Selina Concise Solutions for ICSE Class 7 Chemistry Chapter 7 Air and Atmosphere

Points to Remember:

1. Air is a mixture of many gases, mainly Nitrogen = 78.1%, Oxygen = 20.9%, Carbon dioxide = 0.03 - 0.04%, Inert gases = 0.9%, [Water vapours, Dust particles and Impurities = Variable].
2. Nitrogen is a colourless, an odourless and a tasteless gas. It is slightly lighter than air.
3. The process of conversion of free atmospheric nitrogen into its compounds is called nitrogen-fixation.
4. Oxygen constitutes about 21% of air by volume. It supports life on earth.
5. Carbon dioxide is present in air in a very small quantity, e. 0.03 - 0.04%. It is essential for the process of photosynthesis.
6. Inert gases like neon, argon do not react with any substance.
7. The harmful substances added to air are called pollutants.
8. Some pollutants are suspended particles like pollen grains, oxides of sulphur and nitrogen, oxides of carbon, chlorofluorocarbons etc.
9. Symbol of oxygen = O; atomic number = 8, relative mass = 16, molecule formula = O₂.
10. Oxygen is available in free and combined state.
11. A catalyst is a substance that increases or decreases the rate of a chemical reaction without itself undergoing any chemical change.
12. Oxides are binary compounds formed by the chemical combination of substance with oxygen.
13. Rusting is the process in which iron slowly reacts with oxygen in the air and produces a flaky brown substance.
14. Photosynthesis is a process by which CO₂ and water are used up by green plants in the presence of sunlight to produce glucose and oxygen gases.

A. AIR : A MIXTURE OF GASES

EXERCISE - I

Question 1: Give one use for each of the following inert gases :
(a) argon
(b) helium
(c) neon
(d) radon
(e) krypton
(f) xenon

Answer:
(a) Argon - Argon is filled into electric bulbs to prevent the oxidation of their filaments.
(b) Helium - It is used in filling up weather observation balloons.
(c) Neon - Neon is used for making advertisement sign boards.
(d) Radon - It is used for treatment of Cancer.
(e) Krypton - It is used in photography.
(f) Xenon - It is also used in photography.
Inert gases are very stable and do not easily react with other elements. This property makes them safe and useful for specialized lighting and medical purposes.
Teacher's Tip: Remember that "inert" means "lazy" because these gases don't like to join in any chemical reactions!
Exam Tip: For argon, always specify its use in "electric bulbs" as it is the most common example asked.

Question 2: Answer the questions put against each of the following constituents of air :
(a) Nitrogen : Explain its significance for plants and animals.
(b) Oxygen : What is the percentage proportion of oxygen in air ? Why is oxygen called active air.
(c) Carbon dioxide : “Although carbon dioxide plays no role in respiration, all life would come to an end if there is no carbon dioxide in air.” Support this statement with relevant facts.
(d) Water vapours : Explain their role in modifying the earth’s climate.

Answer:
(a) Plants convert nitrogen into protein. It is an important constituent of proteins, which are necessary for the growth of animals, plants and human beings. Plants convert nitrogen into proteins.
(b) 20.9%, oxygen is called active air because it supports life on earth. It is essential for the process of combustion.
(c) Carbon dioxide is essential for photosynthesis by which green plants prepare their food. It minimises heat loss by radiation. Thus, it balances the temperature on earth.
(d) Water vapour determine the earth’s climate conditions. It causes rain. It controls the rate of evaporation from the bodies of plants and animals.
These various components work together to maintain a stable environment that supports life. Each gas provides a unique service, from feeding plants to controlling the world's temperature.
Teacher's Tip: Think of the atmosphere like a big blanket; Carbon dioxide and water vapor are the parts that keep us warm.
Exam Tip: If asked why oxygen is "active," ensure you mention both "respiration" and "combustion."

Question 3: Define the following terms :
(a) pollutants
(b) acid rain
(c) Global warming
(d) smog

Answer:
(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.
Pollution represents an imbalance in the natural composition of the air caused by human and natural activities. Understanding these terms helps us identify environmental threats and find ways to protect our planet.
Teacher's Tip: Smog = Smoke + Fog. It's a simple word mash-up that explains exactly what it is!
Exam Tip: In the definition of acid rain, make sure to name the specific gases like oxides of sulphur and nitrogen.

Question 4: “Air is a mixture”. Support this statement citing at least three evidences.

Answer:
“Air is a mixture” The following are in evidences which prove 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.
Unlike a compound, the parts of a mixture can be separated by physical means and do not have a fixed formula. These evidences show that air is just a physical combination of different gases.
Teacher's Tip: Think of air like a fruit salad; you can see the different parts, and they aren't permanently stuck together!
Exam Tip: Use the "variable composition" point as your first evidence, as it is the strongest proof.

Question 5: What is air pollution ? What are the harmful effects of sulphur dioxide, nitrogen dioxide and hydrogen sulphide present in the air ?

Answer:
Air Pollution : Air is polluted by natural processes like volcanic eruption, crop pollination, etc. mostly it is polluted by human activities like burning of coal, wood, diesel oil, kerosene, petrol etc.
Fossil fuels contain sulphur and nitrogen as impurities. When fuels burn these substances combine with air to produce gasses like sulphur dioxide, nitrogen oxide and hydrogen sulphide. They cause many serious respiratory problems. They can destroy the ozone layer, which protects us from the ultra violet radiations of the Sun. They also cause acid rain, which damages crops and buildings.
These toxic gases are byproduct of industry and transport that degrade the quality of life on Earth. They not only hurt humans directly but also damage the protective layers of our atmosphere.
Teacher's Tip: "Pollution" usually starts with "People"—remember that human activities are the biggest source of these harmful gases.
Exam Tip: Mention "respiratory problems" as a health effect and "damage to buildings" as an environmental effect to show a complete understanding.

Question 6: (a) What are the causes of air pollution ?
(b) Suggest five measures to prevent air pollution.

Answer:
(a) When fuels burn they produce sulphur dioxide, sulphur trioxide, nitrogen dioxide, hydrogen sulphide when these gases mix with rain water. 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 complete and efficient burning of fuel.
4. By locating industries away from residential areas.
5. By growing more trees.
Pollution can be managed by switching to cleaner energy and using technology to clean the air before it leaves factories. Planting more trees is one of the simplest and most effective natural ways to purify our atmosphere.
Teacher's Tip: Remember the 3 G's for prevention: Green energy, Grow trees, and Good filters!
Exam Tip: When listing preventive measures, try to include a mix of industrial solutions and natural solutions.

Question 7: (a) What is nitrogen-fixation ?
(b) What are the two ways in which nitrogen fixation occurs?
(c) Explain the conversion of nitrogen into nitrates during lightning.

Answer:
(a) Nitrogen fixation : Symbiotic bacteria living in the root nodules of leguminous plants like peas, beans, absorb nitrogen directly from air and convert into nitrates. Thereafter, the plants convert it into proteins. Nitrogen is returned to the soil when plant and animal matter decays. This decomposition work is done by organisms called denitrifying bacteria which reconvert dead organic tissue into its constituent nitrogen.
(b) 1. Natural process. 2. Non-biological fixation.
(c) During lightning, temperatures often reach as high as 3000°C. At such high temperatures, nitrogen and oxygen present in the air combine to form nitric oxide, which further react with oxygen to form nitrogen dioxide
N₂ + O₂ →{electric} 2NO
Oxygen constitutes about 21% of air by volume. It is the active part of air.
Nitrogen dioxide then reacts with the water vapour present in air to form nitrous and nitric acids.
2NO + O₂ →{discharge} 2N₂
Oxygen constitutes about 21% of air by volume. It is the active part of air.
2NO₂ + H₂O → HNO₂ + HNO₃v
Oxygen constitutes about 21% of air by volume. It is the active part of air.
Nitric acid, so formed, reaches the earth along with rain-water, and reacts with metal carbonates to form metal nitrates.
CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂
Oxygen constitutes about 21% of air by volume. It is the active part of air.
Nitrogen fixation makes the nitrogen in the air usable for living organisms by changing it into chemical forms like nitrates. This process happens both through biology in the soil and through high-energy events like lightning strikes.
Teacher's Tip: Think of nitrogen fixation as "Nature's Fertilizer Factory" working in the soil and sky.
Exam Tip: Be sure to write the full balanced equations for the lightning process, as they often carry extra marks.

B. OXYGEN

EXERCISE - II

Question 1: Name :
(a) The most abundant element in the earth’s crust.
(b) A chemical called oxygenated water.
(c) A metal highly resistant to rusting.
(d) A mixture of oxygen and carbon dioxide used for artificial respiration.
(e) Two substances from which oxygen can be obtained at a large scale.
(f) An oxide and a carbonate containing oxygen.
(g) Two substances which undergo rapid oxidation.

Answer:
(a) Oxygen.
(b) H₂O₂ (Hydrogen peroxide).
(c) Tin.
(d) Carbogen.
(e) Air, water.
(f) Mercuric oxide and potassium chlorate.
(g) Sodium, carbon.
Oxygen is not just a gas in the air; it is a major part of the Earth's solid surface and many common chemicals. These terms identify oxygen's role in industrial mixtures and common materials.
Teacher's Tip: Carbogen is like a "Carbon" + "Oxygen" name mashup; use that to remember its two ingredients!
Exam Tip: For "abundant element," remember that oxygen is the winner for the crust, while nitrogen is the winner for the atmosphere.

Question 2:
(a) Taking hydrogen peroxide, how would you prepare oxygen in the laboratory ?
(b) What is the role of manganese dioxide in the preparation of oxygen ?
(c) Write the balanced chemical equation for the above chemical reaction.
(d) Why is hydrogen peroxide preferred in the preparation of oxygen gas ?
(e) Why is oxygen collected by downward displacement of water ?
(f) What happens when a glowing splinter is introduced in a jar containing oxygen ?
(g) What happens when oxygen gas is passed through alkaline pyrogallol solution ?

Answer:

(a) Take manganese dioxide in a round bottom flask and add hydrogen peroxide drop by drop to it, which acts as a catalyst as shown in the figure. Collect oxygen by downward displacement of water.
(b) Manganese dioxide acts as a catalyst.
(c) 2H₂O₂ → MnO₂ 2H₂O + O₂
(d) H₂O₂ is preferred for lab preparation of oxygen because of following reasons:
1. No heating is required.
2. The rate of evolution of oxygen (O₂) is moderate and under control.
H₂O₂ is a safe chemical.
(e) Since the water is displaced downward by the gas collecting in the jar, the process is called downward displacement of water. The reasons are :
1. Oxygen is only slightly soluble in water. Therefore it can be collected over water without fear of excessive dilution.
2. Oxygen is slightly heavier than air, so it cannot be collected over air.
(f) Introduction of glowing splinter in the jar. The glowing splinter rekindles, but the gas does not catch fire.
(g) Alkaline pyrogallol solution turns brown when oxygen is passed through it.
Preparing oxygen with hydrogen peroxide is efficient because it avoids the dangers of high heat. Collecting it over water is the standard method because oxygen won't dissolve away into the liquid.
Teacher's Tip: Think of the catalyst (MnO₂) as a coach; it makes the reaction go faster but doesn't actually play in the game!
Exam Tip: In the equation for lab preparation, always write "MnO₂" over the reaction arrow to show it's the catalyst.

Question 3: (a) What happens when 1. mercuric oxide and 2. potassium nitrate are heated ?
(b) Why is potassium chlorate not used for laboratory preparation of oxygen ?

Answer:
(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 sometime (to a high temperature) before it decomposes.
Heating certain oxides and nitrates can release pure oxygen gas, but it often requires very high temperatures. Because H₂O₂ works at room temperature, these other chemicals are less common in basic lab setups.
Teacher's Tip: Remember that heating chemicals to release oxygen is called "thermal decomposition."
Exam Tip: If asked for a chemical equation for (a), ensure you show the "heat" symbol (Delta) over the arrow.

Question 4: What are oxides ? Give two examples for each of metallic and non-metallic oxides.

Answer:
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).
Oxides are simply what you get when oxygen "bonds" with another element. Metallic oxides often form bases when mixed with water, while non-metallic oxides often form acids.
Teacher's Tip: "Binary" just means the compound is made of exactly TWO different elements.
Exam Tip: Be careful with the chemical formulas (Na₂O vs CaO)—always check the valency of the metal.

Question 5: Name the three types of oxidation processes. In which of these large amount of heat and light energy are produced?

Answer:
Oxidation can be categorised into three types :
1. Spontaneous oxidation
2. Rapid oxidation
3. Slow oxidation
Out of the above said three types, rapid oxidation produces large amount of heat and light energy.
Oxidation is a chemical reaction where oxygen combines with something else, like iron rusting or wood burning. The speed of this reaction determines how much energy is felt as heat or seen as light.
Teacher's Tip: Rapid oxidation is just a fancy scientific way of saying "burning" or "combustion."
Exam Tip: For "slow oxidation," the best example to remember is "rusting of iron."

Question 6: What do you observe when the following substances are heated and then tested with moist blue and red litmus - paper?
(a) Sulphur
(b) Phosphorus
(c) Calcium
(d) Magnesium

Answer:
(a) Sulphur : Blue litmus turns red.
(b) Phosphorus : Blue litmus turns red.
(c) Calcium : Red litmus turns blue.
(d) Magnesium : Red litmus turns blue.
When non-metals like sulphur burn, they create acidic gases that turn blue litmus red. Metals like magnesium create basic oxides that turn red litmus blue when moistened.
Teacher's Tip: Remember the rhyme: "Non-metals make acids (Blue to Red), Metals make bases (Red to Blue)."
Exam Tip: Always specify that the litmus paper must be "moist" because the reaction needs water to show the acid/base change.

Question 7: Complete and balance the following chemical equations.
KNO₃ →heat KNO₂ + _________
KClO₃ →heat _________ + O₂
HgO →heat _________ + _________
CaO + H₂O \rightarrow _________

Answer:
2KNO₃ →heat 2KNO₂ + O₂ (g)
2KClO₃ →heat 2KCl + 3O₂
2HgO → heat 2Hg + O2
CaO + H₂O → Ca(OH)₂
These equations show how oxygen is released from compounds through heat or how oxides react with water. Balancing ensures that the number of atoms on the left side matches the right side.
Teacher's Tip: To balance oxygen equations, look for the O₂ molecule and try to match the counts on the reactant side.
Exam Tip: Don't forget to include the (g) symbol for oxygen to show it is released as a gas.

Question 8: (a) Give four uses of oxygen.
(b) How is oxygen naturally renewed in air ?

Answer:
(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 fuel produces a flame with a very high temperature 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 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.
Oxygen is the "gas of life" used for everything from basic breathing to high-tech space travel. Plants play the most critical role on Earth by constantly recycling carbon dioxide back into the oxygen we need.
Teacher's Tip: Think of plants as the Earth's "charging stations"—we use up the oxygen, and they recharge the air!
Exam Tip: When writing about photosynthesis, mention that it happens in "the presence of sunlight."

Question 9: (a) What is rust ?
(b) State at least two ways of prevent rusting.

Answer:
(a) Rust: Rust is hydrated ferric oxide (Fe₂O₃  x H₂O), which forms a brownish red coating over iron. (x can be any number.)
(b) Two ways of prevention of rusting :
1. Painting with red lead.
2. Oil paint is applied on doors and windows.
3. Enamel coating. Enamel is a mixture of iron, and steel with silicates.
4. Coal tar it is used for coating the lower parts of ships and bridges.
Rust is a chemical result of iron, air, and water coming together to destroy the metal's surface. Coating the metal with paint or oil blocks these elements from touching the iron, stopping the reaction.
Teacher's Tip: Remember that "hydrated" just means "contains water"—without water, iron cannot rust!
Exam Tip: For prevention, "Galvanising" is also a very popular answer to include.

Question 10: State two differences between : Rusting and burning.

Answer:
Difference between rusting and burning:
Rusting:
1. Rusting is the process in which iron slowly reacts with oxygen in the air and produces a flaky substance called rust.
2. Air and moisture are necessary for rusting.
Burning:
1. Burning is fast oxidation process in which large amount of energy is produced.
2. Only air is necessary for burning.
While both involve oxygen, rusting takes a long time and needs water, whereas burning happens instantly and creates visible heat. You won't see a flame during rusting, but you definitely will during burning.
Teacher's Tip: Think of rusting as "invisible fire" that takes years to consume the metal.
Exam Tip: Make a table for difference questions to clearly separate the points for the examiner.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks :
(a) Argon is the most abundant inert gas present in air.
(b) Oxides of sulphur and nitrogen combine with rain water to form sulphuric acid and nitric acid which cause acid rain.
(c) NO₂ and CO are the most common air pollutants.
(d) Joseph Priestly discovered the oxygen gas.
(e) Oxygen occupies about 21% of air by volume.

2. Match the following :

Column A
(a) Global warming
(b) Acid rain
(c) Rust
(d) Catalyst
(e) Photosynthesis

Column B
(i) Hydrated ferric oxide
(ii) Manganese dioxide
(iii) Carbon dioxide
(iv) Methane
(v) Nitrogen dioxide

Answer:
(a) Global warming - (iv) Methane
(b) Acid rain - (v) Nitrogen dioxide
(c) Rust - (i) Hydrated ferric oxide
(d) Catalyst - (ii) Manganese dioxide
(e) Photosynthesis - (iii) Carbon dioxide

MULTIPLE CHOICE QUESTIONS

1. A fuel when used releases least amount of pollutants in the air.
(a) sulphur dioxide
(b) chlorofluorocarbon
(c) smoke
(d) CNG
Answer: (d) CNG

2. The natural way of adding oxygen to air which involves green plants is called
(a) photosynthesis
(b) respiration
(c) burning
(d) dissolution
Answer: (a) photosynthesis

3. Which one of the following is most likely to be corroded?
(a) a stainless steel cup-board
(b) a galvanised iron bucket
(c) an iron hammer
(d) a tin plated iron box
Answer: (c) an iron hammer

4. The process by which oxidation of food in our body takes place is
(a) photosynthesis
(b) respiration
(c) decomposition
(d) combustion
Answer: (b) respiration

ADDITIONAL QUESTIONS
A. Air : A Mixture of Gases

Question 1: What was the ancient belief regarding the nature of matter ?

Answer:
The ancient belief about the nature of matter was that it is compound of four elements:
1. fire
2. air
3. wood
4. earth
Long ago, people didn't know about atoms or the periodic table, so they used simple parts of nature to explain everything. We now know that air is a mixture of many elements, not just one simple thing.
Teacher's Tip: In some cultures, there were 5 elements (including space/ether), but the 4 main ones are most famous in history.
Exam Tip: If this is asked, just list the four elements clearly as shown in the textbook.

Question 2: Name the main components of air.

Answer:
The main components of air are:
1. nitrogen
2. oxygen
3. carbon dioxide
4. water vapour
5. inert or noble gases
Air is made up of these parts in different amounts, with nitrogen and oxygen making up most of it. There are also tiny amounts of other things like dust and smoke that can vary.
Teacher's Tip: Remember that 99% of air is just Nitrogen and Oxygen—the rest are just "minor players."
Exam Tip: List them in order of their percentage to show you really know your stuff!

Question 3: Mention the contribution of the following scientists :
1. Antoine Lavoisier
2. Robert Boyle
3. Joseph Priestley
4. Sir W. Ramsay

Answer:
1. Antoine Lavoisier: Conducted experiments to know the nature of combustion.
2. Robert Boyle: Discovered a rule about how gases behave
3. Joseph Priestley: Prepared oxygen by heating red mercuric oxide with infra red rays from Sun and proved that air was not an element.
4. Sir W. Ramsay: Sir William Ramsay, alongwith Lord Raleigh discovered argon; a noble gas, which is also called as rare gas or inert gas. He also prepared neon, krypton and xenon by distillation of liquid air. In 1910, he discovered radon.
These scientists changed how we view the invisible air around us through careful observation. Their work moved science from ancient guesses to modern chemistry.
Teacher's Tip: Think of Priestley as the one who "poked" at the air to see what was inside!
Exam Tip: Link the scientist's name to a specific discovery, like "Priestley = Oxygen Preparation."

Question 4: State three uses of water vapour in the air.

Answer:
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.
Water vapour is the gas form of water and is vital for keeping the planet hydrated. It acts as a natural regulator that stops life from drying out in the heat of the sun.
Teacher's Tip: Without water vapour, there would be no "Water Cycle" and no rain for our crops!
Exam Tip: Mentioning "climatic conditions" is a high-scoring keyword for this answer.

Question 5: State one method by which :
1. Oxygen is added to air
2. Carbon dioxide is added to air
3. Oxygen is removed from the air
4. Carbon dioxide is removed from the air

Answer:
1. Photosynthesis by green plants
2. Combustion of various compounds
3. Respiration by living organisms
4. Plants- during photosynthesis.
Our atmosphere is constantly being recycled through the actions of living things and industrial processes. This balance ensures that gases don't run out or become too concentrated.
Teacher's Tip: Notice how plants do TWO of these jobs at once; they are the most important gas recyclers!
Exam Tip: For removal of oxygen, "burning" or "combustion" is also a perfectly correct answer.

Question 6: Describe a simple experiment to show that 1/5th of air is oxygen.

Answer:
Aim: To show that 1/5th of the air is oxygen.
Apparatus required: Glass trough, bell jar, stopper, water, phosphorus, evaporating dish
PROCEDURE:
1. a glass trough is filled with water
2. A dry evaporating dish containing white phosphorus is made to float on the water.
3. This apparatus is covered with a well stoppered bell jar with markings on its side.
4. A glowing iron wire is introduced into the bell jar by opening the stopper
5. The phosphorus is carefully ignited.
6. Re-stopper the apparatus
7. The phosphorus burns with a brilliant flame forming dense white fumes of phosphorus pentoxide.
8. Soon the burning ceases and the water level starts rising.
9. The water occupies approximately 1/5th of the original volume, as the fumes dissolve in water.
10. Some phosphorus is left unburnt in the dish. The air left behind in the bell jar does not allow the phosphorus to burn.
INFERENCE: The experiment shows that the volume of air occupied by oxygen is 1/5th.
When the phosphorus burns, it uses up all the oxygen inside the jar to create its chemical reaction. The space left behind by the missing oxygen is filled by rising water, which stops exactly at the 1/5th mark.
Teacher's Tip: This is a classic experiment; remember that the water rises because a "vacuum" is almost created when the oxygen gas turns into a solid/liquid fume.
Exam Tip: Always draw the diagram showing the "Start" and "End" water levels for full marks.

Question 7: Give reasons to support the fact that air is a mixture.

Answer:
Air is a mixture can be supported by the following evidences.
1. Variable composition at different places
2. Lacks distinct properties of its own
3. Has no definite boiling point
4. Components of air cannot be easily separated by physical means.
5. Formation of air does not involve any energy change.
6. Cannot be represented by a chemical formula.
If air were a compound, it would always have exactly the same amount of oxygen everywhere, but it changes depending on if you are in a forest or a city. Also, there is no single chemical symbol (like H₂O) that can describe the whole atmosphere.
Teacher's Tip: Remember that a mixture's ingredients keep their own personalities—oxygen still supports fire even when mixed with nitrogen.
Exam Tip: Mentioning the "lack of a chemical formula" is a very strong concluding point.

Question 8: Name the products formed when a candle burns in air.

Answer:
Carbondioxide and water vapour are formed due to the burning of candle.
Candle wax is made of hydrocarbons, and when it reacts with the oxygen in the air, it breaks down into these two common gases. You can often see the water vapour if you hold a cold glass over the flame.
Teacher's Tip: Most things that burn with a flame produce these same two products: CO₂ and H₂O.
Exam Tip: If asked for a "test" for these products, remember lime water turns milky for CO₂.

OBJECTIVE TYPE QUESTIONS

Question A. Fill in the blank spaces by choosing the correct words from the given list.
List: oxygen, carbonic, sulphur dioxide gas, precipitation, helium
1. Sulphur dioxide gas can cause acid rain.
2. Every nine parts of water by weight contains eight parts by weight of oxygen.
3. Dust particles help in the precipitation of water vapour in air.
4. Helium gas is used for filling weather observation balloons.
5. Solution of carbon dioxide in water is called carbonic acid.

Question B. Statements given below are incorrect. Write the correct statements :

Question 1. Carbon dioxide gas allows the earth to radiate out heat rapidly.
Answer:
Carbon dioxide gas does not allow the earth to radiate out heat rapidly.
Instead of letting heat escape, carbon dioxide acts like a trap that keeps the planet warm. This is why too much of it can lead to global warming.
Teacher's Tip: Think of Carbon dioxide as a "one-way door" for heat; it lets it in from the sun but won't let it out easily.
Exam Tip: Be sure to use the word "trap" or "absorb" when describing how CO₂ affects heat.

Question 2. Helium gas at low pressure is used for filling electric bulbs.
Answer:
Argon gas at low pressure is used for filling electric bulbs.
Argon is used because it won't react with the hot wire (filament) inside the bulb, which prevents it from burning out. Helium is much lighter and is better suited for things that float, like balloons.
Teacher's Tip: Lightbulbs need a "heavy" inert gas like Argon to stay stable inside.
Exam Tip: Don't get confused between Argon and Neon; Neon is for "glow signs," and Argon is for "bulbs."

Question 3. Symbiotic bacteria are present in nodules of cereal plants such as wheat.
Answer:
Symbiotic bacteria are present in nodules of pod bearing plants such as peas, grams, etc.
These special bacteria only live on specific "legume" plants that have root nodules. Wheat and other cereals have different root systems that don't host these nitrogen-fixing bacteria.
Teacher's Tip: Remember: Legumes (Peas/Beans) are the friends of nitrogen bacteria, not cereal grains!
Exam Tip: "Leguminous plants" is the scientific term you should use in your answers.

Question 4. During thunder and lightning, nitrogen and oxygen combine to form nitrogen dioxide gas.
Answer:
During thunder and lightning, nitrogen and oxygen combine to form nitric oxide gas.
The massive heat from a lightning bolt is enough to force these two gases to react. Nitric oxide (NO) is the first stage, which then turns into nitrogen dioxide (NO₂) later on.
Teacher's Tip: Think of nitric oxide as the "baby" step and nitrogen dioxide as the "adult" step of the reaction.
Exam Tip: The question asks for what they form *initially* during the strike, which is NO.

Question 5. The percentage of oxygen in air by volume, varies from 21% to 22%.
Answer:
The percentage of oxygen in air by volume, varies from 20.8% to 20.9%.
The amount of oxygen is very stable in our atmosphere because of the constant balance between breathing and photosynthesis. It rarely goes above 21% in normal outdoor air.
Teacher's Tip: Even though we use " 21% " as a rounded number, the actual measurement is just slightly under that.
Exam Tip: Use 21% for general calculations, but use 20.9% if you want to be precise for "True/False" questions.

Question C. Match the statements in Column A, with those in Column B.

Column A
1. Largest constituent of air, which dilutes the activity of oxygen.
2. A gas used for filling weather observation balloons.
3. An acid formed by the dissolution of sulphur dioxide gas in water.
4. An acid formed by the dissolution of carbon dioxide in water.
5. A gas vital for respiration.
Column B
(a) Oxygen
(b) Carbonic acid
(c) Sulphuric acid
(d) Helium
(e) Nitrogen
Answer:
1. Largest constituent of air, which dilutes the activity of oxygen. — (e) Nitrogen
2. A gas used for filling weather observation balloons. — (d) Helium
3. An acid formed by the dissolution of sulphur dioxide gas in water. — (c) Sulphuric acid
4. An acid formed by the dissolution of carbon dioxide in water. — (b) Carbonic acid
5. A gas vital for respiration. — (a) Oxygen

Question D. Write ‘True’ or ‘False’ in front of following statements.

Question 1. Atmospheric nitrogen is directly absorbed by the plants to form plant proteins.
Answer: False. Atmospheric nitrogen cannot be directly absorbed by the plants to form plant proteins.
Plants are surrounded by nitrogen, but they can't "breathe" it in like they do with CO₂. They must wait for it to be turned into chemicals in the soil by bacteria.
Teacher's Tip: Imagine nitrogen as food in a locked can; plants have the food, but they need the "bacteria opener" to eat it!
Exam Tip: If the statement says "directly," it is almost always false for nitrogen.

Question 2. Water containing dissolved carbon dioxide can dissolve marble.
Answer: True.
When CO₂ dissolves in water, it creates a weak acid that can eat away at rocks like marble. Over many years, this process is what creates limestone caves and wears down statues.
Teacher's Tip: This is a very slow process, which is why marble statues last for hundreds of years before they start to disappear.
Exam Tip: This is related to "Acid Rain" topics, so keep that connection in mind.

Question 3. Nitrogen is a gaseous non-metal essential for respiration.
Answer: False. Oxygen is a gaseous non-metal essential for respiration.
We breathe in nitrogen, but our bodies just breathe it right back out without using it. Oxygen is the only gas our lungs actually "pull" into our bloodstream to keep us alive.
Teacher's Tip: Nitrogen is the "filler" in our lungs, but Oxygen is the "fuel."
Exam Tip: Be careful! Nitrogen is essential for *life* (proteins), but NOT for *respiration* (breathing).

Question 4. The area where a lot of fuel burns has more percentage of carbon dioxide.
Answer: True.
In busy cities with many cars and factories, the air has a higher concentration of CO₂ because of all the burning. This is an example of why air is a mixture with a variable composition.
Teacher's Tip: This is why the air feels "fresher" in the forest—there is less burning and more plants taking COO₂ away.
Exam Tip: This supports the "Air is a Mixture" evidence of variable composition.

Question 5. The trapping of solar heat energy in the earth’s atmosphere is called greenhouse effect.
Answer: True.
The greenhouse effect is a natural process that keeps Earth at the right temperature for life. However, when we add too much CO₂, this "trap" gets too strong and causes the planet to overheat.
Teacher's Tip: A little greenhouse effect is good (we would freeze without it), but too much is bad (global warming).
Exam Tip: Don't confuse "Greenhouse Effect" with "Global Warming"—one is the process, the other is the result.

Question E. Tick the most appropriate answer.

1. A gas which dilutes the activity of oxygen in air is :
(a) carbon dioxide
(b) sulphur dioxide
(c) nitrogen
(d) noble gases
Answer: (c) nitrogen

2. Symbiotic bacteria is not present in the nodules of :
(a) gram plant
(b) rice plant
(c) sweet peas plant
(d) peas plant
Answer: (b) rice plant

3. The presence of water vapour in air is essential for :
(a) health and comfort of animals
(b) rate of evaporation from plants and animals
(c) growth of plants
(d) all the above
Answer: (d) all the above

4. Which is not a polluting gas for the air?
(a) Sulphur dioxide gas
(b) Nitrogen dioxide gas
(c) Carbon monoxide gas
(d) Neon gas
Answer: (d) Neon gas

5. The humans pollute the air by :
(a) burning a vast amount of coal
(b) burning waste materials
(c) using petrol and diesel vehicles
(d) all the above
Answer: (d) all the above

STUDY QUESTIONS

Question 1: Briefly state the importance of following constituents of air :
(a) Oxygen,
(b) Nitrogen,
(c) Carbon dioxide,
(d) Water vapour.

Answer:
(a) Oxygen in the air is essential for respiration and burning.
(b) Nitrogen dilutes the activity of oxygen in air. Its compounds are very useful for the growth of plants.
(c) Carbon-dioxide acts as a food for plants and in the regeneration of oxygen. It also helps in trapping heat radiations on earth.
(d) Water vapour controls climatic conditions and is very essential for the health and comfort of all living beings.
Every gas in our air serves a specific purpose that allows plants to grow, humans to breathe, and the weather to happen. Without just one of these, the Earth's balance would be destroyed.
Teacher's Tip: Think of the atmosphere as a "Life Support System" where every gas is a different vital machine.
Exam Tip: Be sure to mention "dilutes oxygen" for nitrogen—it's a very common exam point.

Question 2: (a) What do you understand by the term polluted air?
(b) State and explain two ways by which air gets polluted in nature.

Answer:
(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 forest fire, a large amount of carbon particles (smoke) and carbon monoxide pollute the air.
- When volcanoes erupt, a huge amount of ash and sulphur-dioxide is added in the air as pollutants.
Pollution isn't always caused by humans; nature can create massive clouds of smoke and ash through heat and geological energy. However, human pollution is much more constant and dangerous over the long term.
Teacher's Tip: Forest fires and volcanoes are "Natural Disasters" that act as "Natural Polluters."
Exam Tip: When explaining "polluted air," focus on the words "harmful" and "concentration."

Question 3: (a) Name and briefly explain four ways by which pollution of air is caused by humans.
(b) Name and briefly explain four ways by which pollution of air caused by humans is minimised.

Answer:
(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 minimise the production of smoke and carbon-monoxide.
4. We must grow more trees as they absorb harmful gases.
By changing our technology and protecting nature, we can undo much of the damage we have done to the air. Growing trees is particularly important because they act as the Earth's natural "filter" system.
Teacher's Tip: Notice how every cause has a matching cure—cars can be made better, and factory smoke can be cleaned!
Exam Tip: If you can't remember "electric precipitators," you can use the simpler term "smoke filters."

Question 4: (a) What is greenhouse effect?
(b) How is greenhouse effect produced?

Answer:
(a) The trapping of solar heat energy in earth’s atmosphere is called greenhouse effect.
(b) The solar heat radiations consist of very short wavelength infrared radiations. These radiations can easily pass through the atmosphere. These radiations are absorbed by the Earth’s surface and temperature of the Earth rises during the daytime. At night, the Earth radiates out heat radiations of very long wavelength which cannot easily penetrate through the atmosphere because carbon-dioxide and clouds act as excellent reflectors. Thus, heat radiations are trapped in the atmosphere which keep the Earth warm during night. This trapping of outgoing solar heat energy in Earth’s atmosphere is greenhouse effect.
The Earth uses gases like carbon dioxide like the glass walls of a greenhouse to hold in warmth from the sun. Without this effect, the Earth would become dangerously cold as soon as the sun went down.
Teacher's Tip: Sunlight comes in "short and fast," but tries to leave "long and slow"—the slow ones get stuck!
Exam Tip: Use the term "infrared radiations" specifically to score higher marks on this question.

Question 5: Discuss harmful effects of greenhouse effect.

Answer:
Large amount of carbon-dioxide in the polluted air results in increase of greenhouse effect which has raised the average temperature of the Earth by 2°C. This results in global warming which, in turn, will melt ice on the polar caps, thereby raising the level of sea water by a few metres. This can result in submerging of coastal areas and islands causing excessive damage.
Even a small rise in the world's temperature can melt massive glaciers and change where animals can live. This rise in sea level threatens millions of people who live near the ocean's edge.
Teacher's Tip: Think of the Earth like a person; a "fever" of just 2 degrees is enough to make the whole world sick.
Exam Tip: "Melting of polar ice caps" and "Rising sea levels" are the two key phrases to remember for this answer.

Question 6: Discuss two useful applications of greenhouse effect.

Answer:
Two useful applications of greenhouse effect are :
1. Growing vegetables and flowers in snow-bound regions: Sheds using glass walls are built which allow smaller wavelength of solar heat radiations to pass into the shed but does not allow the longer wavelength of heat radiations to pass out during night. This keeps the shed reasonably warm for flowers and vegetables to grow.
2. Use of glass in keeping homes warm: In cold countries, the glass windows and doors allow the smaller wavelength of heat radiations of solar energy to pass into the rooms but does not allow longer wavelength of heat radiations to pass out. Thus, greenhouse effect takes place and rooms keep reasonably warm.
Humans use the scientific principles of heat-trapping to survive and grow food in very cold climates. By using glass, we create small "mini-atmospheres" that stay cozy even in the snow.
Teacher's Tip: This is how we get tomatoes in the winter! We use science to trick the plants into thinking it's summer.
Exam Tip: Mention the "glass windows" as a practical example for people living in cold countries.

Question 7: What is the significance of water vapour in air?

Answer:
Significance of water vapour in atmosphere is as follows:
1. Presence of water vapour in air determines the climatic conditions. Excessive water vapour causes rain. They also produce snow, fog, mist, hails and other phenomenon, depending upon the temperature.
2. The presence of water vapour 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.
Water vapour is the fuel for weather; without it, we would have no clouds or precipitation. It also keeps the air from being too dry, which protects our skin and the leaves of plants from shriveling.
Teacher's Tip: High "humidity" just means there is a lot of water vapour in the air today!
Exam Tip: Link water vapour to "climatic conditions" to ensure a comprehensive answer.

Question 8: Name three noble gases present in the air. State their one industrial use.

Answer:
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 on big shops, hotels etc. at night.
Noble gases are valuable because they don't catch fire or explode, making them very safe for human technology. Their unique properties allow us to light up cities and explore the upper atmosphere safely.
Teacher's Tip: "Noble" means they are like kings; they don't react with the "common" elements easily!
Exam Tip: Match the specific gas to its most famous use (Neon = Neon signs) for easy points.

Question 9: What problems are caused by the presence of following in excessive amount in the air?
(a) dust particles
(b) smoke particles
(c) sulphur dioxide gas
(d) nitrogen dioxide gas
(e) carbon monoxide gas
(f) hydrogen sulphide

Answer:
(a) Excess of dust particles can cause serious respiratory problems.
(b) Excess of smoke particles in air can cause serious respiratory problems.
(c) Excess of sulphur dioxide gas can lead to acid rain and cause respiratory problems.
(d) Excess of nitrogen dioxide gas can lead to acid rain damaging plants and properties. They also cause respiratory problems. They lead to thinning of ozone layer too.
(e) Excess of carbon monoxide leads to breathing problem and can be fatal.
(f) Excess of hydrogen sulphide can lead to serious respiratory problems as it is very toxic. It can also affect the nervous system in humans.
These pollutants primarily attack our ability to breathe and the health of the planet's ecosystem. Some, like carbon monoxide, are particularly dangerous because you cannot see or smell them.
Teacher's Tip: Notice how "respiratory problems" (breathing issues) is the answer for almost all of these!
Exam Tip: For Carbon Monoxide, use the word "fatal" to show how dangerous it is compared to the others.

B. OXYGEN

Question 1: Fill in the blanks:
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 1/5 th of air by volume.
6. Oxygen is neutral to litmus testing.

Question 2: (1) What happens when (a) mercuric oxide and (b) potassium nitrate are heated ?
(2) Why is potassium chlorate not used for laboratory preparation of oxygen ?

Answer:
(1) Action of heat on mercuric oxide: Mercuric oxide is a red powder. When it is heated in a test tube, it decomposes to give mercury and oxygen.
2HgO → {heating} 2Hg + O₂ 
Action of heat on potassium nitrate: Potassium nitrate is a white crystalline solid. When it is heated, it melts and then decomposes into molten potassium nitrite and oxygen.
2KNO₃ → {heating} 2KNO₂ + O₂ (g)
(2) Potassium chlorate is a white solid. When it is heated strongly, first it melts and then it begins to boil, giving off oxygen. Potassium chlorate needs strong heating. That is why it is not used for laboratory preparation of oxygen.
Heating chemicals to get oxygen is a good scientific demonstration, but it's not the easiest way. In a school lab, we prefer methods that don't need dangerous high temperatures.
Teacher's Tip: Mercuric oxide starts as "Red" and turns into "Silvery" mercury—it's a very cool color change!
Exam Tip: Be sure to distinguish between Potassium "Nitrate" (KNO₃) and Potassium "Nitrite" (KNO₂ ().

Question 3: What happens when the following substances burn in oxygen ? Also give the balanced chemical equations for each of the reactions.
1. Sulphur
2. Carbon
3. Phosphorus
4. Sodium
5. Magnesium
6. Iron

Answer:
1. Sulphur: Sulphur burns with a bright bluish flame, giving the pungent smell of sulphur dioxide.
S + O₂ → SO₂ + heat

2. Carbon: Carbon burns with bright sparks, forming carbon dioxide
C + O₂ → {heat} CO₂ + heat

3. Phosphorus bums with a dazzing flame, producing dense white fumes of phosphorus pentoxide.
4P + 5O₂ → 2P₂O₅ + heat

4. Sodium bums brightly, with a brilliant yellow flame.
4NA+ O₂ → 2Na₂O + heat

5. Magnesium bums with a bright dazzling light, forming the white powder of
magnesium oxide.
2Mg + O₂ → 2MgO + heat

6. Red hot iron bums with a bright sark, forming an oxide of iron.
3Fe + 2O₂ → Fe₃O₄ + heat
Burning in pure oxygen is much more intense and violent than burning in regular air. Each element reacts to create a specific oxide and releases a different colored light or smell.
Teacher's Tip: Remember the color code: Sulphur = Blue flame, Sodium = Yellow flame, Magnesium = White light!
Exam Tip: If an equation asks for "Burning," always add "+ heat" at the end of the products.

Question 4: Give balanced equations to obtain oxygen from :
1. Red lead
2. Silver oxide
3. Potassium chlorate
4. Hydrogen peroxide
5. Mercuric oxide
6. Lead dioxide

Answer:
(i) Red lead: 2Pb₃O₄ → 6PbO + O₂ ↑
(ii) Silver oxide: 2Ag₂O → 4Ag + O₂ ↑
(iii) Potassium chlorate: 2KClO₃ → 2KCl + 3O₂ ↑
(iv) Hydrogen peroxide: H₂O₂ → MnO₂ 2H₂O + O₂ ↑
(v) Mercuric oxide: 2HgO → 2Hg + O₂ ↑
(vi) Lead dioxide: 2PbO₂ → 2PbO + O₂ ↑
Many metallic oxides are "unstable" when heated, meaning they will let go of their oxygen atoms. This allows scientists to collect pure oxygen for experiments.
Teacher's Tip: The little up-arrow (↑) is the universal symbol for a gas being released!
Exam Tip: Practice these equations often; they are the "math" part of chemistry and very important.

Question 5: State the use of manganese dioxide in the laboratory preparation of oxygen from potassium chlorate.

Answer:
In the laboratory preparation of oxygen from potassium chlorate, manganese dioxide acts as a catalyst i.e. it increase the rate of production of oxygen without getting itself consumed.
Without manganese dioxide, you would have to heat the mixture much longer and much hotter. It saves time and energy while remaining perfectly unchanged at the end.
Teacher's Tip: Think of it like a "chemical helper" that gets the job done faster.
Exam Tip: Always define "catalyst" as something that remains "unchanged in mass and composition."

Question 6: State the precautions while preparing oxygen in the laboratory.

Answer:
The various precautions are :
1. Do not collect the first-few bubbles of the gas produced as it is the air in the delivery tube which is driven out.
2. Method of collection of gas should be appropriate.
3. Keep all inflammable and burning apparatus away as oxygen is a supporter of combustion.
Working with pure oxygen is dangerous because it makes things catch fire much faster than normal. Following safety rules ensures that your experiment is successful and nobody gets hurt.
Teacher's Tip: Remember: Oxygen doesn't burn, it HELPS things burn. It's the ultimate "cheerleader" for fire!
Exam Tip: The "first few bubbles" precaution is a classic question; make sure you explain that those bubbles are just regular air.

Question 7: How will you obtain dry oxygen in the laboratory ?

Answer:
Dry oxygen can be obtained if the gas so collected is passed 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.
Normally, oxygen prepared over water has some moisture in it. Using chemicals that "drink" water (drying agents) leaves us with perfectly pure, dry oxygen gas.
Teacher's Tip: Concentrated sulphuric acid is like a chemical sponge that sucks up all the water vapour.
Exam Tip: Name "Phosphorus pentoxide" or "Calcium chloride" as your primary drying agents.

Question 8: How will you identify oxygen in the laboratory ?

Answer:
Oxygen is a colourless and odourless gas which rekindles a glowing splinter.
This is the most famous test for oxygen; if a piece of wood is glowing but has no flame, putting it into oxygen will make the flame burst back into life. No other common gas does this.
Teacher's Tip: "Rekindles" is the scientific word for "lighting back up again."
Exam Tip: Mentioning that it is "colourless and odourless" is important for a complete physical description.

Question 9: Classify the following oxides. Also give a balanced equation to prepare each.
1. Water
2. Carbon dioxide
3. Calcium oxide
4. Aluminium oxide
5. Copper oxide
6. Carbon monoxide
7. Sulphur dioxide
8. Phosphorus pentoxide
9. Nitric oxide
10. Magnesium oxide

Answer:
(i) Water → neutral oxide: 2H₂ + O₂ → 2H₂O
(ii) carbon dioxide → acidic oxide: C + O₂ → CO₂
(iii) calcium oxide → Basic oxide: 2Ca + O₂ → 2CaO
(iv) aluminium oxide → amphoteric oxide: 4Al + 3O₂ → 2Al₂O₃
(v) copper oxide → Basic oxide: 2Cu + O₂ → 2CuO
(vi) carbon monoxide → neutral oxide: 2C + O₂ → 2CO (insufficient oxygen)
(vii) sulphur dioxide → acidic oxide: S + O₂ → SO₂
(viii) Phosphorus pentaoxide → acidic oxide: 4P + 5O₂ → 2P₂O₅
(ix) Nitric oxide → neutral oxide: N2 + O₂ → 3000°C 2NO
(x) Magnesium oxide → Basic oxide: 2Mg + O₂ → 2MgO
Oxides come in different "flavors" depending on whether the element joined to oxygen is a metal or a non-metal. Some are acidic, some are basic, and some—like water—are neutral and don't change litmus paper at all.
Teacher's Tip: "Amphoteric" (like Aluminium oxide) means it can act as BOTH an acid and a base—it's the "multitool" of oxides!
Exam Tip: Be sure to group your answers by type (Acidic, Basic, Neutral) to show clear classification.

Question 10: State two characteristics of each :
1. Acidic oxides
2. Basic oxides

Answer:
Acidic oxides:
1. Formed when non metals are burnt in oxygen
2. When dissolved in water form acids, eg. carbon dioxide [CO₂]
Basic oxide:
1. Formed when metals are burnt in air
2. turn red litmus to blue.
eg. potassium oxide [K₂O]
The main way to tell these apart is by looking at the type of element they are made of and how they react with water. Acidic oxides are the "litmus turners" for non-metals, while basic oxides do the same for metals.
Teacher's Tip: Just remember: "M" is for Metal and "B" is for Basic. Metal = Basic oxide.
Exam Tip: Always provide an example like CO₂ or MgO to make your answer stronger.

Question 11: Complete and balance the following equation :

(i) P₂O₅ + 3H₂O → 2H₃PO₄
(ii) S + O₂ → SO₂
(iii) N₂ + O₂ → 3000°C 2NO
(iv) 4Al + 3O₂ → 2Al₂O₃
(v) K2O + H₂O → 2KOH
(vi) CaO + H₂O → Ca(OH)₂

Answer:
(i) P₂O₅ + 33H₂O → 2H₃PO₄ (Phosphoric acid)
(ii) S + O₂ → SO₂ (Sulphur dioxide)
(iii) N2 + O₂ → 2NO (Nitric oxide)
(iv) 4Al + 3O₂ → 2Al₂O₃ (Aluminium oxide)
(v) K₂O + H₂O → 2KOH (Potassium hydroxide)
(vi) CaO + H₂O → Ca(OH)₂ (Calcium hydroxide)
These equations show how oxides of non-metals become acids and oxides of metals become bases when they touch water. Balancing ensures the Law of Conservation of Mass is followed correctly.
Teacher's Tip: Notice that the oxides on the left side "grow" into bigger molecules on the right side once water is added!
Exam Tip: For the nitrogen reaction, don't forget to mention the very high temperature (3000°C).

OBJECTIVE TYPE QUESTIONS

Question A. Fill in the blank spaces by choosing the correct words from the given list.
List: sulphurous, nitric acid, red lead oxide paint, oxygen, phosphoric

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 vapour to form nitric acid vapour.

Question B. Statements given below are incorrect. Write the correct statements :

Question 1. Oxygen is prepared in laboratory by treating hydrogen oxide with manganese dioxide.
Answer: Oxygen is prepared in laboratory by treating hydrogen peroxide with manganese dioxide.
"Hydrogen oxide" is actually just another name for water! To get oxygen, you need the "extra" oxygen atom found only in Hydrogen Peroxide H₂O₂.
Teacher's Tip: Peroxide has "per" which means more—it has MORE oxygen than plain water.
Exam Tip: Be very careful with the spelling of "Peroxide" in your answers.

Question 2. Calcium burns in oxygen with a golden yellow flame.
Answer: Sodium burns in oxygen with a golden yellow flame.
Different elements give off specific colors when they burn, which is why fireworks come in so many colors. Sodium is famous in science for its bright, sunny yellow flame.
Teacher's Tip: Sodium starts with "S" but it burns "Sunny yellow."
Exam Tip: Calcium actually burns with a "Brick Red" flame, so don't get them mixed up!

Question 3. The oxygen gas turns moist blue litmus red.
Answer: The oxygen gas does not turn moist blue litmus red.
Pure oxygen is neutral; it is not an acid or a base. It only helps other things become acidic or basic when they react with it.
Teacher's Tip: Litmus paper only changes when it meets something "angry" (Acid) or "bitter" (Base)—neutral oxygen is just friendly!
Exam Tip: If a gas is neutral, the correct observation is "No change in litmus paper."

Question 4. Moist oxygen causes rusting in carbon.
Answer: Moist oxygen causes rusting in iron.
Rusting is a very specific word that only applies to iron and its alloys. While carbon can react with oxygen (burning), we never call that process "rusting."
Teacher's Tip: Rust is a "Metal Problem." Non-metals like carbon don't rust; they just oxidize or burn.
Exam Tip: Remember: No Iron = No Rust. Always associate the two.

Question 5. Potassium nitrate on strong heating decomposes into potassium nitrite and nitrogen gas.
Answer: Potassium nitrate on strong heating decomposes into potassium nitrite and oxygen gas.
Even though "Nitrate" sounds like "Nitrogen," these chemicals are great sources of oxygen because they pack so many oxygen atoms into their crystals. When they get hot, the oxygen is what gets released.
Teacher's Tip: Nitrates are like little oxygen tanks that explode open when heated.
Exam Tip: Always identify Oxygen (O₂) as the gaseous product of nitrate decomposition.

Question C. Match the statements in Column A, with those in Column B.

Column A
1. A mixture of oxygen and carbon dioxide used for artificial respiration.
2. A compound which on strong heating yields oxygen gas.
3. The process of coating zinc on the surface of iron to prevent rusting.
4. A catalyst used in laboratory preparation of oxygen from hydrogen peroxide.
5. A metal which burns with a golden yellow flame in oxygen.

Column B
(a) Manganese dioxide
(b) Sodium
(c) Carbogen
(d) Potassium nitrate
(e) Galvanising
Answer:
1. A mixture of oxygen and carbon dioxide used for artificial respiration. - (c) Carbogen
2. A compound which on strong heating yields oxygen gas. - (d) Potassium nitrate
3. The process of coating zinc on the surface of iron to prevent rusting. - (e) Galvanising
4. A catalyst used in laboratory preparation of oxygen from hydrogen peroxide. - (a) Manganese dioxide
5. A metal which burns with a golden yellow flame in oxygen. - (b) Sodium
This exercise helps students identify specific scientific terms for mixtures, chemical reactions, and industrial processes. Connecting these concepts to their real-world applications is a key step in mastering chemistry.
Teacher's Tip: To remember "Carbogen," think of it as a blend of "Carbon" dioxide and "Oxygen"!
Exam Tip: Always double-check the spelling of "Manganese" as it is frequently misspelled in exams.

Question D. Write ‘True’ or ‘False’ in front of following statements.

Question 1. The catalyst used in laboratory preparation of oxygen gas from hydrogen peroxide gas is magnesium oxide.
Answer: False. The catalyst used in laboratory preparation of oxygen gas from hydrogen peroxide gas is manganese dioxide.
Magnesium oxide (MgO) is a white powder, but Manganese dioxide (MnO₂) is a black powder. They look different and do different jobs in the lab.
Teacher's Tip: Manganese and Magnesium sound similar, but in the oxygen experiment, we only use the "Black Powder" (Manganese dioxide).
Exam Tip: Always double-check your spelling; "Manganese" has an extra 'n' and 'e'!

Question 2. Under laboratory conditions, the oxygen gas is insoluble in water.
Answer: True.
While a tiny bit of oxygen dissolves (so fish can breathe!), most of it stays as a gas. This is why we can collect it by pushing water out of a jar.
Teacher's Tip: If it were truly soluble like sugar, it would just vanish into the water instead of making bubbles!
Exam Tip: The scientific term used in textbooks is often "sparingly soluble" or "practically insoluble."

Question 3. The rust formed on the surface of iron easily crumbles.
Answer: True.
Rust doesn't stick to the metal like a protective coat; it turns into a flaky powder that falls off. This exposes new iron underneath, which then starts to rust too.
Teacher's Tip: This "crumbling" is why rusting eventually destroys the entire piece of metal—it never stops!
Exam Tip: This crumbling nature is why rust is called "non-protective" in chemistry.

Question 4. Enamelling is the process of baking a mixture of silicates on the surface of aluminium at a high temperature.
Answer: False. Enamelling is the process of baking a mixture of silicates on the surface of iron at a high temperature.
Enamel is a tough, glass-like coating that protects iron objects like bathtubs or pots. It acts as a shield that prevents air and water from touching the iron surface.
Teacher's Tip: Think of enamel as "baking a glass coat" onto your metal tools.
Exam Tip: Enamelling is specifically used for iron; aluminium has its own natural protection.

Question 5. Liquid oxygen is used for burning fuel in spaceships.
Answer: True.
In space, there is no air to help fuel burn, so rockets must carry their own oxygen. To fit enough on the ship, they turn the gas into a liquid so it takes up much less space.
Teacher's Tip: Liquid oxygen is extremely cold and takes up hundreds of times less space than gas oxygen!
Exam Tip: Liquid oxygen is often called "LOX" in aerospace technology.

Question E. Tick the most appropriate answer.

1. The catalyst used in the preparation of oxygen from hydrogen peroxide is :
(a) Magnesium oxide
(b) Manganese oxide
(c) Manganese dioxide
(d) Manganese hydroxide
Answer: (c) Manganese dioxide

2. Sodium burns in oxygen with a flame which is :
(a) brick red in colour
(b) golden yellow in colour
(c) blue in colour
(d) crimson in colour
Answer: (b) golden yellow in colour

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

4. Magnesium burns with a dazzling white flame to form magnesium oxide, a powdery mass. The colour of magnesium oxide is :
(a) milk white
(b) grey
(c) silvery
(d) yellow
Answer: (a) milk white

5. When sulphur burns in oxygen, the colour of the flame is :
(a) dazzling white
(b) golden yellow
(c) parrot green
(d) brilliant blue
Answer: (d) brilliant blue

STUDY QUESTIONS

Question 1: Describe your observations when potassium nitrate crystals are heated strongly in a test tube and a glowing splint is held near the mouth of test tube. Write a fully balanced chemical equation for the decomposition of potassium nitrate crystals.

Answer:
The following observations are made when potassium nitrate crystals are heated strongly in a test tube and a glowing splint is held near the mouth of test tube :
1. The white crystals of potassium nitrate slowly melt to form a colourless liquid.
2. The glowing splint bursts into flame, showing that molten potassium nitrate gives off oxygen gas.
2KNO₃} \xrightarrow{heat} 2KNO₂ + O₂
Potassium nitrate — Potassium nitrite + Oxygen
The heat breaks the strong chemical bonds, releasing pure oxygen gas which fuels the glowing splint. The change from a solid crystal to a boiling liquid and a bright flame is a clear sign of a chemical reaction.
Teacher's Tip: This experiment is the classic proof that some solids hide oxygen inside them!
Exam Tip: Be sure to write the full balanced equation, including the names of the reactants and products.

Question 2:
(a) Starting from hydrogen peroxide how is oxygen gas prepared in laboratory?
(b) What is the function of manganese dioxide?
(c) Write fully balanced equation for the liberation of oxygen from hydrogen peroxide.
(d) Why is the above method preferred to any other method?

Answer:
(a) When 20% hydrogen peroxide is poured in funnel and allowed to drip into the flask containing 5g of manganese dioxide, oxygen gas is evolved which is apparent from the bubbles coming out of the water trough. Allow the first few bubbles to escape, as they contain air. Place a glass cylinder filled with water in inverted position over the beehive shelf. The oxygen will collect in the cylinder by the downward displacement of water.
(b) Manganese dioxide does not take part in the chemical reaction. However, it accelerates the rate of decomposition of hydrogen peroxide acting as a catalyst.
(c) H₂O₂ → {MnO₂ Catalyst} 2H₂O + O₂
Hydrogen peroxide → Water + Oxygen
(d) This method is preferred to any other method to prepare oxygen because :
- The oxygen gas is evolved at room temperature. No heating is required.
- The rate of evolution of oxygen is fairly fast.
- Hydrogen peroxide or manganese dioxide are not dangerous chemicals and hence, can be handled easily.
This setup is safe for students because it produces oxygen without needing a burner or dangerous heat sources. The catalyst makes the reaction fast enough to see bubbles forming immediately.
Teacher's Tip: Think of the "drip" method as a way to control how much oxygen you get—one drop equals a few bubbles!
Exam Tip: When asked for "function of manganese dioxide," always use the word "catalyst."

Question 3: State any four physical properties of oxygen gas.

Answer:
Physical properties of oxygen gas are :
1. Oxygen is a colourless, odourless 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 liquid state. Liquid oxygen is slightly bluish in colour.
These properties tell us what oxygen "looks" and "feels" like without it reacting with anything else. Most notably, its heaviness and low solubility determine how we have to store it in the lab.
Teacher's Tip: Remember "COT" (Colourless, Odourless, Tasteless) for almost all common air gases!
Exam Tip: Mentioning the "bluish color" of the liquid form is an excellent way to get bonus points.

Question 4: Describe what you will observe and write fully balanced equations when the following are introduced in a jar of oxygen.
(a) Red hot coal (carbon)
(b) Burning sulphur
(c) Burning sodium
(d) Burning magnesium
(e) Burning phosphorus

Answer:
(a) When red hot coal (carbon) is introduced into a jar of oxygen, it burns brightly producing sparkles and a crackling sound. It forms a colourless gas known as carbon dioxide.
C + O₂ → CO₂ (Carbon dioxide)
(b) When burning sulphur is taken into the jar of oxygen, it burns with a brilliant blue flame forming sulphur dioxide
S + O2 → SO₂ (Sulphur dioxide)
(c) When burning sodium is taken into the jar of oxygen, it burns with a brilliant golden yellow flame to form sodium oxide.
4Na + O₂ → 2Na₂O (Sodium oxide)
(d) When burning magnesium is taken into the cylinder of oxygen, it burns with a dazzling white flame to form magnesium oxide.
2Mg + O₂ → 2MgO (Magnesium oxide)
(e) When burning phosphorus is taken into a jar of oxygen, it burns with dazzling white flame producing dense white fumes of phosphorous pentoxide.
4P + 5O₂ → 2P₂O₅ (Phosphorous pentoxide)
These reactions represent rapid oxidation where the element combines with oxygen very quickly. The bright colors and sounds are the energy being released as the new chemical bonds are made.
Teacher's Tip: Imagine the jar of oxygen as an "energy booster"—everything burns much bigger and brighter inside it!
Exam Tip: Be sure to match the specific observation (like "crackling sound" for coal) to the correct element.

Question 5: How will you prove that (a) oxide formed by burning sodium is basic in nature, (b) oxide formed by sulphur is acidic in nature?

Answer:

(a) The sodium oxide dissolves in water to form sodium hydroxide, which turns red litmus paper blue. Hence, sodium oxide is basic in nature.
Na₂O + H₂O → 2NaOH (Sodium hydroxide)
(b) Sulphur dioxide gas turns moist blue litmus paper red, because it reacts with water to form sulphurous acid. Hence, it is acidic in nature.
SO₂ + H₂O → H₂SO₃ (Sulphurous acid)
Testing with litmus paper is the best way to determine if a chemical is an acid or a base. These reactions show that the oxygen bond changes the "nature" of the element once it meets water.
Teacher's Tip: Remember: S-S-A (Sulphur-Sulphur dioxide-Acidic). It helps keep the non-metals straight!
Exam Tip: Always include the chemical equation to show HOW the acid or base is actually created.

Question 6: Write fully balanced equations for the following :
(a) CO₂ + H₂O →
(b) Ca(OH)₂ + CO₂ →
(c) SO₂ + H₂O →
(d) P₂O₅ + H₂O →
(e) Na₂O + H₂O →
(f) MgO + H₂O →

Answer:
(a) CO₂ + H₂O → H₂CO₃ (Carbonic acid)
(b) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (Calcium carbonate)
(c) SO₂ + H₂O → H₂SO₃ (Sulphurous acid)
(d) P₂O₅ + 3H₂O → 2H₃PO₄ (Phosphoric acid)
(e) Na₂O + H₂O → 2NaOH (Sodium hydroxide)
(f) MgO + H₂O → Mg(OH)₂ (Magnesium hydroxide)
These equations represent the "hydration" of oxides to form either acids (from non-metals) or bases (from metals). Understanding these is key to explaining concepts like acid rain and chemical synthesis.
Teacher's Tip: Notice how the products for (e) and (f) both end in "OH"—that's the signature of a hydroxide base!
Exam Tip: Ensure your coefficients (the big numbers in front) are correct so that the atoms match on both sides.

Question 7: (a) State three uses of oxygen, other than artificial respiration.
(b) Give tests for oxygen gas.

Answer:
(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 burn in a specially designed torch. The oxy- hydrogen 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 colourless nitric oxide gas, it forms reddish brown fumes of nitrogen dioxide.
- It dissolves in alkaline pyrogallol solution and turns it brown.
Oxygen is a powerful industrial tool used for making strong acids and melting tough metals like steel. Because it is so reactive, we can use specific chemicals like nitric oxide to visually prove its presence.
Teacher's Tip: "Rekindles" is the easiest test to remember, but the "Nitric Oxide" test is much more colorful!
Exam Tip: For the use in welding, make sure to name the specific flame: "oxy-hydrogen flame."

Question 8: (a) What is (1) rust (2) rusting?
(b) State two most important conditions for rusting.
(c) State four ways of preventing rusting.

Answer:
(a) 1. The hydrated oxide of iron formed when iron comes in contact with moist air is called rust. This rust is brownish flaky residue and easily crumbles from the metal surface.
2. The slow conversion of iron into its hydrated ferric oxide, in the presence of moisture and air is called rusting.
(b) Two most important conditions for rusting are (1) presence of air and (2) moisture.
(c) Rusting Can be prevented by :
1. Galvanising - iron metal is coated with zinc.
2. Enamelling - iron surface is baked with mixture of silicates at high temperature.
3. Coating with red lead oxide paint or tar on iron surface.
4. Oils and grease coating on iron surface cuts off moist air and prevents rusting.
Rusting is nature's way of recycling iron, but it destroys our tools and buildings in the process. By creating a physical "wall" out of paint, oil, or other metals, we can stop the iron from reacting with the air.
Teacher's Tip: Rust needs TWO friends to start: Air AND Water. If you take away just one, it can't happen!
Exam Tip: When defining rust, always include the word "hydrated" to show that water is part of the chemical formula.