Modern Concept Science and Technology 8 New

Modern Concept Science and Technology – 8 297 Hard water The water that contains the chloride, sulphate and bicarbonate salts of calcium and magnesium is called hard water. It does not produce lather with soap easily. So hard water leads to wastage of soap. The water of wells, rivers, oceans, etc. is hard. Hardness of water is due to the presence of soluble salts of calcium and magnesium, viz. chloride, sulphate and bicarbonate salts of calcium and magnesium. Advantages of hard water i It makes our teeth and bones healthy because they contain minerals. ii. It is used for bathing because its minerals are good for skin. iii. It with desired concentration of minerals is used as drinking water. Disadvantages of hard water i. It is not good for washing clothes because it does not produce lather. ii. It is not good for washing utensil. iii. It is not good for cleaning floors. iv. Hard water can damage washing machine. On the basis of salts dissolved, hardness of water is of two types: a. Temporary hardness b. Permanent hardness a. Temporary hardness The hard water containing bicarbonate salts of calcium and magnesium is called temporary hard water. Such type of water is tasty while drinking and makes our bones healthy. This type of hardness can be removed easily. Cause of temporary hardness of water Temporary hardness of water is caused by bicarbonates of calcium and magnesium. For eample, calcium bicarbonate and magnesium bicarbonate. Removal of temporary hardness of water Temporary hardness of water can be removed easily by boiling the water or treating with lime-water. i. By boiling water : When the temporary hard water is boiled, the soluble bicarbonate salts of calcium and magnesium get converted into water insoluble carbonate salts and settle down at the bottom of the container. These insoluble salts can be filtered away. Calcium bicarbonate heat Calcium carbonate + Water + Carbon dioxide Ca(HCO3 )2 ∆ CaCO3 + H2 O + CO2 ↑ Magnesium bicarbonate heat Magnesium carbonate + Water + Carbon dioxide Mg(HCO3 )2 ∆ MgCO3 + H2 O + CO2 ↑ ii. By treating with lime-water : Temporary hardness of water can also be removed by adding lime water or slaked lime into the hard water. Calcium hydroxide, i.e. lime

298 Water water reacts with bicarbonate of calcium and magnesium and forms insoluble calcium carbonate which makes the water soft. This method is called Clark's method. Calcium bicarbonate + Calcium hydroxide Calcium carbonate + Water Ca(HCO3 ) 2 + Ca(OH)2 2CaCO3 + 2H2 O Magnesium bicarbonate + Calcium hydroxide Calcium carbonate + Water + Magnesium hydroxide Mg(HCO3 )2 + 2Ca(OH)2 2CaCO3 + 2 H2 O + Mg(OH)2 b. Permanent hardness The hard water containing chloride and sulphate salts of calcium and magnesium is called permanent hard water. Cause of permanent hardness of water Permanent hardness of water is caused by presence of water soluble salts such as magnesium chloride, magnesium sulphate, calcium chloride and calcium sulphate. Removal of permanent hardness of water Permanent hardness of water can be removed by treating the hard water with washing soda and permutit process. i. By treating hard water with washing soda : When permanent hard water is treated with washing soda or sodium carbonate, the salts present in it change into carbonates and water becomes soft. The chemical reactions involved in this process are given below: Calcium chloride + Sodium carbonate Calcium carbonate + Sodium chloride CaCl2 + Na2 CO3 CaCO3 + 2NaCl Calcium sulphate + Sodium carbonate Calcium carbonate + Sodium sulphate CaSO4 + Na2 CO3 CaCO3 + Na2 SO4 Magnesium chloride + Sodium carbonate Magnesium carbonate + Sodium chloride MgCl2 + Na2 CO3 MgCO3 + 2NaCl Magnesium sulphate + Sodium carbonate Magnesium carbonate + Sodium sulphate MgSO4 + Na2 CO3 MgCO3 + Na2 SO4 ii. By permutit process : In permutit process, the permanent hard water is passed through sodium aluminosilicate (Na2 Al 2 SiO8 ) or sodium zeolite (Na2 –Z). When hard water passes through the zeolite or permutit, the calcium and magnesium ions of hard water get replaced by sodium ions of permutit. As a result, water becomes soft. Hard water Sodium chloride Zeolite Soft water Column Finegravel Permutit process to remove hardness of water

Modern Concept Science and Technology – 8 299 The chemical reactions involved in permutit method are given below: Sodium zeolite + Calcium chloride Calcium zeolite + Sodium chloride Na2 –Z + CaCl2 Ca – Z + 2NaCl Sodium zeolite + Magnesium chloride Magnesium zeolite + Sodium chloride Na2 – Z + MgCl2 Mg–Z + 2NaCl Sodium zeolite + Calcium sulphate Calcium zeolite + Sodium sulphate Na2 – Z + CaSO4 Ca–Z + Na2 SO4 Sodium zeolite + Magnesium sulphate Magnesium zeolite + Sodium sulphate Na2 – Z + MgSO4 Mg–Z + Na2 SO4 ACTIVITY 1 To demonstrate that temporary hardness can be removed by boiling water • Take a beaker and put some water into it. • Add one spoon of calcium bicarbonate or magnesium bicarbonate into the water and stir with a glass rod. • Now, add a few drops of shampoo or solution of soap into the beaker and observe whether lather is formed or not. • The water does not produce lather. It proves that the water is hard. • Now, boil the water in the beaker and allow it to cool. • Again, add a few drops of shampoo or solution of soap and stir with a glass rod. Observe whether lather is formed or not. The water produces lather with soap after boiling. It proves that temporary hardness of water can be removed by boiling. ACTIVITY 2 To demonstrate that permanent hardness of water can be removed by adding washing soda • Take a beaker and put some water into it. Add some calcium chloride or magnesium chloride into it and stir. • Now add 2-3 drops of shampoo or solution of soap into the beaker and stir it with a glass rod. The water does not produce lather with soap as it is a hard water. • Now, add some sodium carbonate into the beaker and stir the mixture with a glass rod. • Again, put a few drops of solution of soap and stir. Observe whether the water forms lather with soap or not. • After adding sodium carbonate or washing soda, the water produces lather with soap. This activity proves that permanent hardness of water can be removed by using washing soda. Alloy The homogeneous mixture of two or more metals or metals and non-metals is called alloy. The molten mass of two or more metals or non-metals when mixed completely, forms homogenous mixture. On cooling this mixture, we get a solid mass which is called alloy. The alloy which is

300 Water formed by mixing a metal and mercury is called amalgam. The melting point of alloy is always lower than its component metals. But it has greater hardness and more brittleness. Properties of alloys i. Alloys are generally harder and stronger than their components. ii. They have metallic properties like malleability and ductility. iii. They are good conductors of heat and electricity. iv. They are more brittle than their components. v. They have generally low melting point than that of their components. FACTS WITH REASONS Why is an alloy preferred in constructions than metals? An alloy is preferred in constructions than the metals because alloy is harder and stronger than its components. Some alloys and their components S.N. Alloys Components 1. Brass Copper (70%) and Zinc (30%) 2. Bronze Copper (88%) and Tin (12%) 3. Bell metal Copper (80%) and Tin (20%) 4. Artificial gold (Rolled gold) Copper (90%) and Aluminium (10%) 5. Stainless steel Iron (89.4%), Chromium (10%), Manganese (0.35%) and Carbon (0.25%) 6. Duralumin Copper, manganese and magnesium Brass Bronze Bell metal Rolled gold Steel ACTIVITY 3 • Take a few iron nails or a piece of iron. Keep it in a moist place and observe it after 3-4 days. What do you observe? You can see a reddish-brown layer on the surface of the iron nails. The formation of reddish-brown scales or powder on the surface of iron when it comes in contact with moist air is called rusting. As a result of rusting, weight of metal increases. To protect iron from rusting, we apply several methods like painting, forming alloys, galvanizing, etc. 4Fe + xH2 O + 3O2 2Fe2 O3 . xH2 O (Rust)

Modern Concept Science and Technology – 8 301 Brass Brass is an alloy of copper and zinc. It is used to make various objects because of its gold-like appearance, low melting point, durability and conductivity. Some of its uses are: i. It is mostly used to make musical instruments such as trumpet, gong bell and handbells. ii. It is used to make plates, cups, spoons and glasses. It’s wise to use these because of their antimicrobial property. iii. It is used to make idols, flower vases, wands and ornaments. iv. Locks, gears, valves and bearings are usually made from brass because it is tensile. v. Brass is used for making decorative items such as statues, hourglass and watches. Fig: Bell of brass Fig: Brass locks Fig: Musical instrument of brass Fig: Uses of brass Bronze Bronze is an alloy of copper and tin. It is used to make various objects because it is wearresistance and hard. Some of its uses are: i. It is used in the construction of sculptures. ii. Some musical instruments, medals and bearings are made from bronze. iii. Bronze is used for clips, electrical connectors and springs. iv. In architecture, it is found in hardware, doors, entrances, fountains, handrails and column cladding. v. Modern safety tools such as hammers, mallets, and wrenches are also made of bronze. vi. Bronze is used for making statues and weapons. Fig: Medals Fig: Bronze sculptures Fig: Bronze hammers Fig: Uses of bronze

302 Water Steel Steel is an alloy of iron and carbon. Various forms of steel are found based on the addition of other elements. It is used in many ways. i. Steel is used for making cabinets and shelving units. ii. The cutlery and utensils we use every day are mainly manufactured from stainless steel. iii. Doors, engines, suspension and interiors of vehicles consist largely of steel. iv. Stainless steel is used to make bathroom accessories such as taps, bathroom hooks, toothbrush holders etc. v. Steel is used to make railing on the roof and ladders in houses. vi. Most surgical tools are made from stainless steel. Fig: Steel cabinets Fig: Steel utensils Fig: Steel surgical tools Fig: Uses of steel FACTS WITH REASONS Brass is used for making decorative items. Brass is used for making decorative items because it has a beautiful gold-like colour. ACTIVITY 4 What materials in our surroundings are made of brass and bronze? Make a list. HOT SKILL HIGHER ORDER THINKING SKILL 1. Why people prefer adding detergent in water for wahing clothes? People prefer adding detergent in water for washing clothes because detergent can convert hard water into soft water. More lather will come and remove dirt from the clothes easily. 2. Ramesh has a battery to store electricity produced by solar panel. He added tap water in it instead of distilled water? Is it right thing to do? No! it is not right thing to do. The battery needs distilled water. If we add hard water in it, the mineral contents of the hard water will damage the battery. MEMORY TIPS An alloy is a mixture of multiple elements with at least one metal.

Modern Concept Science and Technology – 8 303 3. What happens when temporary hard water is boiled? When temporary hard water is boiled the metal bicarbonates will break down into metal carbonates and the water will become soft water. Ca(HCO3 ) 2 CaCO3 + H2 O + CO2 4. What is the principle of permutit process? The principle of permutit process is to use sodium zeolite to replace magnesium or calcium ions by sodium ions. 5. We should drink mineral water over soft water. We should drink mineral water over soft water because softwater lacks essential salts necessary for our body. Whereas, mineral water is hard water which is useful for drinking because it supplies important salts. It is also low in sodium than soft water. 6. How is brass different from bronze even if both are alloy of copper? Brass is different from bronze even if both are alloy of copper because of their composition. Brass is composed of 70% of copper and 30% of zinc. However Bronze is composed of 88 % of copper and 12% of tin. Brass is usually yellow like a gold but bronze can range from light brown to dark black. 7. Write any two differences between temporary hardness of water and permanent hardness of water. Differences between temporary hardness of water and permanent hardness of water are: SN Temporary hardness of water SN Permanent hardness of water 1 Hardness of water due to presence of bicarbonates of calcium or magnesium is called temporary hardness of water. 1 Hardness of water due to presence of sulphate and chloride of magnesium and calcium is called permanent hardness of water. 2 Temporary hardness of water is usually removed by boiling or using lime water. 2 Permanent hardness of water is usually removed by using washing soda or sodium zeolite. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) Which statement is true for soft water? i. It contains no salt ii. It contains calcium bicarbonate iii. It contains calcium sulphate iv. It can produce more lather with soap

304 Water b) Which of the following salt causes permanent hardness of water? i. sodium chloride ii. calcium chloride iii. calcium bicarbonate iv. magnesium bicarbonate c) What does sodium carbonate do to the permanent hardness of water? i. change into soft water ii. change into temporary hard water iii. increase hardness of water iv. change the colour of water d) Which combination of metals gives bronze? i. copper + zinc ii. copper + tin iii. copper + carbon iv. lead + tin e) What is formed by the combination of carbon with iron? i. steel ii. amalgam iii. brass iv. bronze 2. Write True for the correct and False for the incorrect statement. a) Water is essential for conducting various chemical reactions that occur inside the body. b) Temporary hardness of water is caused by sulphates of calcium and magnesium. c) Permanent hardness of water can be removed by treating the hard water with washing soda and permutit process. d) The melting point of alloy is always higher than its component metals. e) Steel is an alloy of iron and carbon. 3. Select the best answer from the given alternatives. a) Presence of calcium sulphate causes ............ hardness of water. b) Presence of calcium bicarbonate causes ............ hardness of water. c) The homogeneous mixture of copper and zinc forms ............ d) The chemical used for permutit process is ............ e) The homogeneous mixture of metal with mercury is called ............ 4. Very Short Answer Questions a) What is water? b) Name two salts that cause temporary hardness of water. c) What is temporary hardness of water? d) What do you mean by permanent hardness of water? e) Name any two chemicals dissolved in hard water. f) What are the two salts that cause permanent hardness in water? g) Write two examples of soft water.

Modern Concept Science and Technology – 8 305 h) What is permutit method? i) Name two methods to remove permanent hardness of water. j) Name the chemical used during permutit process and write its molecular formula. 5. Define the following terms with required examples. a) Water b) Hard water c) Permanent hard water d) Temporary hard water e) Soft water f) Alloy STEP2 6. Give reasons a) Soft water is good for washing clothes. b) Hard water is good for drinking and bathing. c) Boiling can remove temporary hardness of water. d) Alloy is preferred over component metals. e) It is difficult to wash clothes in hard water. 7. Differentiate between: a) Rain water and Sea water b) Soft water and Hard water c) Temporary hard water and Permanent hard water d) Brass and Steel e) Clark's method and permutit method 8. Answer the following in short. a) What do you mean by soft water? Give an example. b) Define hard water and list its type. c) Name the salts with their molecular formula that cause permanent hardness in water. STEP3 9. Answer the following questions. a) Write any three uses of hard water. b) Write down theree uses of soft water. c) Write any two methods of removing temporary hardness of water. d) How is permanent hardness of water removed by using washing soda? e) What is permutit process? Describe it in brief. f) What is brass? How is it used in daily life? g) What is bronze? Enlist its application. h) Define steel. Write down three uses of steel.

306 The Earth and Universe The Earth and Universe Unit 11 Introduction The universe is a group of everything that exists. Many scientists agree that the universe started from the big bang. We live in the Orion arm of the milky way galaxy. There are billions of stars in the galaxy. As observed from Earth, we can see many constellations in the night sky. Earth is one among many heavenly bodies of the solar system. Meteors, comets, asteroids, dwarf planets and planets are found revolving around the sun. Earth is our home planet. The earth is rich in minerals. Minerals are elements or inorganic compounds. It can be metallic and non-metallic. Minerals can be used for construction. It is used for making houses, roads, utensils, ornaments, cleaning agents, clothes and many more. Iron, copper, limestone, clay, conglomerate and many other minerals are found in various locations in Nepal. Our body is also made up of minerals. Minerals are very important in our body. It is theorized that certain chemicals organized themselves to form DNA and RNA, which later formed protocells. The protocells developed into the first cell. Then the biological evolution of life started on the earth. Each cell absorbed minerals from the surroundings and multiplied. It evolved into Monera, Protista, Fungi, Plantae, Animalia and viruses. Hence earth became full of life over time. The geological time scale is used to track different evolutions that occurred at a certain time. It is divided into four major eras. In this unit, we are going to discuss about the universe, earth, minerals and the evolution of life.

Modern Concept Science and Technology – 8 307 Key terms and terminologies of the unit 1. Minerals : Minerals are naturally occurring inorganic substances in the Earth’s crust which have their own physical and chemical properties. 2. Ores : The minerals from which usable or commercial metal can be extracted in a profitable amount are called ores. 3. Metallic minerals : The minerals that contain significant amount of metals are called metallic minerals. 4. Non-metallic minerals: The minerals that do not contain significant amount of metals are called non-metallic minerals. 5. Lustre : The property of minerals to reflect light is called lustre. 6. Streak : Streak is the colour seen when the mineral is powdered. 7. Specific gravity : Specific gravity is the ratio of the mass of minerals to the mass of the same volume of water. 8. Tenacity : The property of minerals to resist breakage is called tenacity. 9. Energy minerals : The minerals that can be used as fuel are called energy mineral. 10. Mineralogy : The branch of science in which we study about minerals is called mineralogy. Minerals Minerals are useful substances that are found in the crust of the earth. They are metallic or non-metallic solids found naturally in the earth’s crust. The chemical compound which are inorganic, solid and has a definite chemical composition is called mineral. Some of them are metallic, non-metallic and energy minerals. Some minerals are even fossils. All the minerals are unique. Sequence of Curriculum Issued by CDC  Introduction to minerals  Types of minerals (metallic and nonmetallic minerals)  Characteristics and uses of minerals  Important metallic minerals found in different locations of Nepal UNIT Minerals 11.1 Estimated teaching periods Theory Practical 2 1 Hugo Von Mohl is a botanist noted for his research on the anatomy and physiology of plant cells. He was born in Germany on 8th April 1805 and died on 1st April 1872. He is renowned for discovering protoplasm. He pursued botany and mineralogy. He discovered many facts about cell walls, vacuoles and plastids. He discovered that all metabolic activities occur in protoplasm. Most importantly he discovered cell division in 1835 as he worked on green algae. He studied at the University of Tubingen and studied medicine. Later he focused on botany in Munich. Hugo Von Mohl About the Scientist

308 Minerals Some minerals consist of single element such as gold, silver, diamond (carbon), sulphur, etc. Similarly some minerals consist of two or more elements like silica, copper pyrite, horn silver, bauxite, etc. More than two thousands minerals have been identified till today. Mostly these minerals contain inorganic compounds made by various combinations of the eight elements (O, Si, Al, Fe, Ca, Na, K, and Mg). Some minerals contain large amount of metals called ores. Thus, ores are those minerals from which usable or commercial metal can be extracted in a profitable amount. Haematitie, bauxite, copper pyrites, argentite, etc. are some examples of ores. Minerals are the non-renewable natural resources. gold feldspar talc diamond limestone gypsum Fig: Gold, feldspar, talc, diamond, limetone and gypsum Types of minerals Metallic minerals The minerals that contain significant amount of metals are called metallic minerals. However, the minerals from which metals can be obtained profitably are called ores. Ores contain less amount of impurities and higher amount of metals. Metallic minerals are the compounds of metals. For example: iron oxide, aluminium oxide, copper pyrites, limestone, dolomite, etc. Haematitie, magnetite, limonite, etc. are ores of iron, bauxite is an ore of aluminium and copper pyrites is an ore of copper similarly, argentite is an ore of silver. From these ores we can obtain metals easily and profitably. Some metallic minerals are ferrous, which have more iron concentration. Some metallic minerals are non-ferrous. They have no iron concentration or precious minerals such as gold, silver and platinum. Haematite ore Magnetite ore Cuprite ore Chalcopyrite ore Argentite ore Fig: Metallic minerals

Modern Concept Science and Technology – 8 309 Non-metallic minerals The minerals that do not contain significant amount of metals are called non-metallic minerals. Most of the non-metallic minerals are metallic compounds while some are non-metallic compounds. Non-metallic minerals cannot be used to extract metals. They are mostly used for manufacturing cement, fertilizers, pesticides, gems and other decorative items. Examples of non-metallic minerals are granite, slate, sulphur, silica, feldspar, quartz, obsidian, etc. Some non-metallic minerals such as sapphire, topaz, emerald etc, are gemstones. Sulphur Coal Slate Granite Fig: Non-metallic minerals Energy minerals The minerals that can be used as fuel are called energy minerals. It could be petroleum. They are actually fossil minerals. They are carbon compounds which are preserved underground since ancient times. They are used to power vehicles and industries. Uranium is also an energy mineral which is used in nuclear power plant. Differentiate between metallic minerals and non metallic minerals SN Metallic mineral SN Non-metallic mineral 1 The minerals that contain significant amount of metals are called metallic minerals. 1 The minerals that do not contain significant amount of metals are called non-metallic minerals. 2 Metallic minerals are simply metallic compounds. 2 Most of the non-metallic minerals are metallic compounds while some are also non-metallic compounds. ACTIVITY 1 Observe your surrounding and see if you find any minerals. Note its name. FACTS WITH REASONS Non-metallic minerals cannot be used to extract metals. Non-metallic minerals cannot be used to extract metals because they contain very less amount of metals. For example: granite, silica, feldspar, quartz, etc. Granite is a non-metallic mineral. Granite is a non-metallic mineral because it contains lots of nonmetals but very low concentration of metals. Characteristics of minerals Minerals have their own distinct characteristics. Most minerals can be characterized and classified by their unique physical properties such as hardness, lustre, colour, streak, specific gravity, cleavage, fracture, and tenacity.

310 Minerals Hardness Minerals can resist being scratched. Some minerals such as talc and gypsum are soft. They are scratched easily. Some minerals such as quartz, topaz and diamond are hard. They cannot be scratched easily. Lustre The property of minerals to reflect light is called lustre. Metallic minerals such as gold, silver and galena have metallic lustre. Minerals such as Kaolinite and limonite are earthy. They do not have lustre. Colour Minerals are available in various colours. Gold is yellow, copper is red, feldspar is pink and haematite the ore of iron is reddish. Streak Streak is the colour seen when the mineral is powdered. Minerals can make a streak when rubbed on a streak plate. The yellow streak means gold. Black streak means chalcopyrite. It is the ore of copper. Specific gravity Specific gravity is the ratio of the mass of minerals to the mass of the same volume of water. The specific gravity of water is 1, aluminium is 2.7 and gold is 19.32. Tenacity The property of minerals to resist breakage is called tenacity. Quartz, feldspar, calcite and dolomite are brittle minerals. They break easily. Gold and silver are malleable and ductile. They can be hammered into a sheet and drawn into wires. Talc is sectile. It can be carved by knife. Biotite mica is elastic. It can regain its shape if we stop bending it. Gypsum is plastic. It does not regain its shape when we stop bending it. Other characteristics of minerals Some minerals are transparent or translucent or opaque, magnetic or non-magnetic, crystalline or amorphous, salty or bitter or tasteless and break along smooth plane (cleavage) or not. FACTS WITH REASONS Minerals are inorganic substances. Minerals are inorganic substances because they are made of non-living materials only. ACTIVITY 2 Observe the various minerals used in your house. Uses of minerals Minerals are heavily used in construction of infrastructure, manufacture of tools and weapons and making utensils. They are also used to make medicine and fertilizer. Appropriate use of minerals can bring prosperity to the country. MEMORY TIPS The silicate minerals compose more than 90% of earth’s crust.

Modern Concept Science and Technology – 8 311 Golden ornamenets Iron pipes Fungicides Fig: Use of minerals Major uses of minerals are listed below: i. Minerals like gold, silver, platinum and diamond are used to make ornaments. ii. Minerals such as copper and aluminium are used to make electrical wires. iii. Minerals such as iron, copper and aluminium are used in construction of buildings, utensils and furniture respectively. iv. Aluminium is used to make the body of vehicles, cans, foils and cosmetics. v. Iron is used to make agriculture tools, machines and wedges. vi. Fossil minerals such as coal, petrol, natural gases and kerosene are used as a source of fuel for kitchens, vehicles and factories. vii. Limestone, clay and gypsum are used to make cement. viii. Limestone, silica and different other minerals are used to make glasses. ix. Kaolin is used to make ceramics. x. Fertilisers, fungicides and pesticides are manufactured from petrochemicals. xi. Slate, marble, granite and stones are used for roofing, flooring and making walls. xii. Fluorides are used in toothpaste, xiii. Fossil minerals, uranium etc. are used as fuels. FACTS WITH REASONS Gold is used to make ornaments. Gold is used to make ornaments because it is shiny, malleable and rust-free. Important metallic minerals found in different locations of Nepal Minerals are naturally occurring inorganic substances in the Earth’s crust which have their own physical and chemical properties. Minerals could be metallic, non-metallic or energy minerals. More than 60 different types of minerals are discovered in Nepal. Hilly region contains about 83% of minerals in Nepal. Nepal has 21 types of metallic minerals, 23 types of insulators and fertilizer types mineral, 6 types of gemstones types minerals, 9 types of minerals that can be used for construction and 4 types of fossil minerals. Metallic minerals are iron, copper, lead, zinc, etc. Similarly, non-metallic minerals are limestone, marble, slate, silica, etc. Our country has many deposits of metallic and non-metallic minerals. Iron is found in Lalitpur (Phulchowki), Ramechhap (Those) andTanahun (Labdi). Copper is found in Dadeldhura, Bhojpur and Baglung. Lead is found in Lalitpur (Phulchowki), Makwanpur and Solukhumbu. Zinc is found in Lalitpur, Makwanpur and Sankhuwasabha. However, these minerals have not been extracted yet. MEMORY TIPS A rock is a combination of multiple minerals.

312 Minerals Limestone is found in Lalitpur, Dhading, Makwanpur, Udaypur, Sindhuli and Dang. It is the major raw material to manufacture cement. It is the major mineral resource extracted in our county. Similarly, marble is found in Lalitpur, Kavre and Dhading. It is used as quality construction and pavement material. It used to be mined in Phulchowki of Lalitpur but has stopped now. Silica is found in Makwanpur district. It is used for making glass. Magnesite is found in Dolakha and has been extracted in small quantities. It is used for insulation and lining of furnaces. Graphite is found in Ilam, Dhankuta and Sindupalchowk. It is used for making pencils, lubricants and electrodes. Other non-metallic minerals have notbeen extracted so far. Similarly, the Kathmandu valley has deposits of natural gas. Dailekh is also said to have small quantities of natural gas while Surkhet shows prospects of petroleum. Dang has deposits of coal but very little of it is mined. FACTS WITH REASONS We should use natural resources wisely. We should use natural resources wisely because it will bring prosperity to the country. Misuse of resources will deplete them and the future generation will suffer from their scarcity. HOT SKILL HIGHER ORDER THINKING SKILL 1. Despite having lots of mines, Nepal is not developing fast enough. Despite having lots of mines, Nepal is not developing fast enough because of lack of investment, technology and skilled manpower. 2. Minerals are found in different colour and hardness. Minerals are found in different colours and hardness because each minerals have different compositions from each other. Their composition determines their colour and hardness. 3. Write any two differences between metallic minerals and non-metallic minerals. Differences between metallic minerals and non-metallic minerals are: Metallic mineral Non-metallic mineral The minerals that contain significant amount of metals are called metallic minerals The minerals that do not contain significant amount of metals are called non-metallic minerals. Metallic minerals are simply metallic compounds Most of the non-metallic minerals are metallic compounds while some are also non-metallic compounds 4. Wise use of minerlas can bring prosperity in the country. Minerals resources available within the territory is wealth of the country. It can be utilized for the well being of the country. If used wisely it can bring prosperity in the country. i. Minerals are to be mined, processed, stored, transported, used to make things and sold. So it creates employments within the country. ii. It is good source of revenue for the country. iii. Minerals based industrial products can be exported from the country. iv. Locals can use minerals to construct infrastructures.

Modern Concept Science and Technology – 8 313 5. Every ore is a mineral but every mineral is not ore. Minerals contains elements or their compounds. Ores also contains elements or their compounds. Therefore an ore is a mineral. Ore is a rock that contains higher percentage of metals. Most minerals contain lower percentage of metals. So most of the minerals are not called ores. 6. We must recycle the minerals to conserve the environment. Minerals are non-renewable natural resources. Once finished from mines, it is lost for ever. So we should be recycling minerals as much as possible. Recycling will reduce pressure on mines. Minerals will last for many years in mines for future generation. 7. Spoon is made from steel. Steel is made from iron and carbon. Iron is obtained from the ore haematite. Haematite is mineral. What other things are made up of minerals around us? . Many things we use are made from minerals. Spoon for example is made from minerals (iron obtained from haematite). Similarly, ornaments are made from silver and gold. They are obtained from argentite and alluvial soil. Door knobs are made from brass. Brass is an alloy. It is made from copper and zinc. House is made from bricks and cements. Bricks are made from soil. Cement is made from clay and limestone. Toothpaste is made from water, fluorides and abrasives (crushed aluminium ores, limestone, white sands and zinc chlorides). 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) Which of the following is a hard mineral? i. topaz ii. graphite iii. talc iv. gypsum b) Which one is a soft mineral? i. Topaz ii. talc iii. quartz iv. diamond c) What is the mineral from which usuable metal can be extracted called? i. super minerals ii. rock iii. blister iv. ore d) Which one is a metallic mineral? i. haematite ii. quartz iii. diamond iv. slate e) Which one is non-metallic mineral? i. limonite ii. bauxite iii. obsidian iv. argentite 2. Write True for the correct and False for the incorrect statements. a) Minerals are the renewable natural resources. b) The minerals that do not contain significant amount of metals are called metallic minerals. c) Some minerals such as talc and gypsum are soft.

314 Minerals d) The property of minerals to resist breakage is called tenacity. e) Kaolin is used to make ceramics. 3. Fill in the blanks using a suitable word. a) The chemical compound which are inorganic, solid and has a definite chemical composition is called ............ b) Metallic minerals are the compounds of ............ c) The colour seen when the mineral is powdered is called ............ d) The ability of metal to be hammered into thin sheet is called ............ e) The ability of metal to be drawn into thin wire is called ............ 4. Answer the following questions in very short. a) Write the name of rock that contains large amount of metal in it? b) Give any two examples of metallic minerals. c) What is lustre? Give any two examples of minerals that have lustre. d) What is the characteristics of metal that does not allow it to regain its original shape when we stop bending it? e) Where is iron mainly found in Nepal? STEP2 5. Give reasons a) Some minerals are called ores. b) All of the ores are called minerals. c) Minerals are available in various colours. d) Minerals are non-renewable natural resources. e) We should recycle minerals. f) Wise use of minerals can bring prosperity in the country. 6. Differentiate between the following. a) Ores and minerals b) Metallic minerals and non-metallic minerals 7. Answer the following in short. a) What is minerals? Give two examples. b) What are metallic minerals? Give two examples. c) What are non-metallic minerals? Give two examples. d) What are ores? Write any two examples. e) Write names of few major minerals found in Nepal. f) Define fuel minerals with any two examples. STEP3 8. Answer the following questions. a) What are the uses of ores? b) Write down the physical properties of the minerals. c) Minerals are the foundation of physical development of the countries. How does it help us? Enlist. d) Write an essay on minerals found in Nepal.

Modern Concept Science and Technology – 8 315 Key terms and terminologies of the unit 1. Universe : The vast surrounding space which consists of everything seen or unseen including all planets, satellites, stars, galaxies, constellations, comets, dust particles, matter, energy, etc. is called the universe. 2. Big bang : The great explosion in which matter was formed in the universe for the first time about 13.8 billion years ago is called Big Bang. 3. Asteroids : The rocky and metallic objects which are present in between the orbits of Mars and Jupiter that revolve around the sun are called asteroids or minor planets. 4. Comet : A broom-shaped shining object with a bright head that revolves around the sun in a highly elliptical orbit is called a comet. 5. Meteors : The pieces of stone or metal which burn after entering into the earth’s atmosphere are called meteors. 6. Meteor shower : The event when multiple meteors fall through atmosphere is called meteor shower. 7. Meteorite : The meteor that does not burn completely in the atmosphere and reaches the earth surface is called a meteorite. 8. Stony meteorites : The meteorites which contain more amount of silica-based stones and small amount of iron and nickel are called stony meteorites. 9. Irony meteorites : The meteorites which contain more amount of iron and less amount of silicates are called irony meteorites. Sequence of Curriculum Issued by CDC  Introduction to the universe  Big Bang Theory  Introduction to comet and asteroids  Introduction and examples of constellations, galaxy, meteors and meteorites UNIT The Earth in the Universe 11.2 Estimated teaching periods Theory Practical 3 1 Georges Lemaitre is well known for the Big Bang theory. He was born in Belgium on 17th July 1894 and died on 20th June 1966. He was a cosmologist, astrophysicist and mathematician. He was the first to realize that the universe is expanding which was later confirmed by Edwin Hubble. He first derived Hubble’s law and Hubble constant which is now called Hubble-Lemaitre law. He proposed the Big Bang theory of the origin of the universe calling it the hypothesis of the primaeval atom. He was honoured with Francqui Prize and Eddington medal. He was educated at the Catholic University of Louvain, St Edmund’s House Cambridge and the Massachusetts Institute of Technology. Georges Lemaitre About the Scientist

316 The Earth in the Universe 10. Stony iron meteorites: The meteorites which contain nearly equal ratio of iron and silicates are called stony iron meteorites. 11. Galaxy : A galaxy is a stable system of thousands to billions of stars, dust and gases bounded together by the gravitational force. 12. Spiral galaxy : The galaxy which is spiral in shape forming a dense centre and long arms is called spiral galaxy. 13. Elliptical galaxy : The galaxy which is nearly round or elliptical in shape is called elliptical galaxy. 14. Irregular galaxy : The galaxy that does not have a regular geometrical shape is called an irregular galaxy. 15. Milky Way Galaxy : The spiral galaxy where solar system is present is called the Milky Way Galaxy. 16. Constellation : A group of stars having fixed shape and appears in night sky is called constellation. 17. Meteoroids : The rocks in space smaller than asteroids are called meteoroids. 18. Asteroid belt : The region in-between Mars and Jupiter where millions of asteroids are found is called asteroid belt. 19. Near-earth asteroid : The asteroids which are close to earth and intersect the earth's orbit are called near-earth asteroids. 20. Zodiac : The 12 constellations that lie in the solar path are called zodiac constellations. Introduction We live in the earth. The earth is one of the members of the vast region of the space called universe. The word universe is made up of two words: ‘uni’ meaning one and ‘versus’ meaning turned into. The vast surrounding space which consists of everything seen or unseen including all planets, satellites, stars, galaxies, constellations, comets, dust particles, matter, energy, etc. is called the universe. Universe is a huge secret. It is because the most fundamental questions like the shape, size, origin and expansion are not fully understood by science. The branch of science that studies the universe is called Astronomy. The branch of physics that studies the physical nature, laws and principles of astronomical objects and phenomena in the universe is called Astrophysics. The part of universe that is visible to us through telescope is called observable universe. Big Bang Theory The Big Bang theory was proposed by Georges Lemaitre in 1927. According to this theory, all the matter in the universe was originally concentrated in one vast lump called a primeval atom. About 13.8 billions years ago, this super dense and extremely hot body called primeval atom exploded with a big bang. After that, the matter started flying in all directions through space, leading to the formation of galaxies of the stars and other heavenly bodies. Now, we are nearly 13.8 billion years after the Big-Bang explosion, and it is believed that the universe is expanding continuously and will continue in the future as well. Universe at the moment is already so vast that it needs millions of years for light to travel from one galaxy to another. Edwin Powell Hubble discovered through his telescope that spiral nebula was moving farther away from the earth. From this he suggested that universe is expanding. Stephen Hawking supported the theory of creation of universe in 1970 through mathematical singularity called big bang. He published it in his book 'a brief history of time: from the big bang to black holes' in 1988.

Modern Concept Science and Technology – 8 317 Asteroids The millions of rocky and metallic heavenly bodies that range from few kilometres to hundreds of kilometres in diameter are called asteroids. They lie in between Mars and Jupiter. Thus, the rocky and metallic objects which are present in between the orbits of Mars and Jupiter that revolve around the sun are called asteroids or minor planets. The region in-between Mars and Jupiter where millions of asteroids are found is called asteroid belt. They revolve around the sun at a distance of 1.7 AU to 4 AU. The asteroids have two kinds of shape. i. Nearly spherical: which are now known as minor or dwarf planets ii. Irregular: Some of the irregular asteroids are Juno, Vesta, Pallas, Iris, Flora, Victoria, Eros etc. Beyond the solar system, behind the orbits of Neptune, numerous rocks formed during the origin of solar system still exist. It also contains asteroids, comets and dwarf planets. This region is called Kupier belt. Some celestial objects revolve around the sun by intersecting the orbit of major planets. These are called Trojans. For example, Pluto. The asteroids which are close to earth and intersect the earth's orbit are called near-earth asteroids. Ceres is the largest asteroid, now also considered as a dwarf planet because it is larger and almost spherical. Asteroids are sometimes referred to as baby planets and planetoids. Comets Comets are tailed flying bodies that can be observed in the sky. They are small bodies in the solar system which are made up of ice, dust, rocks and trapped gases. Thus, a broom shaped shining object with a bright head is called a comet. The comets that are present and formed inside the solar system are called endocomet. The comets that are formed outside the solar system are called exocomet. Exocomet can also revolve around the sun near by Kupier belt. It is made of gases, dust and ice. There are about 3296 known comets. A person may not see a particular comet twice in his/her lifetime. This is because it has a highly elliptical orbit around the sun due to which it takes many decades to revolve around the sun. A comet is generally a ball of ice, dust and gases. But, when it approaches near the sun, a tail is produced. The tail of the comet increases on approaching to the sun due to the solar radiation and solar wind. Actually a comet has two tails. They are dust tail and ion tail. Dust tail is made from dust. It is thicker and white. The ion tail is thinner and bright. It is formed from charged gas Fig: Asteroid belt Gas tail Dust tail Nucleus Coma Fig: Parts of a comet Fig: A comet revolving around the sun in highly elliptical orbit

318 The Earth in the Universe ions. The rays of the sun make its gas glow, which spread, out to form a tail millions of kilometres long. Comets have no light of their own. In a comet, there are three parts. They are: i) Nucleus or solid centre ii) Coma or the envelope of the burnt gases iii) Tail of the gases and dust Comets were discovered and recorded thousands of years earlier. But, for the first time, it was studied in detail by an English Astronomer Edmond Halley, after whom a comet is named Halley’s Comet. It is one of the most clearly visible comets from the earth. Some of the comets, their year of discovery and the revolution period are given below: Comets Year of discovery Revolution period (years) Halley’s comet 240 BC 76 Tempel-tuttle 1366 AD 33 Enke 1786 AD 3.3 Schwasman-washman 1927 AD 15 Bennett 1969 AD 1679 Shoemaker Levy 1993 AD Destroyed Hyakutake 1996 AD 70,000 A comet might also collide with other heavenly bodies in its path. In July 1994 AD, Shoemaker Levy collided with Jupiter due to the gravitational pull of Jupiter and extinct from its orbit. Hence, it will never be seen again. It is named so because it was discovered by Carolyn and Eugene M. Shoemaker (wife and husband respectively) and David Levy. FACTS WITH REASONS Comets produce a tail on approaching the sun. When a comet approaches the sun, some of its frozen matter begins to evaporate into gases due to the heat of the sun. These gases rush out due to the effect of solar wind and high speed of the comet. In this way, a long tail of a comet is formed when a comet approaches the sun. The tail of the comet becomes shorter on going farther from the sun. The tail of the comet becomes shorter on going farther from the sun. It is because the heating effect of the sun increases on approaching the sun which causes more ice and gases to become vaporized. But, when the comet goes far away from the sun, the heating effect decreases and thus the tail of the comet becomes shorter. The comets have a longer revolution period. Comets have a longer revolution period because their path around the sun is highly elliptical, due to which they spend a long period of time to revolve once around the sun. Some comets are never seen twice in our lifetime. Some comets are never seen twice in our lifetime because: i. The revolution time of the comet might be longer due to which it is not possible to see it again. ii. They might collide themselves with another heavenly body in their path causing them to be destroyed forever. iii. Some of them are melted permanently by sun. They leave a small rocky core which becomes asteroid or meteor. They are called extinct comet. MEMORY TIPS It is believed that a comet with rocky core that hit the earth wiped out dinosaurs 65 million years ago from the earth during Cretaceous period of Mesozoic era.

Modern Concept Science and Technology – 8 319 Meteoroids, Meteors and Meteorites Meteoroids are rocks that are smaller than the asteroids. They float in the solar system. They were probably formed during the formation of solar systems. They keep floating in space or inside solar system. Sometimes they enter the gravitational field of planets and change into meteors. A meteor is a space rock ranging from the diameter of a grain to several metres. It floats freely inside the solar system. Scientists believed that they are the leftovers of the formation of the solar system. If a meteor comes nearer to the earth, the gravity of the earth pulls it towards its atmosphere. Due to the pull of gravity, they strike with the earth’s atmosphere and burn as a result of friction. Thus, the pieces of stone or metal which enter the earth’s atmosphere are called meteors. Meteors drag themselves in the atmosphere and produce bright light. So, they are also known as ‘shooting stars’, falling stars’, or ‘fireball’. Meteors generally start burning at an altitude of 120 km to 7 km from the earth’s surface and accelerate with a speed of about 35 km/s to 95 km/s. This makes them visible from the surface of the earth as streaks of glowing gases, during the night. Sometimes, a large number of meteors strike the atmosphere at once, which produce a shower like appearance. Thus, the event when multiple meteors fall through atmosphere is called meteor shower. An extremely large and bright meteor is called fire ball. Most of the meteors get destroyed in the upper atmosphere when they pass from thermosphere to mesosphere. But, some bigger ones do not burn in the atmosphere completely and collide with the earth’s surface. This collision often produces a drastic effect upon the earth surface. The meteor that does not burn completely in the atmosphere and reaches the earth surface is called a meteorite. Such a collision occurred about 50,000 years ago in Arizona, USA. It created a large crater of length 1275 m and depth 175 m. The mass of the meteorite ranges from 100 grams to 60,000 kg. A meteorite called Hoba found in Namibia weighed more than 60,000 kg. About 200 meteors collide with the earth’s atmosphere every day but they do not reach the earth's surface because they completely burn in the upper layers of the atmosphere. On the basis of composition, meteors are classified into three types. i) Stony meteorites : The meteorites which contain more amount of silica-based stones and small amount of iron and nickel are called stony meteorites. ii) Irony meteorites : The meteorites which contain more amount of iron and less amount of silicates are called irony meteorites. iii) Stony iron meteorites : The meteorites which contain nearly equal ratio of iron and silicates are called stony iron meteorites. Differences between Comets and Meteors. S.N. Comets S.N. Meteors 1. A broom-shaped shining object with a bright head that revolves around the sun is called a comet. 1. The pieces of stone or metal which enter the earth’s atmosphere are called meteors. 2. They have a long glowing tail-like structure. 2. They have streaks of light falling down towards the earth surface.

320 The Earth in the Universe FACTS WITH REASONS Meteors called falling stars, but they are not actually stars. The light produced in the meteors is not due to nuclear reaction but it is due to the frictional force with the earth’s atmosphere. So, meteors are called falling stars but they are not actually stars. Meteors burn brightly on striking the earth’s atmosphere. The high velocity of the meteors with which they strike the earth’s atmosphere produces tremendous amount of friction and heat. It causes their molecules to become vaporized and burnt. Due to this, meteors burn brightly. A meteor is finished before reaching the earth's surface. When a meteor comes towards the earth's surface at a high speed, it burns completely due to the friction between the meteor and earth’s atmosphere. Therefore, a meteor is finished before reaching the earth's surface. Galaxy The universe is made up of trillions of stars. They are distributed non-uniformly as clusters. These clusters are bounded strongly due to gravitational force. The cluster of stars along with dust and gases is called galaxy. Thus, a galaxy is a stable system of thousands to billions of stars, dust and gases bounded together by the gravitational force. The diameter of the galaxy ranges from 1000 light years to 10,000 light years. The centre of galaxy around which the stars rotate is called a galactic centre. The observable universe consists of more than 1011 galaxies. Each galaxy is believed to have about 1011 stars. Therefore, the estimated number of stars in the visible universe is 1022. 1 Light year is the total distance travelled by light in vacuum in a year. It's magnitude is 9.46 × 1015 m. Milky Way Galaxy The galaxy where solar system is present is called Milky Way Galaxy. It is called Milky Way Galaxy because it appears as a milky circle. The nearest galaxy to Milky Way is Andromeda. The distance between the Milky Way Galaxy and the Andromeda is about 2.5 million light years. The time taken by the sun to revolve around the galactic canter of the Milky Way Galaxy is called cosmic year. It is equal to 225 million to 250 million years. Milky Way Galaxy contains about 1011 stars and also interstellar clouds. The Milky Way is about 1.33 × 1011 times larger than our sun. Types of Galaxies On the basis of shape of the galaxies, they can be divided into three major groups. They are: a) Spiral galaxy The galaxy which is spiral in shape forming a dense centre and long arms is called spiral galaxy. A spiral galaxy contains a large amount of interstellar dust and gases concentrated on their arms. Such galaxies are the Fig: Milkyway galaxy Fig: Spiral galaxy

Modern Concept Science and Technology – 8 321 brightest galaxies in the sky. Examples: Milky Way, Andromeda, etc. b) Elliptical galaxy The galaxy which is nearly round or elliptical in shape is called elliptical galaxy. It contains less amount of interstellar matter like dust and gases. It is brighter at the centre and gradually dim towards the edges. Examples: NGC 4486, IC 1101, etc. c) Irregular galaxy The galaxy that does not have a regular geometrical shape is called an irregular galaxy. It is less bright than the other galaxies. Examples: IC 559, Large Magellanic Cloud, etc. Constellations The word constellation is derived from the Latin word ‘constellatio’ which means ‘a set of stars’. When we observe in the clear night sky, we can see many stars arranged randomly. Among them some stars make a group. The stars present in the group do not change their relative positions with respect to each other. They resemble an animal, god or any other shape. These groups of stars are called constellation. Thus, a group of stars having fixed shape and appears in the night sky is called constellation. They are named after the shape they seem to resemble. Throughout the sky, altogether 88 constellations are discovered. Among them, 36 lie in the northern sky and 52 in the southern. Some of the northern constellations are Leo, Pisces, Taurus etc. and some of the southern constellations are Musca, Tucana, Telescopium, etc. Among the 88 constellations, there are 12 zodiacs. The 12 constellations that lie in the solar path are called zodiac constellations. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Some other constellations are Draco, Perseus, Cepheus, Ursa Major, Ursa Minor, Orion, Cassiopeia, Andromeda, etc. The constellations were named in the past from 1400 AD to 1700 AD by sailors. They named them after objects, gods and animals. Examples: Telescopium (telescope), Musca (housefly), Orion (hunter), etc. Hydra is the largest and the crux is the smallest constellation. Fig: Elliptical galaxy Fig: Irregular galaxy Fig: Zodiac constellation Fig: Constellation in northern sky

322 The Earth in the Universe The significance of the constellations is: i. The 12-zodiac constellations have astrological significance. ii. The position of the constellations in the sky at a given time helps to map the path of the sun across the sky. iii. Constellations help to predict the change in weather in the earth. iv. They help to study about the position of different stars in the sky. v. The stars in the constellations help the sailors to find their positions and destination in the sea. HOT SKILL HIGHER ORDER THINKING SKILL 1. Comets that are seen once may not be seen again. Comets have a highly elliptical orbit around the sun. Some comets even go out of the solar system and come back inside during their revolution. During their revolution, sometimes comets come near to the massive bodies like Jupiter, Saturn etc. and may collide with them. Similarly, some comets burn completely due to the solar winds and become invisible due to their smaller size. 2. Differentiate between Galaxy and Constellation. S.N. Galaxy S.N. Constellation 1. It is a collection of billions of stars. 1. It is a collection of limited number of stars. 2. It has no regular pattern. 2. It has a regular pattern. 3. There are about 1011 galaxies in the universe. 3. There are 88 constellations found so far. 3. Differentiate between Comets and Asteroids. S.N. Comets S.N. Asteroids 1. Broom-shaped shining objects with a bright head made up of ice are called comets. 1. Asteroids are those heavenly bodies which revolve around the sun mainly between Mars and Jupiter. 2. They produce long glowing tail-like structure when approaching to the sun. 2. They do not glow or make tail like structure when approaching sun. 4. Differentiate between Meteors (falling stars or shooting stars) and Stars. S.N. Meteors S.N. Meteorites 1. They are made up of stone, iron or both. 1. Meteorites are usually made of iron. 2. Metoers are the rocks from space that enters earth atmosphere due to earth’s gravity. 2. Meteors are the rocks from space that enters earth’s atmosphere and hit the earth. 5. Constellations help the sailors to find the direction in oceans. As constellations appear fixed in a certain direction throughout the year, the sailors will be able to find the direction by observing them. The pole star of the Ursa Minor is towards the north direction. Observing the pole star sailors find the direction.

Modern Concept Science and Technology – 8 323 6. Most meteors do not reach earth. Do you know why? Give your thoughts. Most meteors do not reach earth. It is because of the atmosphere. Atmosphere is the layer of air around of the earth. A meteors falls towards the earth with a great velocity. As a result there will be lots of friction between the meteor and the air. The friction generates sufficient temperature that starts ignition of the meteor. Most meteor burn to ashes before reaching the earth surface. 7. The Big Bang theory is considered plausible until today. As observed by Hubble, the galaxy are moving far away from each other in every direction. Therefore the expansion must have the origin some where. The Big Bang also suggest that the universe started from the explosion of primeval atom and is expanding ever since. Here the theory of Big Bang and the Hubble's observation support each other. Therefore Big Bang theory is plausible until today. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) What is universe? i. family of sun and its planets ii. group of everything that exists iii. collection of billions of stars iv. collection of few stars with definite shape b) Why do comets have tail? i. ice melts due to heat from sun ii. they pull materials behind them due to gravity iii. tail never leave comet iv. comet collects tail along the path c) What are the rocks from outer space that has reached earth surface called? i. meteorides ii. meteors iii. meteorites iv. meteor shower d) What is the group of billions of stars called? i. solar system ii. constellation iii. universe iv. galaxy e) Why is milky way called spiral galaxy? i. it moves in spiral path ii. it has spiral motion iii. it has arms around it iv. it is spherical 2. Write True for the correct and False for the incorrect statements. a) The branch of science that studies the universe is called chemistry. b) A meteor is generally a ball of ice, dust and gases. c) A meteor is a space rock ranging from the diameter of an atom to several metres. d) A meteorite called Hoba found in Namibia weighed more than 60,000 kg. e) The diameter of the galaxy ranges from 1000 light years to 10,000 light years.

324 The Earth in the Universe 3. Fill in the blanks using a suitable word. a) The ............ consists of more than 10¹¹ galaxies. b) Andromeda is about 2.5 million light years far away from ............ c) Spiral galaxies are the ........... galaxies in the universe. d) A group of stars resembling the shape of a god, animal or an object is called ............ e) Throughout the sky, altogether ............ constellations are discovered. 4. Answer the following questions in very short. a) Which heavenly object grows tail when comes closer to the star? b) What are the rocks that are not planets but revolves around the sun from the orbits between Mars and Jupiter. c) What is the cloud of evapourated ice present around the core of comet called? d) Which comet collided with Jupiter on 1994? e) Write the name of largest meteorite found in the earth. 5. Define the following. i) Universe iii) Comet v) Meteor shower vii) Galaxy ix) Constellation xi) Big bang xiv) Exocomet ii) Asteroids iv) Meteors vi) Meteorite viii) MilkyWay Galaxy x) Zodiac xii) Endocomet xv) Meteoroid STEP2 6. Give reasons a) Ceres is called a dwarf planet. b) Comets produce a tail on approaching the sun. c) The tail of the comet becomes shorter on going farther from the sun. d) Comets have a longer revolution period. e) Some comets are never seen twice in our lifetime. f) Meteors are called falling stars but they are not actually stars. g) A meteor is finished before reaching the earth's surface. h) Milky Way galaxy is called spiral galaxy. i) Constellations are important. 7. Differentiate between the following. a) Comets and Meteors b) Galaxy and Constellation

Modern Concept Science and Technology – 8 325 c) Asteroids and comets d) Meteors and meteorites e) Constellation and Asteroids. 8. Answer the following in short. a) Write the name of following cellestial objects. i. A ball of ice with a tail that revolves around sun ii. An irregular rock that revolves around the sun from space between mars and jupiter. iii. A group of few stars iv. A group of billions of stars v. Space rocks falling towards the earth b) What is an asteroid belt? Where is it located? c) What do you mean by coma of the comet? d) What is spiral galaxy? Give any two examples. STEP3 9. Answer the following questions. a) How did universe came to be according to big bang theory? b) Describe the types of meteors based on their composition. Which one of them has higher probability to hit earth? c) What is galaxy? Explain the types of galaxies. d) Explain in short about Milky Way Galaxy. e) How is constellation useful to us? Enlist some of its importance. Project Work Project Work Look in the clear night sky and observe different types of constellations like Ursa Major, Ursa Minor, Orion, etc. Sketch their diagrams and write a short description.

326 History of Earth Key terms and terminologies of the unit 1. Hypothesis : An early theory of supposition which is formed on the basis of scientific ideas and reasoning but might lack experimental proof is called a hypothesis. 2. Nebula : The molecular clouds of dust, gases and hydrogen is called nebula. 3. Old planetisimal hypothesis : The hypothesis in which solar system was formed due to collision between star and a comet is called old planetesimal hypothesis. 4. New planetesimal hypothesis: The hypothesis in which the passing of a giant star nearby caused tides in the sun, which cooled down to form planetismal that later changed into planets and satellites is called new planetesimal hypothesis. 5. Nebular hypothesis : The hypothesis in which the gravity condenesed nebula into rotating core which changed into sun, whereas the surrounding nebula changed into planets and satellites is called nebular hypothesis. 6. Tidal hypothesis : The hypothesis in which a passing giant star pulled a cigar shaped tide from sun, which detachded from sun, cooled down and formed the planets and satellites is called tidal hypothesis. 7. Binary star hypothesis : According to this hypothesis ' our solar system had two stars. They were rotating each other. There was another star far bigger than the sun. That star suddenly flew very close to the sun and formed tides on the sun. The tides formed in the sun at that moment break from the sun. It cooled down and changed into planets and satellites. Sequence of Curriculum Issued by CDC  Introduction to the history of the earth  Some hypotheses about origin of the earth (George wofan hypothesis, Nebular hypothesis, Tidal hypothesis and Dust and cloud hypothesis)  Some important facts about the earth  Geological time scale, introduction of four era  Evolution of life UNIT History of Earth 11.3 Estimated teaching periods Theory Practical 2 1 George Buffon is well known for his old planetesimal hypothesis. He was born in France on 7th September 1707 and died on 16th April 1788. He was a French naturalist, mathematician, cosmologist and encyclopaedist. He was the first naturalist to recognize ecological succession, geological history and animal evolution. He also proposed that earth, along with other planets and satellites was formed when a comet moving around the universe stroke the sun millions of years ago. Georges Buffon About the Scientist

Modern Concept Science and Technology – 8 327 8. Geological time scale : Geological time scale is the representation of a series of major geological and biological changes throughout 4.5 billion years' history of the earth. 9. Eon : An eon is the largest unit of geological time scale consisting of millions of years. 10. Era : An era is the smaller unit of geological time scale which represents the events that occurred during an eon. 11. Pre cambrian era : The first era that lasted from 4.5 billion years ago to 570 million years ago is called pre-cambrian era. 12. Paleozoic era : The era of fishes that lasted from 570 million years ago to 250 million years ago is called palaeozoic era. 13. Mesozoic era : The era of reptiles that lasted from 250 million years ago to 65 millions years ago is called mesozoic era. 14. Cenozoic era : The era of mammals that started 65 millions years ago till today is called cenozoic era. 15. Evolution of life : The process in which primitive living being slowly but gradually modify, adapt and change into newer species after millions of years is called evolution of life. Introduction It is very difficult to say exactly when the earth originated. It is because the history of human survival is very recent but some studies suggest that the earth was originated about 4.5 billion years ago. Scientists differ in their opinions about how the earth originated. But, most of them agree that the sun and the earth were formed in the past tentatively at the same time. Yet, there is no concrete evidence about how the solar system came into existence. There are different hypotheses regarding the origin of the solar system. Some of the hypotheses regarding the origin of solar system are given below. 11.3.1 Hypothesis on Origin of the Earth Old planetesimal hypothesis or George Buffon’s hypothesis According to this hypothesis, a comet collided with a huge star resulting the scattering of matters around it. The matters cooled down to form planetesimals. The planetesimals combined to form the planets. The remaining mass formed the sun. This hypothesis was proposed by Georges Buffon in 1749 A.D. New planetesimal hypothesis New planetesimal hypothesis was proposed by Moulton and Chamberlin in 1900. According to this hypothesis, a giant star almost collided with the sun. During near-collision, hot gases were pulled out from both stars and the gas condensed to form planetesimal (small space rocks). The planetesimal combined to form the planets and satellites. The debris formed asteroids and comets. Features of Planetesimal hypothesis i. The planets were very hot in the beginning but cooled down later as they went far from sun. Fig: Nebular hypothesis

328 History of Earth ii. All the celestial objects in solar system must have come out of sun because there total mass does not sum up to even 1% of solar mass. Nebular hypothesis or Laplace-Kant hypothesis More than 4.5 billion years ago, there were no sun, planets and the satellites. In the space there were molecular clouds of dust and hydrogen. The molecular clouds of dust, gases and hydrogen is called nebula. Immanuel Kant in 1755 AD suggested nebular hypothesis regarding the origin of the solar system and it was modified by Pierre-Simon Laplace in 1796 A.D. Due to some reasons, the clouds of nebula pulled themselves together towards their common centre. This caused the whole cloud structure to rotate around increasing mass. As mass increased, the gravitational force also increased. This process continued and most of the mass concentrated at the centre creating extreme temperature and pressure. Due to extreme temperature and pressure, nuclear reaction started. Finally, the sun was formed at the centre. The remaining matters started revolving around the sun and cooled down later to form the planets. Features of Nebular hypothesis i. Planets were cold to begin with. Some of them might have melted in early stage due to collision with meteorites. ii. Since the planets and dwarf planets were formed from the ejected materials of rotating nebula, all of them revolve around the sun. Tidal hypothesis or Jeans and Jeffreys hypothesis This hypothesis was put forward by Sir James Jeans and Harold Jeffrey in 1917 A.D. It is also called Jeans and Jeffreys hypothesis. Its main idea is that before the solar system existed, there was a huge star, many times bigger than the current size of the sun. When another star approached near the sun and passed by, it pulled the mass of the sun in the form of hot gases and formed tides. These tides detached from the sun and started revolving round the remaining sun due to gravitational force. Finally, the detached matters cooled down to form the planets. Features of Tidal hypothesis i. The tide was tapered on both end. So planets near the sun and the planets far away from the sun are small. Similarly, the planets in the middle regions are bigger. ii. Mars was supposed to be bigger than the earth but it is smaller. It may be smaller than the earth because its certain part broke off from it. Binary star hypothesis The binary hypothesis was proposed by Thomas Chrowder Chamberlin and Forest Ray Moulton. According to this hypothesis ' our solar system had two stars. They were rotating each other. There was another star far bigger than the sun. That star suddenly flew very close to the sun and formed tides on the sun. The tides formed in the sun at that moment break from the sun. It cooled down and changed into planets and satellites. However, there is no sign that the sun ever had a binary star. Fig: Tidal hypothesis

Modern Concept Science and Technology – 8 329 About the Earth Estimated age 4.5 billion years Total surface area 509,700,000 square km Area of the lithosphere 148,400,000 square km Area of the hydrosphere 361,300,000 square km Average surface temperature 15o C Average relative density 5.5 Mass 6 × 1021 metric tonne or 6 × 1024 kg Satellite Moon (1) Diameter 12756.3 km 11.3.2 Geological Time Scale Scientists tried to estimate the age of the earth by different methods like studying the age of fossils, age of the rocks, examining the earth’s crust, etc. They also tried to understand about the incidents that occurred at different time scales in the history. During the dramatic 4.5 billion year history of the earth, it has gone through a series of major geological and biological changes. Geologic time scale describes the order and duration of major events on the earth that have occurred since its origin. Thus, geological time scale is the representation of a series of major geological and biological changes throughout 4.5 billion year history of the earth. Geological time scale covers a vast expanse of time. It is divided into various time intervals. According to chronology, they are arranged in the order of eon, era, period and epoch. An eon is the largest time unit and epoch is the smallest time unit in the geological time scale. Eon An eon is the largest unit of geological time scale consisting of millions of years. The whole geological time scale is divided into four major eons they are Hadeon, Archaen, Proterozoic and Phanerozoic eons. Some geologists also classify geological time scales into two eons. They are Cryptozoic eon (4.6 billion years ago to 570 million years ago) and Phanerozoic eon (570 million years ago onward). They divide cryptozoic eon into Archeozoic era, Proterozoic era and Eozoic era. Sometimes cryptozoic eon is also called azoic era or Precambrian era. Phanerozoic eon is divided into three eras. They are Paleozoic era, Mesozoic era and Cenozoic era. Era An eon is divided into subdivisions called eras. They are the smaller units of eons. Thus, an era is the smaller unit on geological time scale which represents the events that occurred during an eon. The geological time scale is divided into four eras. They are Precambrian era, Palaeozoic era, Mesozoic era and Cenozoic era. In this unit, we will learn about some of the major geological and biological events in these four eras. Fig: Geological time scale

330 History of Earth a) Pre-Cambrian Era : This era refers to the time scale from the origin of the earth (4.5 billion years ago) to 57 crore years ago. During the early years (4.5-3.8 billion), there was no sign of life. Only organic and inorganic compounds were found in the vast lifeless oceans. The first sign of life, which was a bacterium, was found in this era about 3.8 billion years ago. Similarly, algae were also one of the first living organisms to evolve about 3.2 billion years ago. Several geological and meteorological changes also occurred in this era. The igneous rocks like gneiss and granite were formed from the cooling of hot molten materials released from the frequently erupting volcanoes. At the end of this era, several marine invertebrates evolved which ruled the oceans for thousands of years. But, most of those creatures are extinct today. b) Paleozoic Era : This era marks the time period from 57 crore years ago to 25 crore years ago. During this era, several terrestrial and marine invertebrates evolved and died. The fossils found on the strata of the sedimentary rocks provide the evidence of the existence of different plants and animals in this era. The atmosphere was formed and climate was significantly changed. Several terrestrial and marine invertebrates and vertebrates such as winged insects, fishes, amphibians and first reptiles were evolved. Different kinds of green plants like bryophytes and pteridophytes were evolved. There are six periods in this era. They are Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. This era is also called the age of fishes as fishes evolved, developed and flourished in this era. FACTS WITH REASONS Palaeozoic era is called the era of fishes. As fishes of various species evolved, developed and flourished in the oceans of Palaeozoic era, it is called age of fishes. c) Mesozoic Era : The Mesozoic era extends from 25 crore years ago to 6.5 crore years ago. In this era, mountains and hills were formed. The climate slowly became favourable for aquatic, terrestrial and aerial animals. In this era, different types of dinosaurs evolved, developed, flourished over a large area and finally became extinct. So, it is also called the age of dinosaurs. The Mesozoic era is divided into three periods. They are Triassic, Jurassic and Cretaceous. Dinosaurs evolved in the Triassic period, flourished in the Jurassic period and became extinct in the Cretaceous period. Not just the dinosaurs, different types of other reptiles also dominated this era. Hence, this era is also called the age of reptiles. The reptiles in the Mesozoic era were larger than the reptiles of the current age. Coniferous plants evolved in Mesozoic era. Coniferous plants such as ginkgo, ephedra, cycas, pines, etc. were developed in this era. It is also called age of conifers. Most dinosaurs went extinct due to meteor shower, natural disasters, forest fire and air pollution. Dust and smoke covered the sky and made air poisonous for many years, which killed almost all dinosaurs. Fig: Mesozoic era

Modern Concept Science and Technology – 8 331 FACTS WITH REASONS Mesozoic Era is called the era of creepers (or age of reptiles). Different kinds of large and small reptiles evolved, developed, flourished and dominated the Mesozoic era. So, the Mesozoic Era is called the era of creepers (or age of reptiles). No single dinosaur is present today. Most of the large creatures including dinosaurs were destroyed by the hit of the meteorites, volcanic eruptions and climate change in the Cretaceous period of the Mesozoic era. So, no single dinosaur is present today. d) Cenozoic Era : This era extends from 6.5 crore years ago to the present time. In this era, fragmentation and displacement of the rocks occurred. Similarly, mountains also grew in height along with the change in climate and the earth crust. The mountain peaks were covered with ice. In the middle of this era, the huge ice age began, which caused the extinction of the several creatures in the earth. Along with the end of ice age, new organisms evolved. Mammals like horses, elephants, whales, rat, cat, humans, chimpanzees, etc. evolved in cenozoic era. Therefore, Cenozoic era is also known as the era of mammals. Apes (e.g. human, gorilla, chimpanzee and orangutan) were dominant in this era. So, this era is also called the Age of apes. Gymnosperms and angiosperms were also evolved in this era. Evolution of early human beings took place in this era about 6 million years ago. New species of humans that walked in two feet evolved 4 million years ago. Modern humans, Homo sapiens or cro-magnons evolved 56,800 years ago. Human ancestors passed through Stone Age, Bronze Age, Iron Age and lastly the agriculture and animal husbandry since last 11,700 years ago. 11.3.3 Evolution of Life The earth is 4.5 billion years old. According to the geological data it will last for another 4.5 billion years. When the earth first originated, it was hot and molten. There was a large ocean of molten rocks but no water, oxygen, ozone or life on it. Over millions of years, the earth slowly started to cool down. Millions of comets carrying ice crystals struck the earth forming oceans. In the Pre-Cambrian Era, the ocean along with the hot chemical soup at its floor allowed to form the first prokaryotic life, i.e. a bacterium about 3.8 billion years ago. These bacteria lived in colonies and Fig: Cenozoic era MEMORY TIPS In cenozoic era, first human beings, toothed animals, gymnosperms and angiosperms evolved and developed. Trilobites fossil Brachiopods fossil

332 History of Earth were called stromatolites. They started photosynthesis and produced oxygen at a large scale. Slowly, algae evolved and dominated the oceans about 3.2 billion years ago. For the first time, marine invertebrates evolved in the Palaeozoic era. Some of them were called trilobites, brachiopods etc. Gradually, the ocean was full of marine invertebrates. These marine invertebrates slowly evolved into marine vertebrates and terrestrial organisms. In Palaeozoic era, green plants and forests evolved in the land. Different fishes evolved in water and several creatures of smaller size also evolved. Most of the animals were extinct towards the end of Palaeozoic era due to volcanic eruption and low temperature in Polar regions. After millions of years, in the Mesozoic era, large reptiles evolved, developed and ruled the world. Dinosaurs were dominant. In the Mesozoic era, there were flying dinosaurs, carnivorous dinosaurs, herbivorous dinosaurs and so on. But, at the end of the Mesozoic era, in the Cretaceous period, it is believed that a large meteorite struck the earth and killed all the dinosaurs. Only few small organisms living on land, oceans and underground could survive. At the same time, coniferous plants evolved and developed. After that, flowering plants, insects and small mammals evolved. Among them, few organisms developed to form large mammals. For the first time, in the Cenozoic era, different kinds of small to big mammals evolved. They were whale, horse, rat, cat, dog, monkey, crocodiles, etc. In Pliocene epoch, early human beings evolved. These early human beings looked like humans partially. Finally, in the Holocene epoch, the modern man evolved. The scientists believe that humans first originated in Africa and then moved to Asia, America and Europe. It is also evident that the evolution of organisms is an ever going process. Geological history and evolution of life Era Duration Evolution of life Precambrian 4.5 billion years ago to 570 million years ago Origin of the earth Evolution of unicellular organisms and development of multicellular organisms Palaeozoic 570 million years ago to 250 million years ago Development of a few marine organisms Development of vertebrates in marine water Evolution of life on land, green vegetation and development of fishes Evolution of winged insects, development of fishes and plants Evolution of amphibians and reptiles, development of fern plants Extinction of many invertebrates and vertebrates, development of reptiles Carnivore dinosaur Flying dinosaur

Modern Concept Science and Technology – 8 333 Mesozoic 250 million years ago to 65 million years ago Evolution of dinosaur, reptiles and coniferous plants Dominance of reptiles, well-developed dinosaurs, birds and other organisms Evolution of flowering plants and different insects, extinction of dinosaurs and other large reptiles Cenozoic 65 million years ago to the present Evolution of first mammal and horse Development of various mammals like elephant, whale, etc. and development of owl Evolution of sharp-toothed cats and monkeys Development of grasses Evolution of human beings Development of Stone Age human beings and giant hairy elephants, i.e., mammoths Development of modern human beings and other mammals, dominance of human beings FACTS WITH REASONS Living beings evolved millions of years after the formation of the earth. After the formation, the earth took a long time to cool down and to form essential substances like amino acids, nucleic acids, water, oxygen, carbon dioxide, etc. as these substances are the bases for life. So, living beings were evolved millions of years after the formation of the earth. HOT SKILL HIGHER ORDER THINKING SKILL 1. Do people know how exactly universe came to be? What are those hypothesis on origin of the earth and solar systems? Are they facts? No! people do not know how exactly universe came to be. The available hypothesis on origin of the earth and the solar systems are educated guesses based on what humans have learnt by observing the visible universe. These hypothesis are not facts but educated predictions only. 2. Palaeozoic era is called age of fishes. Palaeozoic era is called age of fishes because different species of fishes were evloved during that era. Fishes were dominant beings on the water. 3. The earth and the human beings are not of the same age. Is this statement true? Justify your answer. The age of the earth is determined by the study of rocks of different places and depths. Rocks are the oldest substances on earth. So, they give a good estimate of the age of the earth. From the study of rocks, scientists found that the age of the earth is about 4.5

334 History of Earth billion years. But, when the same kind of study was conducted on the fossils of ancient humans, scientists found that these fossils were only few crore years old. Hence, the earth and humans are not of the same age. 4. How can we tell that the first life originated in the water? The only way to know about the organisms that lived in the earth millions of years ago is by the study of fossils. The study of fossils revealed that the oldest fossils belonged to the aquatic animals. So we can conclude that first life must have originated in water in the earth. 5. Explain the trend of evolution and extinction of life from 57 crore to 25 crore years ago. The time interval from 57 crore to 25 crore years ago represents the Palaeozoic era. In this era, for the first time, marine invertebrates were evolved in the oceans. Some of them were called trilobites, brachiopods, etc. Gradually, the ocean was full of other marine invertebrates most of which are extinct today. These marine invertebrates slowly evolved into marine vertebrates and terrestrial organisms. In the Palaeozoic era, green plants and forests evolved on the land. Different fishes evolved in water and several creatures of smaller size also evolved. Most of the animals were extinct towards the end of Palaeozoic era due to volcanic eruption and low temperature in the polar regions. 6. Why are there so many different speceis of living things in earth? There are so many different species of living things in the earth because of evolution. As the millions of years kept passing by, the environment, climate, presence of resources also changed accordingly. And the living things also evolved differently to adapt to their changing environment. As a result, varities of species of living things evolved in different periods of different eras. 7. Differentiate between pre-cambrian era and cenozoic era based on origin of life and geological activities. The differences between cenozoic era and pre-cambrian era are: S.N. Pre-cambrian era S.N. Cenozoic era 1. Origin of first life occured in precambrian era. 1. Evolution of mammals occured in cenozoic era. 2. Formation of igneous rocks were common. 2. Fragmentation and displacement or rocks, increase in height of mountains, ice age started and ended in this era. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) In which hypothesis a comet strikes with a star to form solar system? i. nebular hypothesis ii. tidal hypothesis

Modern Concept Science and Technology – 8 335 iii. dust and cloud hypothesis iv. old planetesimal hypothesis b) What is the estimated age of the earth? i. 4.5 billions years ii. 10000 years iii. 6000 years iv. 3.8 billion years c) Which era is known as age of fishes? i. Precambrian era ii. Palaeozoic era iii. Mesozoic era iv. Cenozoic era d) What is a mesozoic era popularly known as? i. golden age of pisces ii. golden age of amphibians iii. golden age of reptiles iv. golden age of mammals e) In which era did modern humans evolve? i. cenozoic ii. pliocene iii. holocene iv. mesozoic 2. Write True for the correct and False for the incorrect statements. a) New planetesimal hypothesis was proposed by Immanuel Kant in 1900. b) The nebula is the cloud of dust and gases in the space. c) An eon is the largest time unit and epoch is the smallest time unit in thegeological time scale. d) The mesozoic era is divided into triassic, jurassic and cretaceous period. e) Mammals like horses, elephants, whales, rat, cat, humans, chimpanzees, etc. evolved in cenozoic era. 3. Fill in the blanks using a suitable word. a) The planetesimal will combine together to form ............ b) The representation of a series of major geological and biological changes throughout 4.5 billion year history of the earth is called ............ c) The smaller unit on geological time scale which represents the events that occurred during an eon is called an ............ d) The fossils found on the strata of the .................... rocks provide the evidence of the existence of different plants and animals in this era. e) Dinosaurs evolved in the Triassic period, flourished in the Jurassic period and became extinct in the ............ 4. Answer the following questions in very short. a) Who proposed old planetesimal hypothesis? b) Who and when introduced tidal hypothesis? c) What is the mass of the earth? d) In which era did horse evolve? e) In which era did dinosaur evolve? 5. Define the following terms with required examples. i. Universe ii. Big Bange theory iii. Old planetesimal hypothesis iv. New planetesimal hypothesis

336 History of Earth v. Tidal hypothesis vi. Nebula hypothesis vii. Binary star hypothesis viii. Geological time scale ix. Eon x. Era xi. Precambrian era xii. Palaeozoic era xiii. Mesozoic era xiv. Cenozoic era STEP2 6. Give reasons a) Palaeozoic era is called the era of fishes. b) Mesozoic era is called the era of creepers (or age of reptiles). c) No single dinosaur is present today. d) Living beings evolved millions of years after the formation of the earth. e) Cenozoic era is called age of mammals. f) Study of fossil is very important to learn about history of the earth. 7. Differentiate between the following. a) Old planetisimal hypothesis and tidal hypothesis b) Pre-cambrian era and palaeozoic era c) Mesozoic era and cenozoic era 8. Answer the following in short. a) Write down the time periods of Pre-cambrian era, Palaeozoic era, Mesozoic era and Cenozoic era. b) Write in short about the facts of the earth. c) In which era did the following evolve? fish, fern, first human, dinosaur, mammals, and reptiles d) How do we know that dinosaurs existed in the past? STEP3 9. Answer the following questions a) Explain the formation of solar system based on new planetisimal hypothesis. b) Explain Laplace-Kant’s Nebular hypothesis about the formation of the solar system. c) Describe the Tidal hypothesis. d) Explain George Wofan hypothesis. e) How did solar system formed according Binary star hypothesis? f) Explain when dinosaurs evolved, flourished and became extinct. g) Describe the evolution of life in different eras. h) How did dinosaurs become extinct?