting of iron.8. Both carbon dioxide and oxygen are essential for respiration by plants

and animals.Ans. Only oxygen is essential for respiration by plants and animals.

9. The components of air can be separated by physical means and havea specific set of properties.

Ans. The components of air can be separated by physical means and havedifferent sets of properties.

10. Carbon dioxide is the main pollutant from an automobile exhaust.Ans. Carbon monoxide is the main pollutant from an automobile exhaust.

11. A catalyst is a chemical substance which increases the rate of areaction.

Ans. A catalyst is a chemical substance which alters the speed of areaction.

12. Dilute sulphuric acid can be used instead of dilute hydrochloric acidfor the laboratory preparation of carbon dioxide from calciumcarbonate.

Ans. Dilute sulphuric acid cannot be used instead of dilute hydrochloricacid for the laboratory preparation of carbon dioxide from calciumcarbonate.

13. The milkiness in lime water by the passage of carbon dioxide is dueto the formation of calcium hydrogen carbonate.

Ans. The milkiness in limewater on the passage of carbon dioxide is due tothe formation of calcium carbonate.

14. Limestone dissolves in water containing dissolved carbon dioxide andforms limewater.

Ans. Limestone does not dissolve in water containing dissolved carbondioxide.

15. Carbon dioxide is prepared in the laboratory by treating marble chipswith dilute sulphuric acid.

Ans. Carbon dioxide is prepared in the laboratory by treating marble chipswith dilute hydrochloric acid.

H. Name the following:


1. A constituent of air which acts as nuclei for precipitation of rainwater.2. A pollutant in air formed during lightning discharge.3. A metallic oxide which on thermal decomposition, i.e., heating, gives

a metal and oxygen gas.4. A non-metal which burns in oxygen with a pale blue flame.5. Two products of oxidation of hydrocarbons.6. The component of air whose percentage by volume is about 21%.7. The air pollutant which causes warming of the earth’s surface.8. The chemical name of rust.9. The gas other than oxygen present in an anaesthetic.

10. A white solid formed when carbon dioxide solidifies on cooling atvery low temperature.

11. The second lightest gas which does not burn.12. It changes the rate of reaction.13. Component of air that supports burning.14. Priestley discovered this gas.15. Lavoisier explained the role of oxygen in this process.

Ans. 1. Dust particles 2. Oxides of nitrogen 3. Mercuric oxide 4. Sulphur5. Carbon dioxide and water vapour 6. Oxygen 7. Carbon dioxide8. Hydrated ferric oxide 9. Nitrous oxide 10. Dry ice 11. Helium12. Catalyst 13. Oxygen 14. Oxygen 15. Acid formation

I. Write the uses of the following.1. Dry ice 2. Noble gases3. Nitrogen 4. Oxygen 5. Catalytic converter

Answers1. Dry ice — It is used in the refrigeration of sea food.2. Noble Gases — (a) Helium is used for filling weather observation

balloon.(b) Argon is used for filling electric bulbs.(c) Neon is used for making advertising glow tubes.

3. Nitrogen — (a) It dilutes the activity of oxygen.(b) It is used in the manufacture of proteins.

4. Oxygen — (a) It is essential for respiration.(b) It is essential for combustion of fuels.

5. Catalytic converter — The catalytic converter converts

14


hydrocarbons, nitrogen dioxide and carbon monoxide into harmlesswater vapour and carbon dioxide.

J. Match the statements in Column A with those in Column B.

Column A Column B1. Largest constituent of air which (a) Helium

dilutes the activity of oxygen.2. A gas used for filling weather (b) Nitrogen

observation balloons.3. A gas vital for respiration. (c) Sulphur dioxide4. A pollutant in air responsible for acid (d) Carbogen

rain.5. A mixture of oxygen and carbon dioxide (e) Carbon dioxide

used for artifical respiration in hospitals.6. A gas extensively used in making (f) Oxygen

soft drinks.

Ans. 1. (b) 2. (a) 3. (f) 4. (c) 5. (d) 6. (e).K. Define these terms.

1. Catalyst 2. Humidity 3. Oxidation 4. Ignition temperature 5. PollutantsAnswers

1. Catalyst — A chemical substance that alters the rate of a reactionwithout undergoing any change in itself is called a catalyst.

2. Humidity — The amount of water vapour present in the atmosphereis called humidity.

3. Oxidation — The process of addition of oxygen is known asoxidation.

4. Ignition temperature — The lowest temperature at which asubstance burns is known as ignition temperature.

5. Pollutants — Harmful substances present in the air are calledpollutants.

****

2 air and its constituents - testlabz.com

Documents