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VIII-EM-AIR-EVALUATION -QUESTION AND ANSWERS
AIR
I. Choose the best answer.
1. Which of the following is true about oxygen?
a) Completely burning gas b) Partially burning gas
c) Doesn’t support burning d) Supports burning
Ans: d) Supports burning
2. Aerated water contains
a) air b) oxygen
c) carbon dioxide d) nitrogen
Ans: c) carbon di oxide
3. Solvay process is a method to manufacture
a) lime water b) aerated water
c) distilled water d) sodium carbonate
Ans: d) sodium carbonate
4.Carbon dioxide with water changes
a) blue litmus to red b) red litmus to blue
c) blue litmus to yellow d) doesn’t react with litmus
Ans: a) blue litmus to red
5. Which of the following is known as azote?
a) Oxygen b) Nitrogen
c) Sulpher d) Carbon dioxide
Ans: b) Nitrogen
II. Fill in the blanks.
1. ___________ is called as vital life.
Ans: oxygen
2. Nitrogen is ___________ than air.
Ans: slightly lighter
3. ___________ is used as a fertilizer.
Ans: Nitrogen
4. Dry ice is used as a ___________.
Ans: refrigerant
5. The process of conversion of iron into hydrated form of oxides is called
___________.
Ans: rusting
III. Match the following.
Ans:
IV. Answer briefly.
1. Mention the physical properties of oxygen.
Oxygen is a colourless, odourless and tasteless gas.
It is a poor conductor of heat and electricity
Oxygen dissolves readily in cold water.
2. List out the uses of nitrogen.
Liquid nitrogen is used as a refrigerant.
It provides an inert atmosphere for conducting certain chemical reactions.
It is used to prepare ammonia (by Haber’s process) which is then converted into fertilizers and nitric acid.
Nowadays it is used as a substitute for compressed air in tyres.
3. Write about the reaction of nitrogen with non metals.
Nitrogen reacts with non-metals like hydrogen, oxygen etc., at high temperature to form their corresponding nitrogen compounds.
Δ
Non-metal + Nitrogen → Nitrogen compound
Example
Δ
3H2 + N2 → 2NH3
Hydrogen Nitrogen Ammonia
4. What is global warming?
CO2, N2O, CH2, CFC gases are called as greenhouse gases. The increases in the levels of these gases results in the gradual increase of temperature of the earth’s surface. This increased greenhouse effect caused due increase in the air pollutants and results in the average increase of temperature of the atmosphere.
This is called global warming.
5. What is dry ice? What are its uses?
Dry ice is a solid form of carbon dioxide.It gets this name because it does not melt into a liquid when heated; instead, it changes directly into a gas (a process known as sublimation).
It is used as a refrigerant.
CO2 gas is so cold that moisture in the air condenses on it, creating a dense fog which is used in stage shows and movie effects.
V. Answer in detail.
1. What happens when carbon dioxide is passed through lime water? Write the equation for this reaction.
When a limited amount of CO2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH)2 + CO2 → CaCO3 + H2O
When an excess amount of CO2 is passed through lime water, it first turns milky and the milkyness disappears due to the formation of soluble calcium hydrogen carbonate, Ca(HCO3)2.
2. Name the compounds produced when the following substances burn in oxygen.
a) Carbon b) Sulphur c) Phosphorous d) Magnesium e) Iron f) Sodium
3. How does carbon dioxide react with the following?
a) Potassium b) Lime water c) Sodium hydroxide
a.Reaction with potassium:
Lighter metals like potassium combine with CO2 to form sodium carbonates.
4K + 3CO2 → 2K2CO3 + C
(pot. carbonate)
b.Reaction with lime water:
When a limited amount of CO2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH)2 + CO2 → CaCO3 + H2O
Calcium carbonate
When an excess amount of CO2 is passed through lime water, it first turns milky and the milkyness disappears due to the formation of soluble calcium hydrogen carbonate, Ca(HCO3)2.
c.Reaction with sodiu:m hydroxide:
Sodium hydroxide (base) is neutralized by carbon dioxide (acidic) to form sodium bicarbonate (salt) and water.
NaOH + CO2 → NaHCO3 + H2O
Sodium bicarbonate
4. What are the effects of acid rain? How can we prevent them?
Acid rain affects us in many ways.
It irritates the eyes and skin of human beings.
It inhibits germination and growth of seedlings.
It changes the fertility of the soil, destroys plants and aquatic life.
It causes corrosion of many buildings,bridges etc.
Preventive measures:
Acid rain and its effects can be controlled by the following ways.
Minimizing the usage of fossil fuel such as petrol, diesel etc.,
Using CNG (Compressed Natural Gas).
Using non-conventional source of energy.
Proper disposal of the industrial wastes.
VI. Higher Order Thinking Questions.
1. Soda bottle bursts sometimes when it is opened during summer. Why?
The coefficient of expansion of gases is large. Due to this property the gas in the bottle expands which inturn leads to increase the pressure inside the bottle. So, the bottle may burst sometimes during summer.
2. It is said that sleeping beneath the tree during night is not good for health. What is the reason?
It is because,during the night , plants take in oxygen and release carbon dioxide. It will increase the CO2 level in the atmosphere. When one sleeps under the tree, he will not get sufficient oxygen and experience suffocation. Hence, it is not advisable to sleep under the tree at night.
3. Why does the fish die when it is taken out of water?
Gills are the respiratory organs of fish. Gills are supplied with blood capillaries which can absorb oxygen dissolved in water.
When fishes are taken out of water the supply of oxygen to the fishes will stop and fishes cannot breathe using oxygen present in the atmosphere. Hence, the fish die after some time from being taken out of water.
4. How do astronauts breathe when they go beyond earth’s atmosphere?
Oxygen produced in the spacecraft by three different ways, using oxygen generators, pressurized oxygen tanks or solid fuel oxygen generators.
2H2O → 2H2 + O2
The electricity is generated by the station's solar panels by electrolysis. The hydrogen gas made in the electrolysis process gets vented into space and the oxygen gas is circulated into the cabin air.
