8th Science Guide Unit 10 Changes Around Us

8th Std Science Guide Lesson 10 Changes Around Us | English Medium

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8th Science Guide Unit 10 Changes Around Us

I. Choose the best answer

1. Burning of paper is a______ change

1. Physical
2. chemical
3. physical & chemical
4. neutral

Ans: chemical

2. The burning of matchstick is an example for chemical reaction based on ______

1. Contact
2. electricity
3. light
4. catalyst

Ans: Contact

3. ______ metal undergoes rusting.

1. tin
2. sodium
3. copper
4. iron

Ans: iron

4. The pigment responsible for browning of apples is______.

1. Hydrated iron (II) oxide
2. melanin
3. starch
4. ozone

Ans: melanin

5. Brine is a concentrated solution of ______.

1. Sodium sulphate
2. sodium chloride
3. calcium chloride
4. sodium bromide

Ans: sodium chloride

6. Limestone contains ______ mainly.

1. Calcium chloride
2. calcium carbonate
3. calcium nitrate
4. calcium sulphate

Ans: calcium carbonate

7. Which of the following factor induces electrtolysis?

1. Heat
2. light
3. electricity
4. catalysis

Ans: electricity

8. In Haber’s process of producing ammonia ______ is used as a catalyst.

1. Nitrogen
2. hydrogen
3. iron
4. nickel

Ans: iron

9. Dissolved gases like sulphur dioxide, nitrogen oxides in rain water causes______

1. Acid rain
2. base rain
3. heavy rain
4. neutral rain

Ans: Acid rain

10. ______ is responsible for Global warming.

1. Carbon di oxide
2. methane
3. chlorofluoro carbons
4. all the above

Ans: all the above

II. Fill in the blanks.

1. The Reactants → __________ Ans : Products

2. Photosynthesis is a chemical reaction that takes place in presence of ______. Ans : Sunlight
3. Iron objects undergo rusting when exposed to ______ and ______. Ans : Water and Oxygen.
4. ______ is the basic material to manufacture urea. Ans : Ammonia
5. Electrolysis of Brine solution gives ______ gases. Ans : hydrogen
6. ________ is a chemical substance which alters the speed of a chemical reaction. Ans : Catalyst
7. ________ is the enzyme responsible for browning of vegetables, fruits Ans : Polyphenol oxidase or tyrosinase

III. True or False

1. A chemical reaction is a temporary reaction. Ans : False

Cu. St : A chemical reaction is a permanent reaction.

2. Change in colour may take place during a chemical reaction. Ans : True
3. Formation of slaked lime from quicklime is a endothermic reaction. Ans : False

Cu. St : Formation of slaked limed from quicklime is a exothermic reaction.

4. CFC is a pollutant. Ans : True
5. Browning of some vegetables and fruits is due to tannin formation. Ans : False

Cu. St : Browning of some vegetables and fruits is due to melanin formation.

IV. Match the following:

1.

1. Rusting – photosynthesis
2. Electrolysis – Haber’s process
3. Thermolysis – Iron
4. food – Brine
5. Catalysis – Decomposition of limestone

Ans: 1 – c, 2 – d, 3 – e, 4 – a, 5 – b

2.

1. Rancidity – Decomposition
2. Ozone – biocatalyst
3. Tarnishing – oxygen
4. Yeast – chemical reaction
5. Calcium Oxide – fish

Ans: 1 – e, 2 – c, 3 – d, 4 – b, 5 – a

V. Give Short Answers For The Following Questions.

1. Define a chemical reaction.

• A chemical change is a permanent, irreversible change and produces anew substance.

2. Mention the various conditions required for a chemical reactionto occur

1. Physical contact
2. Solution of reactants
3. Electricity
4. Heat
5. Light
6. catalyst

3. Define catalysis.

• Chemical substance used to alters the speed of the reaction is called catalyst and the process is called catalysis.

4. What happens when an Iron nall Is placed In copper sulphate solution?

• 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.

5. What is pollution?

• Due to human activity there is an unwanted change in physical, chemical and biological properties of the environment. Thisis 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 is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide.

8. On heating, calcium carbonate gives calcium oxide and oxygen. Is it exothermic reaction or endothermic reaction?

• It is endothermic reaction

9. What is the role of a catalyst in a chemical reaction?

• some chemical substances are used to alter the speed of a chemical reaction. These substances are called catalysts.

10. Why photosynthesis is a chemical reaction?

• photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon-di-oxide and water.
• The sunlight in uses the chemical reactions between carbon-di-oxide and water, which finally ends up in the production of starch.
• These chemical reactions in used by light are called as photo chemical reactions.