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

Very Short Answer Questions

Question 1. Name the gas which absorbs ultraviolet rays.
Answer:
Ozone (\(\ce{O3}\)).
In simple words: Ozone absorbs harmful UV rays.

Teacher's Note:
1. Ozone is present in the stratosphere.
2. Without it, UV rays would harm plants and animals.

Question 2. Name the principal products of combustion of glucose.
Answer:
Carbon dioxide and water, along with the release of energy.
\(\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy}\)
In simple words: Burning glucose gives carbon dioxide, water and energy.

Teacher's Note:
1. This is the same reaction as respiration in our cells.
2. Respiration is slow oxidation of glucose.

Question 3. Mention the significance of water vapour in atmosphere.
Answer:
Water vapour along with dust particles helps in the formation of clouds, fog, mist and rain.
It also acts as a greenhouse gas.
In simple words: Water vapour forms clouds, fog and rain.

Teacher's Note:
1. It controls the weather.
2. The amount of water vapour in air varies from 0 to 4%.

Question 4. In which way is greenhouse effect useful to us?
Answer:
Greenhouse gases trap heat and keep the earth's surface warm enough for life.
In simple words: The greenhouse effect keeps the earth warm so that life is possible.

Teacher's Note:
1. Without it the earth would be very cold.
2. Trouble starts only when greenhouse gases increase too much.

Question 5. Name the gases which are responsible for acid rain.
Answer:
Sulphur dioxide (\(\ce{SO2}\)) and nitrogen dioxide (\(\ce{NO2}\)).
In simple words: \(\ce{SO2}\) and \(\ce{NO2}\) dissolve in rain to form acids.

Teacher's Note:
1. They form sulphuric acid and nitric acid with water.
2. Their sources are vehicles, factories and power plants.

Question 6. What happens when potassium nitrate is subjected to heating?
Answer:
Crystals melt and decompose to give potassium nitrite and a colourless gas, oxygen.
\(\ce{2KNO3 -> 2KNO2 + O2 ^}\)
In simple words: Heated potassium nitrate gives potassium nitrite and oxygen.

Teacher's Note:
1. The gas relights a glowing splinter.
2. The method is slow and needs strong heating, so it is not used in the laboratory.

Question 7. Define a catalyst.
Answer:
A catalyst is a substance that changes the rate of a chemical reaction without itself being used up in the reaction.
In simple words: A catalyst speeds up (or slows down) a reaction and is left unchanged.

Teacher's Note:
1. Example: \(\ce{MnO2}\) in the breakdown of \(\ce{H2O2}\).
2. The catalyst can be recovered at the end.

Question 8. Give the balanced chemical equation for the laboratory preparation of oxygen gas.
Answer:
\(\ce{2H2O2 ->[MnO2] 2H2O + O2 ^}\)
In simple words: Hydrogen peroxide breaks into water and oxygen in the presence of \(\ce{MnO2}\).

Teacher's Note:
1. Count atoms on both sides: 4 H and 4 O on each side.
2. The catalyst is written over the arrow.

Question 9. What is the action of litmus on oxygen?
Answer:
Oxygen is neutral, so it has no effect on either blue or red litmus.
In simple words: Oxygen does not change litmus.

Teacher's Note:
1. Acidic gases turn blue litmus red.
2. Neutral gases leave both papers unchanged.

Question 10. What happens when a paper soaked in acidified potassium dichromate is exposed to \(\ce{SO2}\) gas?
Answer:
The orange paper turns green because chromium(III) sulphate forms.
\(\ce{3SO2 + K2Cr2O7 + H2SO4 -> K2SO4 + Cr2(SO4)3 + H2O}\)
In simple words: The paper turns from orange to green.

Teacher's Note:
1. This is the test for sulphur dioxide.
2. The green colour is due to \(\ce{Cr2(SO4)3}\).

Question 11. Mention the factors which influence rusting of iron.
Answer:
Rusting needs oxygen (air) and moisture (humid atmosphere). Iron does not rust in dry air.
In simple words: Iron rusts when both air and moisture are present.

Teacher's Note:
1. Keeping iron dry or covered stops rusting.
2. Moist air is the key factor.

Question 12. What is alloying?
Answer:
Alloying is the process of mixing small amounts of other metals or non-metals with a molten metal to get a homogeneous mixture with improved properties.
In simple words: Alloying means melting a metal with other substances to improve its properties.

Teacher's Note:
1. Stainless steel is iron alloyed with carbon and chromium.
2. An alloy is more resistant to corrosion.

Question 13. Name two metal carbonates from which carbon dioxide cannot be obtained by heating.
Answer:
Sodium carbonate and potassium carbonate.
In simple words: Sodium and potassium carbonates do not decompose on ordinary heating.

Teacher's Note:
1. Most other carbonates, such as \(\ce{CaCO3}\) and \(\ce{CuCO3}\), give \(\ce{CO2}\) on heating.
2. These two are the exceptions given in the notes.

Question 14. What is carbogen?
Answer:
Carbogen is a mixture of 95% oxygen and 5% carbon dioxide. It is given to patients who have trouble in breathing.
In simple words: Carbogen is 95% oxygen with 5% carbon dioxide, used to help breathing.

Teacher's Note:
1. The carbon dioxide stimulates breathing.
2. It is used for asthma and pneumonia patients.

Question 15. Aeroplanes usually fly in the zone of mesosphere.
Answer:
False. Aeroplanes usually fly in the stratosphere.
In simple words: Aeroplanes fly in the calm stratosphere, not the mesosphere.

Teacher's Note:
1. Check the statement against the facts in the notes.
2. If a statement is false, always correct it to see why.

Question 16. Meteorites burn up in thermosphere.
Answer:
False. Meteorites burn up in the mesosphere.
In simple words: Meteors burn in the mesosphere, where the air is thick enough.

Teacher's Note:
1. Check the statement against the facts in the notes.
2. If a statement is false, always correct it to see why.

Question 17. The percentage of noble gases present in air is more than that of \(\ce{CO2}\).
Answer:
True. Noble gases are about 0.94% (mostly argon), while \(\ce{CO2}\) is about 0.03%.
In simple words: Noble gases are present in a larger amount than carbon dioxide.

Teacher's Note:
1. Check the statement against the facts in the notes.
2. If a statement is false, always correct it to see why.

Question 18. Oxygen is neutral towards litmus.
Answer:
True. Oxygen is neither acidic nor basic, so litmus does not change colour.
In simple words: Oxygen does not change litmus.

Teacher's Note:
1. Check the statement against the facts in the notes.
2. If a statement is false, always correct it to see why.

Question 19. Gases like nitrogen dioxide, sulphur dioxide present in atmosphere contribute to acid rain.
Answer:
True. They form nitric acid and sulphuric acid with rain water.
In simple words: These two gases make rain acidic.

Teacher's Note:
1. Check the statement against the facts in the notes.
2. If a statement is false, always correct it to see why.