Samacheer Kalvi 8th Science Solutions Term 1 Chapter 5 Changes Around Us
Samacheer Kalvi 8th Science Solutions Term 1 Chapter 5 Changes Around Us
Tamilnadu Samacheer Kalvi 8th Science Solutions Term 1 Chapter 5 Changes Around Us
Samacheer Kalvi 8th Science Solutions Changes Around Us Text Book Exercises
I. 8th Science Changes Around Us Multiple choice questions
Changes Around Us Class 8 Question 1.
Burning of paper is a …………….. change.
(a) Physical
(b) Chemical
(c) Physical & chemical
(d) Neutral
Answer:
(b) Chemical
8th Science Guide Matter Around Us Question 2.
The burning of matchstick is an example for chemical reaction based on
(a) Contact
(b) Electricity
(c) Light
(d) Catalyst
Answer:
(c) Light
Matter Around Us Class 8 Samacheer Kalvi Question 3.
…………….. metal undergoes rusting.
(a) Tin
(b) Sodium
(c) Copper
(d) Iron
Answer:
(d) Iron
Changes Around Us Class 8 Question 4.
The pigment responsible for browning of apples is …………….
(a) Hydrated iron (II) oxide
(b) Melanin
(c) Starch
(d) Ozone
Answer:
(b) Melanin
Samacheer Kalvi Guru 8th Standard Science Question 5.
Brine is a concentrated solution of ………………
(a) Sodium sulphate
(b) Sodium chloride
(c) Calcium chloride
(d) Sodium bromide
Answer:
(b) Sodium chloride
Samacheer Kalvi Guru 8th Science Book Solutions Question 6.
Limestone contains ………………. mainly.
(a) Calcium chloride
(b) Calcium carbonate
(c) Calcium nitrate
(d) Calcium sulphate
Answer:
(b) Calcium carbonate
Kalvi Guru 8th Science Question 7.
Which of the following factor induces electrolysis?
(a) Heat
(b) Light
(c) Electricity
(d) Catalysis
Answer:
(c) Electricity
Samacheer Kalvi Guru Science 8th StandardQuestion 8.
In Haber’s process of producing ammonia is used as a catalyst.
(a) Nitrogen
(b) Hydrogen
(c) Iron
(d) Nickel
Answer:
(c) Iron
8th Science Solution Guide Question 9.
Dissolved gases like sulphur dioxide, nitrogen oxides in rain water causes
(a) Acid rain
(b) Base rain
(c) Heavy rain
(d) Neutral rain
Answer:
(a) Acid rain
Changes Around Us Question 10.
…………….. is responsible for Global warming.
(a) Carbon di oxide
(b) Methane
(c) Chlorofluoro carbons
(d) All the above
Answer:
(a) Carbon di oxide
II. Fill in the blanks
- Reactants → ……………….
- Photosynthesis is a chemical reaction that takes place in presence of Iron objects undergo rusting when exposed to and ……………..
- Iron objects undergo rusting when exposed to …………….. and ……………..
- ……………… is the basic material to manufacture urea.
- Electrolysis of Brine solution gives gases.
- ……………… is a chemical substance which alters the speed of a chemical reaction.
- ………………. is the enzyme responsible for browning of vegetables, fruits.
Answer:
- Products
- Sunlight
- Water, oxygen
- Ammonia
- Chlorine and hydrogen
- Catalyst
- Poly phenol oxidase or tyrosinase
III. Write True Or False for the following
8th Science Matter Around Us Guide Question 1.
A chemical reaction is a temporary reaction.
Answer:
False.
Correct statement:
A chemical reaction is permanent.
Changes Around Us Question 2.
Change in colour may take place during a chemical reaction.
Answer:
True.
10 Changes Around Us Question 3.
Formation of slaked lime from quicklime is a endothermic reaction.
Answer:
False.
Correct statement:
Formation of slaked limed from quicklime is a exothermic reaction.
Std 8 Science Chapter 5 Question 4.
CFC is a pollutant.
Answer:
True.
Question 5.
Browning of some vegetables and fruits is due to tannin formation.
Answer:
False.
Correct statement:
Browning of some vegetables and fruits is due to melanin formation.
IV. Match the following
Question 1.
| A | B | ||
| 1. | Rusting | (a) | photosynthesis |
| 2. | Electrolysis | (b) | Haber’s process |
| 3. | Thermolysis | (c) | Iron |
| 4. | food | (d) | Brine |
| 5. | Catalysis | (e) | Decomposition of limestone |
Answer:
- c
- d
- e
- a
- b
Question 2.
| A | B | ||
| 1. | Rancidity | (a) | Decomposition |
| 2. | Ozone | (b) | biocatalyst |
| 3. | Tarnishing | (c) | oxygen |
| 4. | Yeast | (d) | chemical reaction |
| 5. | Calcium Oxide | (e) | fish |
Answer:
- e
- c
- d
- b
- a
V. Give Short Answers For The Following Questions
Question 1.
Define a chemical reaction. .
Answer:
Chemical changes are otherwise called as chemical reactions, because one or more substances (Reactants) undergo a reaction to form one or more new substances (Products).
Reactant(s) → Product(s)
Question 2.
Mention the various conditions required for a chemical reaction to occur.
Answer:
The conditions required for a chemical reaction to take place:
- Physical contact
- Solution of reactants
- Electricity
- Heat
- Light
- Catalyst
Question 3.
Define catalysis.
Answer:
Chemical substance used to alters the speed of the reaction is called catalyst and the process is called catalysis.
Question 4.
What happens when an iron nail is placed in copper sulphate solution?
Answer:
When iron nail is dipped in the solution of copper sulphate, copper is displaced by iron and forms ferrous sulphate. Thus, the blue colour of copper sulphate changes into green because of formation of ferrous sulphate.
Question 5.
What is pollution?
Answer:
Unwanted change in physical, chemical and biological properties of the environment. This is termed as pollution.
Question 6.
What is Tarnishing? Give an example.
Answer:
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 on exposure to atmospheric air.
Similarly, brass vessels which contain copper as one of constituents develop a greenish layer on exposure to air for a long time. This is due to a chemical reaction between copper and moist air to form basic copper carbonate and copper hydroxide.
Question 7.
What happens to the brine during electrolysis?
Answer:
A concentrated solution of sodium chloride called Brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide. This is a very important reaction to produce chlorine industrially.
Question 8.
On heating, calcium carbonate gives calcium oxide and oxygen. Is it exothermic reaction or endothermic reaction?
Answer:
Endothermic reaction.
Question 9.
What is the role of a catalyst in a chemical reaction?
Answer:
Substances used to alter the speed of a chemical reaction, are called catalysts. For example, metallic iron is used as a catalyst in the manufacture of ammonia by Haber’s process.
Question 10.
Why photosynthesis is a chemical reaction?
Answer:
1. Photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon dioxide and water.
2. The sunlight uses the chemical reaction between carbon dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions in used by light are called as photochemical reactions.
Hot Questions
Question 1.
Explain the role of yeast in making cakes?
Answer:
- Yeast is a key ingredient in the production of baked goods.
- Yeast is a bio – organic catalyst carbon dioxide is generated by the yeast as a result of the breakdown of fermentable sugars in the dough and makes cakes to rise.
Question 2.
Justify the statement. Burning of fossil fuels is responsible for global warming.
Answer:
The combustion of fossil fuels also releases a large amount of carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas which is responsible for global warming. Humans bum fossil fuels, releasing huge amounts of carbon pollution and trapping more and more heat in the atmosphere.
Question 3.
Discuss acid rain occurs due to emission of smoke from vehicles and industries.
Answer:
- Rain becomes acidic in nature due to the presence of certain pollutants in the air released by cars and industrial processes.
- Acid rain is caused by emissions of sulphur dioxide and nitrogen oxide which react with the water molecules in the atmosphere.
Question 4.
Is rusting good for Iron materials? Explain.
Answer:
No. Rusting is not good for iron materials. Rust is a form of iron oxide. It occurs when iron combines with the oxygen in the air causing it to corrode. Rust can affect iron and its alloys. It makes them weaker, by replacing the strong iron with flaky powder.
Question 5.
Do all the fruits and vegetables undergo browning? Explain.
Answer:
No, not all the fruits. E nzymic browning can be observed in fruits such as apricots, pears, bananas, grapes and avocados and vegetables such as aubergines, potatoes, lettuce. The enzyme polyphenol oxidase or tyrosinase that when in contact with oxygen catalyses a biochemical reaction of plants phenolic compounds to brown pigments known as melanin, which results in the fruit or vegetable turning brown.
Question 6.
Classify the following day to day activities based on chemical reactions by physical contact, solutions of reactants, heat, light, electricity and catalyst.
- Burning of crackers during festivals
- Addition of water to quicklime to make it slaked lime
- Silver ornaments become black on exposure to air for a longtime
- Copper vessel kept in open air for long time
Answer:
- Chemical reaction.
- Exothermic reaction.
- Tarnishing.
- Rusting.
VI. Answer In Detail
Question 1.
Explain how food items are spoilt due to chemical reactions?
Answer:
1. Spoilage of food and vegetables:
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 taste and odour, deterioration and loss of nutrients.
Example 1 : Rotten eggs develop a bad smell due to formation of hydrogen sulphide gas.
Example 2 : Decaying of vegetables and fruits due to microbes.
2. Rancidity of fishes and meat:
Fishes and meat containing high levels of polyunsaturated fatty acids that undergo oxidation causes bad odour when exposed to air or light. This process is called rancidity.
3. Apples and fruits turn brown when cut:
- Apples and some fruits turn brown due to chemical reaction with oxygen in air. This chemical reaction is called browning.
- The cells of apples, fruits and other vegetables contain an enzyme called polyphenol oxidase or tyrosinase, when come in contact with oxygen catalyses a biochemical reaction of plants, phenolic compounds to brown pigments known as melanins.
Question 2.
Explain the three types of pollution.
Answer:
Generally there are three types of pollutions viz air, water and land pollution. Due to increasing human activities lot of chemical substances are produced artificially which harm all the living and non – living things.
Types of chemical substances and their effects:
| S. No | Type of pollution | Chemical substances responsible for the pollution | Effects |
| 1. | Air pollution | Carbon dioxide, Carbon monoxide, oxides of sulphur, oxides of nitrogen, Chlorofluorocarbons, methane etc. | Acid rain, Global warming, respiratory problems etc. |
| 2. | Water pollution | Waste water containing chemical substances (e.g dyeing industries), detergents, oil spillage etc. | Decrease in quality of water, skin diseases etc. |
| 3. | Land pollution | Fertilizers like urea, various pesticides, herbicides etc. | Spoilage of land, cancer, respiratory diseases etc. |
Question 3.
Explain any three conditions that is required for a chemical reaction to take place by citing one example each.
Answer:
Conditions required for a chemical reaction to take place:
Chemical reactions can be done through:
- Physical contact
- Solution of reactants
- Electricity
- Heat
- Light
- Catalyst
1. Chemical Reactions Based On Solution Of Reactants:
When we mix two substances (Reactants) in solution form, the chemical reaction takes place to form new substances (Products).
Example:
- Take small amount of solid silver nitrate and sodium chloride in a test tube. The reactants in solid state have no reactions.
- Dissolve the same reactants in water in separate test tubes.
- Mix both the solutions. Silver nitrate solution reacts with sodium chloride solution to form a white precipitate of silver chloride and sodium nitrate solution.
- From the above reaction, we infer that some chemical reactions proceed only in solution form not in solid form.
2. Chemical Reaction Based On Electricity:
- When electricity is passed through water containing small amounts of sulphuric acid, hydrogen and oxygen gases are liberated.
- Similarly, a concentrated solution of sodium chloride called Brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide.
- This is a very important reaction to produce chlorine industrially.
- From the above two reactions, we infer that some chemical reactions proceed only by the passage of electricity.
- Hence, such reactions are called as electrochemical reaction or electrolysis.
3. Chemical Reactions Based On Light:
Sunlight is important not only for us but also for plants as welll. As you know photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon dioxide and water.
Example:
The sunlight uses the chemical reaction between carbon dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions is used by light are called as photochemical reactions.
VII. Value Based Questions
Question 1.
Kumar is going to build a house. To purchase the iron rods required for construction, he visited an Iron & steel shop nearby. The seller showed him some Iron rods which are fresh and good. He also showed him little older Iron rods which are brownish in appearance. The price of fresh rods are costlier than the older ones* the seller also gave some offer to older ones. Kumar’s friend Ramesh advised him not to buy the cheaper rods.
- Is Ramesh right in his suggestion?
- Could you explain the reason for his suggestion?
- What are the values shown by Ramesh?
Answer::
- Yes, Ramesh is right in his suggestion because old iron rods were rusted.
- The older rods are brownish and they are rusted. Rusting destroys the quality of the iron rods and harmful for construction.
- Responsible behaviour, correct decision, caring and awareness.
Question 2.
Palanikumar is a Lawyer. He lives in a costly flat. Due to high rent, he wants to shift his residence to a place where he has a chemical industry nearby. There the rent is very cheap and the area is less populated also. Rajasekar, his son studying VIII does not like this and likes to go to some other place.
- Is Rajasekar right in his attitude?
- Why did he refuse to go there?
- What are the values shown by Rajasekar?
Answer:
- Yes, Rajasekar is more concerned over his family health than saving money.
- Rajasekar know that the air pollution from nearby chemical industry would be harmful and affect his family health.
- Environmentally sensitive, awareness, caring and correct decision.
Samacheer Kalvi 8th Science Solutions Changes Around Us Additional Questions
I. Choose the correct answer
Question 1.
When carbon dioxide is passed through lime water –
(a) Calcium hydroxide is formed
(b) White precipitate of CaO is formed
(c) Lime water turns milky
(d) Colour of lime water disappears
Answer:
(c) lime water turns milky
Question 2.
When a magnesium ribbon is burnt in air, the ash formed is –
(a) Black
(b) White
(c) Yellow
(d) Pink
Answer:
(b) White
Question 3.
Rancidity can be prevented by –
(a) Adding antioxidants
(b) Storing food away from light
(c) Keeping food in refrigerator
(d) All of these
Answer:
(d) All of these
Question 4.
The reaction of H2 gas with oxygen gas to form water is an example of –
(a) Combination
(b) Redox reaction
(c) Exothermic reaction
(d) All of these
Answer:
(a) Combination
Question 5.
On immersing an iron nail in CuSO4 solution for few minutes, you will observe –
(a) No reaction takes place
(b) The colours of solution fades away
(c) The surface of iron nails acquire a black coating
(d) The colour of solution changes to green
Answer:
(d) the colour of solution changes to green
Question 6.
Chemically rust is –
(a) Hydrated ferrous
(b) Only ferric oxide
(c) Hydrated ferric oxide
(d) None of these
Answer:
(c) Hydrated ferric oxide
Question 7.
Both CO2 and H2 gases are –
(a) Heavier than air
(b) Colourless
(c) Acidic in nature
(d) soluble in water
Answer:
(b) Colourless
Question 8.
Fatty foods become rancid due to the process of –
(a) Oxidation
(b) Corrosion
(c) reduction
(d) hydrogenation
Answer:
(a) Oxidation
II. Fill in the Blanks
- ……………… is the chemical name of rust.
- ……………… is chemical name of slaked lime
- ……………… is chemical name of limestone.
- ……………… is a chemical substance is used in white washing.
- The head of the matchstick contains ……………… and ………………
- The sides of the matchbox contains ………………
- The term electrolysis was introduced by ………………
- When milk is mixed with coffee decoction, the colour of milk and decoction changes due to ………………
- Acid rain is a result of ………………
- Fishes and meat containing high levels of ……………….
Answer:
- Iron oxide
- Calcium hydroxide
- Calcium carbonate
- Calcium hydroxide (slaked lime)
- Potassium chlorate, antimony tri sulphide
- Red phrosphorous
- Michael Faraday
- Chemical reaction
- Air pollution
- Polyunsaturated fatty acids
III. True or False – if false give the correct statement
Question 1.
The head of a matchstick contains red phosphorous.
Answer:
False.
Correct statement:
The head of a matchstick contains potassium chlorate and antimony tri sulphide.
Question 2.
Lead nitrate produces brown gas on heating.
Answer:
True.
Question 3.
Photochemistry deals with chemical reactions involving light.
Answer:
True.
Question 4.
Ammonia is an important fertilizer in agriculture.
Answer:
False.
Correct statement:
Urea is an important fertilizer in agriculture, but ammonia is the material used for the production of urea.
Question 5.
Decaying of vegetables and fruits due to microbes.
Answer:
True
Question 6.
Catalyst inhibit the chemical reaction.
Answer:
False.
Correct statement:
Catalysts are used to alter the speed of a chemical reaction and increase the reaction rate.
IV. Match the following
Question 1.
| i | Quick lime | (a) | Calcium hydroxide |
| ii | Slaked lime | (b) | Calcium oxide |
| iii | Thermolysis | (c) | Light |
| iv | Photo chemical | (d) | Heat |
Answer:
i. b
ii. a
iii. d
iv. c
Question 2.
| i | Ammonia | (a) | Nickel |
| ii | Dalda | (b) | Fermentation |
| iii | Alcoholic beverages | (c) | Biocatalysts |
| iv | Yeast | (d) | Urea |
Answer:
i. d
ii. a
iii. b
iv. c
Question 3.
| i | Apples | (a) | Oxygen |
| ii | Fatty acids | (b) | Browning |
| iii | Acid rain | (c) | Rancidity |
| iv | Solid camphor | (d) | Air pollution |
Answer:
i. b
ii. c
iii. d
iv. a
V. Very short Answers
Question 1.
What are the conditions required for a chemical reaction to take place?
Answer:
Chemical reactions can be done through;
- Physical contact
- Solution of reactants
- Electricity
- Heat
- Light
- Catalyst.
Question 2.
What is electrolysis?
Answer:
Some chemical reactions proceed only by the passage of electricity. Hence, such reactions are called as electrochemical reaction or electrolysis.
Example:
When electricity is passed through water containing small amounts of sulphuric acid, hydrogen and oxygen gases are liberated.
Question 3.
What is brine commonly known as?
Answer:
Brine is a solution of sodium chloride (NaCl) and water (H2O). Brine is a high concentration solution of salt in water.
Question 4.
How is chlorine prepared industrially?
Answer:
A concentrated solution of sodium chloride called BRINE is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide. This is a very important reaction to produce chlorine industrially.
Question 5.
What is thermo chemical reactions?
Answer:
Some of the chemical reactions can be achieved by the supply of heat only. These reactions are called thermo chemical reactions or thermolysis. In industries limestone rocks are heated to get quicklime (calcium oxide).
Question 6.
What are catalystic reactions?
Answer:
Speed of the certain reactions is influenced by the catalysts and such reactions are called catalytic reactions.
Question 7.
Define the term exothermic and endothermic reactions.
Answer:
Chemical reactions accompanying evolution of heat are called exothermic reactions whereas reactions involving absorbtion of heat are called endothermic reactions.
VI. Short Answer
Question 1.
What is the difference between reactants and products?
Answer:
- Reactants are the inputs whereas products are output.
- When we mix two substances(Reactants)in solution form, the chemical reaction takes place to form new substances(Products).
- There is no existence of products without reactants.
Question 2.
Write the effects of chemical reactions.
Answer:
Chemical reaction requires a specific condition to occur. When chemical reactions take place there will be production of heat, light, sound, pressure etc.
Effects of chemical reactions are:
- Biological effects
- Environmental effects
- Production of heat, light, sound and pressure
- Evolution of gas, changes in colour and change in state.
Question 3.
What happens to the steel benches and tables during rainy season?
Answer:
Tarnishing of metal articles:
This is because the iron metal comes into contact with water and oxygen, it undergoes a chemical reaction called Rusting. Shiny metal surfaces and other articles lose their shining appearance due to chemical reactions on the surface.
VII. Long Answer
Question 1.
Explain the process of corrosion and rusting.
Answer:
Corrosion:
- Corrosion is the natural process by which a naturally pure metal converts into a more stable form of itself like hydroxide or oxide.
- It is an actual degradation of the metal as it renders the metal useless of its usual metallic character.
Rusting:
- Rusting is the specific name given to the corrosion of iron. Rust can be termed with the chemical formula of iron oxide with its general formula Fe2O3.
- Iron when in contact in the air moisture or water, corrodes and forms rust.
Question 2.
Write a note on catalytic reactions.
Answer:
- In industries some chemical substances are used to alter the speed of a chemical reaction. These substances are called catalysts.
- For example, metallic iron is used as a catalyst in the manufacture of ammonia using Haber’s process.
- This ammonia is the basic material for the production of urea, an important fertilizer in agriculture.
- In Vanaspati ghee (dalda) preparation finely divided nickel is used as a catalyst.
- Thus, speed of the certain reactions is influenced by the catalysts and such reactions are called catalytic reactions.
- Enzymes and yeasts are called biocatalysts.
Question 3.
Write the effects of chemical reactions.
Answer:
The effects of chemical reactions are:
1. Evolution of gas:
Gas evolution may take place as a result of chemical reactions. Example: When dilute hydrochloric acid is added to a solution of sodium carbonate or sodium bicarbonate carbon dioxide gas is evolved.
2. Change in colour:
Certain chemical changes produce change in colour. Example : When you place a iron nail in a solution of copper sulphate, the blue colour of copper sulphate slowly changes into green due to chemical reaction between iron copper sulphate solution.
3. Change in state:
Certain chemical reaction change of state is observed. Example : When you bum a piece of camphor, smoke comes out as result of chemical reaction between solid camphor and oxygen. Here, there is a change of state from solid to gas.
VIII. Reason out
Question 1.
Speed of the certain reaction is influenced by the catalyst.
Answer:
Reason : Some chemical substances are used to alter the speed of a chemical reaction. Example : metallic iron is used as a catalyst in the manufacture of ammonia.
Question 2.
Chlorine is produced by Brine.
Answer:
Reason : Brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide. Such reactions are called as electrolysis.
Question 3.
Ozone is produced naturally in the stratosphere.
Answer:
Reason : Most of the earth’s ozone is contained in the stratosphere, a layer of the atmosphere 10 – 40 km above the surface of the earth.
Question 4.
Photolysis occurs in the atmosphere.
Answer:
Reason : The UV rays from the sun break ozone (O3 molecules in the stratosphere into oxygen and atomic oxygen.
IX. Creative questions : HOTS
Question 1.
Name the nitrate which does not produce brown gas on heating?
Answer:
- The nitrates of alkali metals do not produce reddish brown nitrogen dioxide on heating.
- The most common ones are sodium nitrate and potassium nitrate.
Question 2.
What are the chemical reactions occur in everyday life?
Answer:
- The rusting of iron.
- Combustion (burning) of wood.
- The metabolism of food in the body.
- Mixing an acid and a base, such as hydrochloric acid and sodium hydroxide.
- Cooking an egg.
- Digesting sugar with the amylase in saliva.
- Mixing baking soda and vinegar to produce carbon dioxide gas.
- Using a chemical battery.
- The explosion of fire works.
- Rotting bananas.
- Photosynthesis.
Question 3.
Is rotting fruit a chemical change?
Answer:
The rotting of fruit is a chemical reaction. It’s not a physical change. Because an enzymatic reaction causes molecules to change when rotting.
Question 4.
An element ‘A’ on exposure to moist air turns reddish – brown and a new compound ‘B’ is formed. The substance ‘A’ and ‘B’ are:
Answer:
A = Fe (Iron)
B = Fe2O3 (Iron oxide)
Question 5.
Which substance is flushed in bags of potato chips to prevent rancidity?
Answer:
Nitrogen.
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