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VIII-EM-Changes around us
CHANGES AROUND US
TEXTBOOK EXERCISE
I. CHOOSE THE BEST ANSWER.
1. Burning of paper is a __________ change.
a) physical
b) chemical
c) physical and chemical
d) neutral
Ans:b) chemical
2. Burning of matchstick is an example for chemical reaction caused by __________
a) contact in physical states
b) electricity
c) light
d) catalyst
Ans:a) contact in physical states
3. __________ undergoes rusting.
a) Tin
b) Sodium
c) Copper
d) Iron
Ans:d) Iron
4. The pigment responsible for browning of apples is __________.
a) hydrated iron (II) oxide
b) melanin
c) starch
d) ozone
Ans:b) melanin
5. Brine is a concentrated solution of __________.
a) sodium sulphate
b) sodium chloride
c) calcium chloride
d) sodium bromide
Ans:b) sodium chloride
6. Limestone contains __________ mainly.
a) calcium chloride
b) calcium carbonate
c) calcium nitrate
d) calcium sulphate
Ans:b) calcium carbonate
7. Which of the following factors induces electrolysis?
a) Heat
b) Light
c) Electricity
d) Catalysis
Ans:c) Electricity
8. In Haber’s process of producing ammonia __________ is used as a catalyst.
a) nitrogen
b) hydrogen
c) iron
d) nickel
Ans:c) iron
9. Dissolved gases like sulphur dioxide and nitrogen oxides in rain water causes
__________
a) acid rain
b) base rain
c) heavy rain
d) neutral rain
Ans:a) acid rain
10. __________ is/are responsible for global warming.
a) Carbon dioxide
b) Methane
c) Chlorofluorocarbons
d) Carbon dioxide, Methane,Chlorofluorocarbons
Ans:d) Carbon dioxide, Methane,Chlorofluorocarbons
II. FILL IN THE BLANKS.
1. Photosynthesis is a chemical reaction that takes place in the presence of __________.
Ans : Sunlight
2. Iron objects undergo rusting when exposed to __________ and __________.
Ans : air, water
3. __________ is the basic material to manufacture urea.
Ans : Ammonia
4. Electrolysis of brine solution gives __________ gases.
Ans : chlorine and hydrogen
5. __________ is a chemical substance which alters the speed of a chemical reaction.
Ans : Catalyst
6. __________ is the enzyme responsible for browning of vegetables and fruits.
Ans : Polyphenol oxidase
III. SAY TRUE OR FALSE. IF FALSE, CORRECT THE STATEMENT.
1. A chemical reaction is a temporary reaction.
Ans : False. Correct statement: A chemical reaction is a permanent change
2. Decomposition of lead nitrate is an example for a chemical reaction caused by light.
Ans : False. Decomposition of lead nitrate is an example for a chemical reaction caused by heat.
3. Formation of slaked lime from quicklime is an endothermic reaction.
Ans : False. Correct statement: Formation of slaked lime from quicklime is a exothermic reaction.
4. CFC is a pollutant.
Ans : True
5. Light energy may come out due to chemical reactions.
Ans : True
IV. MATCH THE FOLLOWING.
a.
Ans:
B.
Ans:
V. ANSWER BRIEFLY.
1. Define a chemical reaction.
A chemical change is a permanent, irreversible change and produces a new substance.
2. Mention the various conditions required for a chemical reaction to occur.
a. Contact in physical states
b. Solution of reactants
c. Electricity
d. Heat
e. Light
f. Catalyst
3. Define catalysis.
Chemical substances which are used to speed up the chemical reactions are called catalyst and the process is called catalysis. 4. What happens when an iron nail is placed in copper sulphate solution?
When you place an iron nail in the solution of copper sulphate, colour of the solution changes from blue to green, due to chemical reaction between iron and copper sulphate solution.
5. What is pollution?
Due to industrial process and increasing automobiles there is an unwanted change happening in physical, chemical and biological properties of the environment. The unwanted change is termed as pollution.
6. What is tarnishing? Give an example.
Shiny metal surfaces and other articles lose their shining appearance due to chemical reactions on the surface. For example, silver articles become black on exposure to atmospheric air.
7. What happens to the brine during electrolysis?
A concentrated solution of sodium chloride called BRINE produce chlorine and hydrogen gases along with sodium hydroxide, during electrolysis.
8. On heating, calcium carbonate gives calcium oxide and oxygen. Is it an exothermic reaction or an endothermic reaction?
It is an endothermic reaction.
9. What is the role of a catalyst in a chemical reaction?
The role of a catalyst in a chemical reaction is to speed up the reaction by influencing the reactants.
10. Why is photosynthesis a chemical reaction?
Photosynthesis is a process in which plants convert light energy into chemical energy. Along with light energy water and carbon dioxide combine to form starch. Since new chemical species are formed, photosynthesis is clearly a chemical reaction.
VI. ANSWER IN DETAIL.
1. Explain the environmental effects of chemical reactions?
Environmental Effects of Chemical reactions:
a. Pollution
Our environment provides air to breathe,water to drink and the land to roduce food. Due to industrial processes and increasing number of automobiles, our environment is badly affected now-a-days. So, there is an unwanted change in the physical, chemical and biological properties of the environment. This is termed as pollution. The substances which cause these changes are called pollutants. Generally there are three types of pollutions viz air, water and land pollution.
b. Rusting
When the iron metal come into contact with water and oxygen, it undergoes a chemical reaction called rusting.
c. Tarnishing of metal articles
Shiny metal surfaces and other articles lose their shining appearance due to chemical reactions on the surface. For example, silver articles become black when exposed to atmospheric air.
2. Explain how food items are spoiled by chemical reactions?
Food spoilage may be defined as any change that causes food unfit for human consumption. The chemical reactions catalyzed by the enzymes result in the degradation of food quality such as development of bad tastes and odor, deterioration and loss of nutrients.
Eg.
1. Rotten eggs develop a bad smell due to formation of hydrogen sulphide gas
2. Decaying of vegetables and fruits due to microbes.
3. Explain any three conditions that is required for a chemical reaction to take
place. Give example.
Contact in physical state:
When dry wood comes into contact with fire, it burns with the help of oxygen to form carbon dioxide, which is given out as smoke.These changes are due to chemical reactions by contact in physical states. Combination of reactants in their naturally occurring states (solids, liquids, gases) is referred as contact in physical states.
Solution of reactants:
When we mix two substances (reactants) in solution form, a chemical reaction takes place between them to form new substances (products). For example, when milk is mixed with coffee decoction the colour of the milk and the decoction changes due to chemical reaction. From the above reaction, we infer that some chemical reactions proceed only in solution form not in solid form.
Electricity:
Electricity is essential for our living. We use electricity for cooking, lighting, grinding, watching television etc. Many chemical reactions which take place with the help of electricity are industrially very important.
Ex. A concentrated solution of sodium chloride called brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide.
VII. HIGHER ORDER THINKING QUESTIONS.
1. Explain the role of yeast in making cakes and buns in a bakery?
Yeast is used in the baking industry for making bread, pastries and cakes. Carbon dioxide produced during fermentation is trapped in the dough which helps the bread rise when baked, giving the required lightness and soft texture.
2. Burning of fossil fuels is responsible for global warming. Justify the statement.
When fossil fuels are burned, they release large amounts of carbon dioxide, a greenhouse gas, into the air. Greenhouse gases trap heat in our atmosphere, causing global warming. Already the average global temperature has increased by 1°C. Warming above 1.5°C risks further sea level rise, extreme weather, biodiversity loss and species extinction, as well as food scarcity, worsening health and poverty for millions of people worldwide.
3. Discuss how acid rain occurs due to emission of smoke from vehicles and
industries?
The emission of smoke from industries and vehicles contains sulfur dioxide (SO2) and nitrogen oxides. These gases are very harmful and acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground.
4. Is rusting good for iron materials? Explain.
No, rusting is not good for iron objects, because rusting makes iron object weak and useless. Since it is a chemical change it is not possible to get back the original object after rusting.
5. Do all the fruits and vegetables undergo browning? Explain.
No, not all the fruits and vegetables undergo browning. Enzymatic browning can be observed in fruits (apricots, pears, bananas, grapes), vegetables (potatoes, mushrooms) and also in seafood (shrimps, crabs).
Enzymatic browning is a chemical process which occurs in fruits and vegetables when their cut pieces are kept for some time in the open air by the enzyme polyphenoloxidase.
6. Classify the following day to day activities based on chemical reactions by physical contact, solutions of reactants, heat, light,electricity and catalyst.
a ) Burning of crackers during festivals
Ans: Heat
b) Fading of coloured clothes on drying under sunlight.
Ans: Light, solutions of reactants
c) Cooking of eggs.
Ans: Heat
d) Charging of batteries.
Ans: electricity and catalyst
VIII. Value Based Questions.
As it tests your understanding, knowledge on concepts and attitude, it is better to be answered by you. So read the questions carefully and try to answer the questions on your own.
