CBSE Class 7 Science Advanced Chemistry Air and Oxygen Short Answer Questions Set 02

Short Answer Questions

Question 1. What is the composition of water gas and how is it formed?
Answer:
Water gas is a mixture of carbon monoxide and hydrogen (\(\ce{CO + H2}\)).
It is formed by passing superheated steam over red-hot coke (or charcoal): \(\ce{C + H2O -> CO + H2}\).
In simple words: Steam passed over red-hot coke gives water gas, which is CO and \(\ce{H2}\).

Teacher's Note:
1. Water gas is the starting material of the Bosch process.
2. It is also used as a fuel.

Question 2. Mention the action of oxygen with the elements sulphur, phosphorus, sodium and magnesium.
Answer:
Sulphur: burns with a bright blue flame to give \(\ce{SO2}\). \(\ce{S + O2 -> SO2}\)
Phosphorus: burns with dazzling dense white fumes to give \(\ce{P2O5}\). \(\ce{4P + 5O2 -> 2P2O5}\)
Sodium: burns with a golden-yellow flame to give sodium oxide. \(\ce{4Na + O2 -> 2Na2O}\)
Magnesium: burns with a dazzling white flame to give magnesium oxide. \(\ce{2Mg + O2 -> 2MgO}\)
In simple words: Each element burns in oxygen with its own flame to give its oxide.

Teacher's Note:
1. Learn the flame colour along with each equation.
2. The oxides of the non-metals are acidic and those of the metals are basic.

Question 3. Mention the methods for preventing acid rain.
Answer:
1. Use less fossil fuel; replace petrol and diesel with CNG in vehicles.
2. Use non-conventional sources of energy such as solar and wind energy.
3. Dispose of industrial waste safely.
4. Use scrubbers in industries to remove \(\ce{SO2}\) and \(\ce{NO2}\) before they leave the chimney.
In simple words: Burn less fossil fuel and clean the industrial smoke to stop acid rain.

Teacher's Note:
1. CNG is a cleaner fuel than petrol and diesel.
2. Scrubbers use lime to absorb \(\ce{SO2}\).

Question 4. What are the human activities that contribute to air pollution?
Answer:
1. Burning of fossil fuels in homes, vehicles and industries.
2. Deforestation.
3. Emissions from factories and vehicles.
4. Agricultural activities.
5. Lack of civic sense, such as burning of waste.
(Volcanic eruptions, forest fires and pollen are natural causes.)
In simple words: Burning fuel, cutting forests, industry, farming and careless habits add to air pollution.

Teacher's Note:
1. Separate human causes from natural causes.
2. Every one of us can cut down on one of these activities.

Question 5. While a teacher was teaching her class about air and oxygen she emphasized the importance of nitrogen in the atmosphere. A student named Vivek asked the teacher, "What would happen if the percentages of nitrogen and oxygen were interchanged in the atmosphere?" The teacher praised him for the question and explained it. What was the teacher's explanation?
Answer:
If oxygen were about 78% and nitrogen about 21%, there would be far too much of the active gas.
Burning would be much faster, fires would spread and be hard to put out, metals would corrode and rust faster, and respiration would be too rapid, which would harm the human body.
Nitrogen's role is to dilute oxygen, and this would be lost.
In simple words: With more oxygen than nitrogen, everything would burn and rust much faster and living things would be harmed.

Teacher's Note:
1. Nitrogen acts like a brake on the activity of oxygen.
2. Link your answer to combustion, corrosion and respiration.

Question 6. Vish went to an electrical shop along with her father to purchase a bulb and some electrical equipment. After returning home, Vish looking at the bulb keenly asked her father, "Is it filled with air or vacuum?" Her father told her that it was filled with nitrogen or argon. She asked him why the bulb could not be filled with air which is readily available, as nitrogen is obviously a part of air? What was her father's correct explanation to her question?
Answer:
Air contains oxygen, which is an active gas. The hot metal filament would be oxidised by this oxygen and would burn out quickly.
Argon and nitrogen are chemically unreactive, so they do not react with the filament, and the bulb lasts long.
In simple words: Oxygen in air would burn the filament, so unreactive argon or nitrogen is filled instead.

Teacher's Note:
1. The key words are 'active component' and 'inert gas'.
2. Tungsten filaments become white hot, so they must be protected.

Question 7. Vidya went to her grandfather Rajaram's house to spend her summer vacation. Rajaram has coconut fields near his house. In the evening she went to the coconut field along with her grandfather. There the workers offered her coconut water. Her grandfather told the workers to provide a straw to facilitate drinking the coconut water comfortably. Can you explain the principle behind using straw for the above purpose?
Answer:
When we suck at the top end of a straw dipped in coconut water, some of the air in the straw is drawn out and a partial vacuum forms inside it.
The atmospheric pressure acting on the surface of the coconut water outside is greater than the pressure in the straw, so it pushes the water up the straw into the mouth.
In simple words: Sucking lowers the pressure in the straw, and the air pressure outside pushes the liquid up.

Teacher's Note:
1. We do not 'pull' the liquid; atmospheric pressure pushes it.
2. A straw would not work in a vacuum.

Question 8. Shakti along with his parents was going to the railway station by an auto to catch a train. Shakti's father was recollecting his childhood days and said that in those days, autos were run with petrol only. His mother added that diesel was used to run autos in later days. Shakthi said that nowadays autos are preferably run with CNG. Shakti questioned his father regarding the advantage of CNG over petrol and diesel. What was his father's explanation?
Answer:
CNG burns cleanly and does not release harmful gases, so it is a clean fuel and also reduces maintenance cost.
Petrol and diesel contain nitrogen and sulphur compounds, which on burning give \(\ce{NO2}\) and \(\ce{SO2}\), causing acid rain and air pollution. So CNG is a better substitute.
In simple words: CNG is a clean fuel that does not give acid-rain gases, unlike petrol and diesel.

Teacher's Note:
1. Pollution in cities has reduced where vehicles use CNG.
2. Link the answer with acid rain.

Question 9. Arjun's family stays in an area which is in the heart of the city for the sake of convenience. During the summer vacation, Arjun's family stays in their farm house which has all amenities. As the farm house is located in the city outskirts, they could escape from the hot sun in summer. Why is the temperature in the peripheral area relatively lower than that in the heart of the city?
Answer:
The centre of the city has a dense population and a large amount of traffic. This increases the amount of gases such as \(\ce{CO2}\), methane and nitrous oxides, which are greenhouse gases that trap heat.
The outskirts have less traffic and more trees, so the temperature there is lower.
In simple words: More vehicles and people in the city centre add greenhouse gases and heat, so it is hotter.

Teacher's Note:
1. Trees absorb carbon dioxide and give shade.
2. Heat in cities is also due to concrete and machines.

Question 10. Calculate the volume of nitrogen, oxygen and carbon dioxide in 750 ℓ of air.
Answer:
Percentage by volume: nitrogen 78.1%, oxygen 21%, carbon dioxide 0.03%.
Volume of nitrogen = \( \frac{78.1 \times 750}{100} \) = 585.75 ℓ
Volume of oxygen = \( \frac{21 \times 750}{100} \) = 157.5 ℓ
Volume of carbon dioxide = \( \frac{0.03 \times 750}{100} \) = 0.225 ℓ = 225 mℓ
In simple words: Multiply the volume of air by each gas's percentage and divide by 100.

Teacher's Note:
1. Use percentage × volume ÷ 100.
2. Remember 1 ℓ = 1000 mℓ for the last step.

Question 11. Troposphere is extended up to 6–20 km altitude from the earth's surface but the amount of oxygen is less at higher altitudes, i.e., on mountains or hill tops. Justify.
Answer:
Oxygen molecules are heavier than the average molecule of air (vapour density of oxygen is 16, that of air is about 14.4). Heavier molecules are pulled more strongly by the earth's gravity, so they stay closer to the ground.
In addition, the air becomes thinner and its pressure falls with height. So at hill tops the amount of oxygen in each breath is less.
In simple words: Oxygen is relatively heavy and the air thins out with height, so less oxygen is available on mountains.

Teacher's Note:
1. This is why mountaineers carry oxygen cylinders.
2. Thin air means fewer molecules per unit volume.

Question 12. Both galvanizing and tinning are adopted for preventing rusting of iron but only tinned iron containers are used to preserve food material. Justify.
Answer:
Zinc (used in galvanisation) is a reactive metal, and it reacts with acids present in food, so the food gets spoiled and cannot be eaten.
Tin is much less reactive and does not react with the acids in food. Hence only tinned iron sheets are used for making containers to store edible substances.
In simple words: Zinc reacts with food acids, but tin does not, so tin-coated cans are safe for food.

Teacher's Note:
1. Compare the reactivity of zinc and tin.
2. Galvanised iron is used for buckets and trunks, not food tins.

Question 13. Why are people advised not to sleep in a closed room where coal is burnt in chulhas?
Answer:
In a closed room the supply of oxygen is limited, so coal burns incompletely and produces carbon monoxide, a poisonous gas.
CO combines with haemoglobin about 200 times more strongly than oxygen does, forming carboxyhaemoglobin. This causes suffocation and can even be fatal.
In simple words: A coal fire in a closed room makes poisonous carbon monoxide, which can stop us from getting oxygen.

Teacher's Note:
1. Keep a window open whenever a fire is burning inside.
2. Incomplete combustion gives CO, complete combustion gives \(\ce{CO2}\).

Question 14. What is the role of water vapour in the formation of fog?
Answer:
Water vapour in the atmosphere affects the climate. On cold nights, especially in winter, the vapour condenses on dust particles because of the low temperature.
Depending on the temperature it forms snow, fog, mist or hail. When the condensed droplets remain suspended in the air on dust particles, it is called fog.
In simple words: On cold nights water vapour condenses on dust particles and hangs in the air as fog.

Teacher's Note:
1. Dust particles are essential for condensation.
2. Fog is a cloud near the ground.

Question 15. An inverted glass jar is placed over a burning candle. After a while, the candle is removed and the inverted glass jar is placed in a trough of water. What do you observe and justify your observations.
Answer:
Observation: The water level rises slightly inside the jar.
Justification: The burning candle uses oxygen and produces carbon dioxide and water vapour. When the jar is inverted in water, the carbon dioxide dissolves in water, and the water level rises a little. This shows that \(\ce{CO2}\) is soluble in water.
In simple words: The water rises a little in the jar because the carbon dioxide formed dissolves in water.

Teacher's Note:
1. The rise is small because only part of the gas is \(\ce{CO2}\).
2. Candle burning gives \(\ce{CO2}\) and water.

Question 16. Acid formed as a result of lightening reaches the earth's surface through precipitation, thus contributing to the acid rain as well as to the growth of plants. Justify.
Answer:
During lightning, nitrogen and oxygen combine to give nitric oxide, then nitrogen dioxide, which dissolves in cloud droplets to form nitric acid. This very dilute acid falls with rain and adds to the acidity of rain.
In the soil, it reacts with metal carbonates to give nitrates: \(\ce{CaCO3 + 2HNO3 -> Ca(NO3)2 + CO2 + H2O}\). These nitrates are excellent nutrients, which plants absorb for their growth.
In simple words: Nitric acid from lightning makes rain slightly acidic but also forms nitrates that feed the plants.

Teacher's Note:
1. Nitrates act as natural fertiliser.
2. Link lightning, rain and soil in the answer.