CBSE Class 7 Science Advanced Chemistry Air and Oxygen Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. Ultraviolet rays are absorbed by ______ of atmosphere.
Answer:
Ozonosphere (the ozone layer in the stratosphere).
In simple words: The ozone layer of the stratosphere takes in harmful UV rays.

Teacher's Note:
1. The ozonosphere lies about 25 km to 40 km above the ground.
2. It acts as a shield for living things.

Question 2. Auroras appear in the layer of atmosphere which is known as ______.
Answer:
Thermosphere (also called the ionosphere).
In simple words: Auroras are lights seen in the upper atmosphere, the thermosphere.

Teacher's Note:
1. Charged particles from the sun collide with gas molecules here.
2. The thermosphere is also known as the ionosphere because it contains ions.

Question 3. ________ depletes the ozone layer.
Answer:
Chlorofluorocarbons (CFCs).
In simple words: CFCs from refrigerators and air conditioners destroy ozone.

Teacher's Note:
1. CFCs rise to the stratosphere and react with ozone.
2. Using CFC substitutes helps to protect the ozone layer.

Question 4. The ______ alters the rate of reaction.
Answer:
Catalyst.
In simple words: A catalyst changes how fast a reaction goes without being used up.

Teacher's Note:
1. Manganese dioxide is a catalyst for the breakdown of hydrogen peroxide.
2. A catalyst takes no permanent part in the reaction.

Question 5. In the laboratory preparation of oxygen from hydrogen peroxide, _______ is used as a catalyst.
Answer:
Manganese dioxide (\(\ce{MnO2}\)).
In simple words: \(\ce{MnO2}\) speeds up the breakdown of \(\ce{H2O2}\) at room temperature.

Teacher's Note:
1. The reaction is \(\ce{2H2O2 ->[MnO2] 2H2O + O2 ^}\).
2. \(\ce{MnO2}\) is found unchanged at the end.

Question 6. _____ gas is evolved when metal carbonates are treated with dil acids.
Answer:
Carbon dioxide (\(\ce{CO2}\)).
In simple words: Carbonates give carbon dioxide gas with dilute acids.

Teacher's Note:
1. For example, \(\ce{CaCO3 + 2HCl -> CaCl2 + H2O + CO2 ^}\).
2. The gas turns lime water milky.

Question 7. The solid carbon dioxide is commonly called _______.
Answer:
Dry ice.
In simple words: Solid carbon dioxide is dry ice.

Teacher's Note:
1. It turns directly from solid into gas without becoming liquid.
2. It is used to keep food cold.

Question 8. _______, an inert gas is used to fill weather observation balloons.
Answer:
Helium.
In simple words: Helium is light and unreactive, so it fills weather balloons.

Teacher's Note:
1. Helium is an inert gas of the atmosphere.
2. It is much lighter than air, so the balloon rises.

Question 9. The chemical formula of rust is _________.
Answer:
\(\ce{Fe2O3.xH2O}\) (hydrated iron(III) oxide).
In simple words: Rust is iron oxide combined with a variable amount of water.

Teacher's Note:
1. The x shows that the number of water molecules is not fixed.
2. Rust forms only when iron meets both oxygen and moisture.

Question 10. The trapping of solar heat energy by certain gaseous molecules present in the earth's atmosphere is called _______
Answer:
Greenhouse effect.
In simple words: Gases in the air trap the sun's heat and keep the earth warm; that is the greenhouse effect.

Teacher's Note:
1. Greenhouse gases include \(\ce{CO2}\), \(\ce{CH4}\), \(\ce{N2O}\) and water vapour.
2. The effect is useful, but too much of it causes global warming.

Teacher's Note:
1. Match each gas with its main effect or use.
2. Column B can have extra or repeated items, so read every option.

Teacher's Note:
1. Surgical instruments are made of stainless steel (an alloy), so option c matches none of these.
2. Tar and red lead paint are both used on bridges and poles.

Teacher's Note:
1. Two reactants can give the same gas, as here with oxygen.
2. Nascent hydrogen is a form of hydrogen, not a separate product of these reactants.

Question 14. Name the different layers of atmosphere.
Answer:
The five layers, from the ground upwards, are the troposphere, stratosphere, mesosphere, thermosphere (ionosphere) and exosphere.
In simple words: There are five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.

Teacher's Note:
1. A good memory order from the ground is T, S, M, T, E.
2. Different layers have different temperature patterns.

Question 15. What is atmosphere?
Answer:
The envelope of gases surrounding the earth is called the atmosphere.
In simple words: The atmosphere is the blanket of gases around the earth.

Teacher's Note:
1. It becomes thinner with height.
2. It contains air, moisture and dust.

Question 16. Name any two inert gases present in atmosphere
Answer:
Argon and neon (helium is also present).
In simple words: Argon and neon are inert gases found in air.

Teacher's Note:
1. Inert gases do not react easily with other substances.
2. They occur only in traces in air.

Question 17. Mention the percentage by volume of inactive and active components of atmosphere.
Answer:
Inactive component (nitrogen): about 78% by volume.
Active component (oxygen): about 21% by volume.
In simple words: Nitrogen is about 78% and oxygen about 21% of air.

Teacher's Note:
1. Lavoisier gave the ratio of oxygen to nitrogen as 1 : 4.
2. Together they make nearly 99% of dry air.

Question 18. Name the device used to measure the atmospheric pressure.
Answer:
The barometer.
In simple words: A barometer measures atmospheric pressure.

Teacher's Note:
1. Atmospheric pressure at sea level is 760 mm of mercury.
2. Its SI unit is the pascal.

Question 19. How are the delicate colours of auroras caused?
Answer:
High-energy electrons from the sun collide with molecules such as \(\ce{N2}\) and \(\ce{O2}\) in the thermosphere.
The electrons of the atoms are excited to higher energy levels, and when they return to lower levels they emit light of beautiful colours.
In simple words: Excited atoms give out coloured light as they return to their normal energy.

Teacher's Note:
1. Auroras are seen near the polar regions.
2. Earth's magnetic field guides the sun's electrons to those regions.