 Questions with solution for Holly faith publication for class 8 Learnwell Science Question: Why plastic should not be burnt? Answer: Burning of plastic release toxic gases which causes air pollution that result various health hazard. This is why we should avoid burning plastic. Question: Why does handle of cooking utensils are made up of Bakelite? Answer: Bakelite is a kind of thermosetting plastic that do not get deform on heating at high temperature. This is why handle of cooking utensils are made up of Bakelite. Question: Avoid wearing synthetic clothes while cooking. Give reason? Answer: We should avoid wearing synthetic clothes while cooking because synthetic clothes catch fire easily and melt before burning. Question: Give reason; Handle of heating pans are made up of plastics? Answer: Handle of heating pans are made up of plastics because plastic being bad conductor of heat do not allow heat to pass into body. Question: Electric wires are covered with plastics. Give reason? Answer: Since plastics are poor conductor of electricity, do not allow current to pass through them. This is why electric wires are covered with plastics Question: Why do electric plug not melt even the wire inside then get overheated? Answer: Electrical plugs are made up of Bakelite that do not get deform on heating at high temperature. Question: Why does rayon smell like burning paper; although it is a synthetic fibre? Answer: Rayon smell like burning paper; although it is a synthetic fibre because rayon are obtain on mixing celuse a extract of wood pulp into chemicals. Question: Why are plastic considered as environmental hazard? Answer: Plastic considered as environmental hazard because of the following reason: (i) Plastics do not decay in soil by natural process and cause soil pollution (ii) Dumping of plastic into water cause threat to aquatic life. (iii) Plastic bags thrown in to drain choke it out and support growth of disease causing pathogen like mosquito. Question: Distinguish between Thermosetting plastics and Thermoplastic Plastic? Answer: Thermosetting Plastic Thermosetting plastics are kind of plastic which once moulded cannot moulded again by heating. Thermosetting plastics are kind of plastic whose polymers form highly cross linked chain on heating. Bakellite , melanin, Formaldehyde are some of the examples of Thermosetting Plastic Thermoplastic Plastic Thermoplastic are kind of plastic which can be moulded again and again by heating and easily reused. PVS, Polysterine , nylon , polythine are some of the examples of Thermoplastic Plastic Polymers of thermoplastic Plastic have long chain without cross linked to each other. Question: What is a synthetic fibre? Answer: Fibres made by human beings using chemical substance is called synthetic fibre. A synthetic fibre is a chain of small unit of chemical substance joined together. These small units combine to form a large single unit called a polymer. The word ‘polymer’ comes from two Greek words; poly meaning many and mer meaning part/unit. So, a polymer is made of many repeating units. Question: List four measures to reduce the use of plastics and pollution caused by it? Answer: (i) Do not throw plastic bags in the water bodies or on the road. (ii) Take a cotton carry-bag or a jute bag while going for shopping. (iii) Try to minimise the use of plastic materials e.g., use a steel lunch box instead of a plastic one. (iv) Dispose plastic properly and store separately in bin at home. Learnwell science class 8 chapter force and pressure and Friction Read  
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A. Answer these questions 1. Q. what do we do to change the speed or direction of the motion of a moving body Answer: We have to apply force to change the speed or direction of the motion of a moving body. 2. What makes a moving ball to rest in a while even is no force is applied Answer: Force of friction brings the moving ball to rest in a while even is no force is applied. 3. What makes a ball thrown up into the air fall back to the ground? Answer: Gravitational force of earth causes the ball to fall back to the ground 4. Mention the three types of force that can act from a distance? Answer: Three types of force that can act from a distance are: (a) Gravitational force (b) Frictional force (c) Magnetic force 5. Mention the three disadvantages of friction between the parts of a machine? Answer: Three disadvantages of friction between the parts of a machine are as follows: (a) It causes surfaces to wear down. (b) It produces heat. (c) It wastes energy 6. what is pressure? what is the SI unit of pressure? Answer: Pressure is defined as the perpendicular force acting on unit area of an object. Presure = Threust/Area The SI unit of pressure is N/m2 or pascal 7. Define 1 pascal? Ans: 1 pascal is pressure act on applying 1 n force on area of 1m2 8. What makes a coaster placed over the rim of a glass of water to stick to the glass even when the glass is inverted. Answer: Atmospheric pressure causes a coaster placed over the rim of a glass of water to stick to the glass even when the glass is inverted. 9.Why is cutting edge of knife made sharp? Ans: The cutting edge of knife made sharp so that force distributed over small area creating large pressure. This makes cutting easier. 10. What is the SI unit of force? Answer: The SI unit of force is Newton (N). 1 N = kg x m/s2 11. What is the CGS unit of force? Ans: The CGS unit of force is the dyne which is defined as 1 g x cm/s2, B. Answer these questions 1. Give two example of situation in which it is helpful to increase friction? Ans: (i) applying brakes (ii) walk on the ground (ii) Write with pencil on paper 2. What is streamline body? How it is helpful for a body to have such a shape? Answer A streamlined body is defined as a body with narrow front and back and brooder at middle. This reduces the air and water resistance to its movement. 3. How does oil and ball bearing help to reduce friction? Answer (a) Oil reduces friction by filling up the dents on the surfaces that move against each other in a machine. It also prevents direct contact between the surfaces by forming a film between them. (b) Ball bearing converts sliding friction into rolling friction, which is less than sliding friction. This helps in reducing friction. 4. When you push something heavy, the resistance you feel decreases as the body starts moving. Explain with examples? Answer : When we push something heavy, the resistance offered by friction force we feel decreases as the body starts moving because friction force has limit strength. For example when we apply force to move table , force applied at first used to manage the friction force. As soon as applied force became greater than the frictional force body stat moving and we feel least resistant. 5. (a) What are the characteristics of magnetic, electrostatic and gravitational forces have in common? (b)What happen when a body leaves the earth gravitational field? Answer : (a) Magnetic, electrostatic and gravitational forces are non contact force that can act from a distance. (b) When a body leaves the earth's gravitational field, feel weightless. For example astronauts in space feel weightlessness because the earth's gravity has less effect on them. 6. (a) If every object attract every other object in the universe, why do we not attract each other? (b) Why is moons force of gravity is lesser than that of earth? Answer Answer (a) Gravitational force depends on the mass of two bodies concerned. Therefore, gravitational force act between the bodies on the earth is so small that it cannot be felt. (b) Gravitational force depends on the masses of the two bodies. Moon has considerably less mass than the earth itself. Therefore, force of gravity of moon is less than that of the earth. 7. Calculate the weight of 1 Kg mass. Answer: Mass = 1 Kg, g = 9.8 m/s2 Weight = Attraction force on a body by earth = m x g (Acceleration due to gravity) W = 1 x 9.8 = 9.8 N Thus, Weight of 1 Kg mass is 9.8 N. 8. What is Centripetal force? Answer: When a body moves in a circular path, it changes its direction at every point. The force which keeps the body in the circular path acts towards the centre of the circle. This force is called centripetal force. If there is no centripetal force, the body will move in a straight line tangent to the circular path. 9. If the moon attracts the earth, why does the earth not move towards the moon? Answer: The earth is much larger than the moon so, the acceleration produced on the earth surface cannot be noticed 10. The gravitational force acts on all objects in proportion to their masses. Why, then, a heavy object does not fall faster than a light object? Answer: Acceleration due to gravity does not depend on mass of object . Hence, all bodies fall with the same acceleration provided there is no air or other resistance C. Answer these questions 1. What does a spring balance measure? How do you demonstrate the principal on which it works? Answer Spring balance measures the weight of a body. Take a spring and hang it on a nail and note the length. Now, hang a small stone from the other end of the spring and note the increase in length. Hang a bigger stone and note the change in length of the spring again. It would be observed that the bigger the stone, greater is the extension of the spring, as a bigger stone has more mass. Therefore, it is pulled by the earth with a greater force. The extension of the spring gives the weight of the body. This is the principle on which a spring balance works. 2. How is pressure in liquid related with depth? How do you demonstrate this? Answer: Pressure increases with the depth of a liquid. Let us prove it with the following example. Take a plastic bottle filled with water. Now make four holes one above another from top to bottom in a line. The flow of water from the bottom whole travel furthest because pressure exerted at his point is the heighest. Or, Utube and fix it on a board. Fill half of the tube with water such that both the arms of the tube have equal water level. This will work like a manometer. Now, attach one end of one meter rubber tube over one end of Utube and attach funnel to the other end of the rubber tube. Stretch a thin balloon over the mouth of the funnel and fix it with thread or rubber band. Lower the funnel slowly into a bucket of water. The difference between the heights of water in the two arms of manometer will increase as the funnel goes deeper into the bucket of water. This shows that pressure increases with the depth of the liquid. 3. How does friction help and creates problems when you cycle? Answer: Friction helps us in cycling in the following ways: 1. It helps us in applying brakes. 2. It helps the cycle to move. 3. It keeps our feet on the pedals. 4. It helps us in gripping the handlebar. 5. It helps us to sit on the saddle. Friction creates problems while cycling in the following ways 1. Friction between moving parts wastes energy. 2. Friction causes wearing down. 3. Air resistance wastes our energy. D. Complete the sentence 1. Rolling friction is less then the sliding friction. 2. A body has weight because of gravitational force. 3. The force which opposes the motion of one body over another is frictional force. 4. The smoother the surface, the lesser is the frictional force. 5. Kinetic friction is less than the static friction. 6. The smaller the area over which a force is applied, the higher is the pressure. 7. The pressure at a particular depth in a liquid is same in every direction E .Choose the correct options 1. (c) between all bodies in the universe Gravitational force acts between all bodies in the universe because it is a universal force. 2. (c) 1 kgf = 9.8 N The relation between the SI units of force and weight of a one kilogram mass is given by the formula: 1 kgf = 9.8 N 3. (b) 10 N/m2 If a force of 100 N acts on an area of 10 m2, the pressure equals 10 N/m2 because: Pressure = Force/Area 4. (d) water reduces friction between the road and the tyres A car skids on a wet road because water reduces the friction between the road and the tyres 5. (c) Friction helps a ship move through water There is no friction between a ship moving through water F. Match the columns Answer : 1. Gravitational force B, t, n 2. Frictional force A, o, s 3. Magnetic force B, p 4. Electrostatic force B, q A. Contact force B. Non contact force m. Makes a pencil work n. Acts between any two objects o. Always opposes motion p. Can act on iron but not on paper q. Can act on paper r. A non contact force that depends on mass s. Reduces when a body starts moving t. Prevents objects from flying away from earth Next Class 8 Basic science Chemistry chapters  We are providing you solution Science [Chemistry ] of so that you can check your study
chapters are: 1. Food production 2.The cell 3. Micro organism 4. Reproduction 5. Synthetic Material 6. Metal and non metals 7.Combustion and Flame 8.Force and Pressure  We are providing you solution Science [Chemistry ] of so that you can check your study chapters are: 1. Food production 2.The cell 3. Micro organism 4. Reproduction 5. Synthetic Material 6. Metal and non metals 7.Combustion and Flame 8.Force and Pressure Class 08 Basic sciences synthetic-materials
A. Answer these Questions 1. What is polymers? Answer : A polymer is made up of small units called monomers that combine under specific conditions. Example: Cotton, jute, silk and wool. 2. What is the process of the formation of the formation of polymers from its monomers called? Answer : The process of formation of a polymer from its monomer is called polymerisation. 3. Name three rayon? Answer : Viscose, cuprammonium and acetate rayon are three widelyused rayons. 4. Mention the five uses of synthetic fibre Other than making clothes? Answer : Other than making clothes, synthetic fibres are used in making towels, mats, curtains, cushions, furniture covers and mosquito nets 5. What are plastic? Answer : Synthetic materials that can be moulded into a permanent shape are called plastics 6. What are the polymers is used for making bubblegums , thermocol and Synthetic erasers ? Answer : Poly (styrenebutadiene) is used for making bubblegums. Polystyrene is used for making thermocol. Synthetic erasers are made from vinyl rubber, neoprene and poly (styrenebutadiene). 7. Name the three polymers that can be obtain from chemical derived from natural gas? Answer : Polythene, polyvinyl chloride (PVC) and Bakelite are three polymers that can be obtained by using chemicals derived from natural gas. 8. What do you mean by tensile strength of material? Answer : The force or stress that is required to break a material is called its tensile strength. 9. What are those material called that (a) that attract water (b) repel water Answer : (a) Materials that attract water are called hydrophilic. (b) Materials that repel water are called hydrophobic. B. Answer these Questions 1. Why are acrylics not spun from the melt? Answer : Acrylics are so named because they are made from polymer polyacrylonitrile. They are not spun from the melt because these polymers decompose without melting. Thus, they are dissolved in a specific solvent and the solution is forced through spinnerets to obtain filaments that are spun into yarns. 2. What are Thermoplastics and Thermosetting plastics ? Answer : Thermoplastics are those plastics that can retain their plasticity even after repeated heatings and coolings. They can also be moulded again and again. Thermosetting plastics are those plastics that cannot be moulded again after they are set into a particular shape 3. To what use can a polythene and acetylene be put? Answer : Acetylene is obtained from methane. It is used for making polythene, polyvinyl chloride and acrylics. Propylene is obtained from petroleum and is used for making polypropylene (a plastic). C. Answer these Questions 1. How are polymers classified? What are the raw material used to manufacture synthetic polymers? Answer : Polymers are classified into three types depending upon their sources: (a) Natural polymers (b) Semisynthetic polymers (c) Synthetic polymers The raw materials used to manufacture the synthetic polymers are obtained from natural gas and petroleum, either directly or indirectly. Methane is present in natural gas, which is converted into acetylene and formaldehyde. Acetylene is used for making polythene, PVC and acrylics and formaldehyde is used for making bakelite. Fractional distillation and cracking of petroleum gives acetylene, propylene, butylene, benzene and naphthalene. Benzene is used for making nylon, butylene is used for making poly (butadienestyrene), naphthalene is used for making polyester and propylene is used for making polypropylene. 2. Mention three advantages and disadvantages of synthetic fibre over natural fibre? Answer : Advantages of synthetic fibres: (a) They do not depend on agricultural crops and animal farming unlike natural fibres that are obtained by these methods. (b) They are cheaper than natural fibres. (c) They are stronger and more durable than natural fibres. Disadvantages of synthetic fibres: (a) They are hydrophobic and do not absorb moisture whereas natural fibres are hydrophilic and are more comfortable to wear. (b) They are very lustrous unlike natural fibres that have soothing colours. (c) They melt before burning and may cause severe injuries in case of an accident. 3. Describe how youwould compare the tensile strength of thread of different material? Answer : Tensile strengths of different materials can be compared by the following activity: Take a cotton thread. Tie its one end to a hook that is fixed with a rigid body and the other end to the pan of a balance. Now, add some weights to the pan. Keep on increasing the weight in small amounts till the thread breaks. Note the total weight including the weight of the pan required to break the thread. Repeat the same activity with other materials and note the different weights required to break the threads. By comparing the values of the weights, one can compare the tensile strengths of different materials. D. Complete sentence. 1. Acrylic fibres compete with wool. 2. Polythene carrybags block drains and sewers. 3. Plastics are generally nondegradable, i.e., they are not environmentally broken down to simpler, harmless substances. 4. On being burnt, plastics give harmful gases. 5. Nylon has a higher tensile strength than copper. 6. On being heated, polyester and nylon melt before they burn. E. Chose the correct options with explanation 1. (c) Cellulose Cellulose is a natural polymer. 2. (a) polyester Polyester is a synthetic polymer that can be stretched several times of its original length. 3. (d) polyester Polyester is a synthetic fibre and it is hydrophobic. 4. (c) polythene Polythene is a bad conductor of heat because it is a synthetic polymer. 5. (a) plastic Plastic is a bad conductor of electricity because it is a synthetic polymer F. Match the columns Answer : A B (a) Cellulose (v) Glucose (b) Nylon (iv) Parachutes (c) Acrylics (i) Artificial wool (d) Acetate rayon (ii) Semisynthetic fibre (e) Polythene (iii) Carrybags Class 08 Basic sciences Metal and non Metal A. Answer these Questions 1. Name ten metals? Answer: Ten examples of metals are potassium (K), iron (Fe), platinum (Pt), tin (Sn), copper (Cu), calcium (Ca), sodium (Na), magnesium (Mg), mercury (Hg) and silver (Ag). 2. How tensile strength of metal used? Answer: Metals have a high tensile strength, i.e., they cannot be broken easily. Example: This property of metals is used to make steel cables for carrying heavy loads. 3. Why cant you (a) draw wires from a piece of wood or coal (b) use nylon or jute ropes for electrical transmission (c) use a cooking utensil made of cardboard Answer: (a) We cannot draw wires from a piece of wood or coal because they are brittle. They break down on stretching. (b) We cannot use nylon or jute ropes for electrical transmission because they are bad conductors of electricity. (c) We cannot use a cooking utensil made of cardboard because it is a bad conductor of heat and it will start burning itself if kept on fire. 4. Name the five non metal found in gaseous form? Answer: Hydrogen (H), nitrogen (N), oxygen (O), chlorine (Cl) and argon (Ar) are five nonmetals that are gaseous at room temperature 5. Name two metals react vigorously with cold water. Name one metal which,if ignited continue to burn in steam? Answer: Potassium and sodium are two metals that react vigorously with cold water. Magnesium is a metal that continues to burn in steam, if it is ignited. 6. Mention three uses of oxygen. Answer: Three uses of oxygen: (a) It is required in all combustion processes. (b) It is used in the productions of nitric acid and sulphuric acid. (c) It is used in extraction of metals from their ores. 7. Mention three uses of nitrogen? Answer: Three uses of nitrogen: (a) It is used by plants for making proteins. (b) It is used in manufacturing ammonia and urea. (c) It is used for filling food packets. 8. Define an alloy? Answer: An alloy is a homogenous mixture of metals or metals and nonmetals. B.Answer these Questions 1. Mention five physical properties of metals. Answer : Five physical characteristics of metals are: (a) They have a high tensile strength; therefore, they cannot be broken down easily. (b) They are malleable; i.e., they can be beaten into thin sheets. (c) They are ductile; i.e., they can be stretched into thin wires. (d) They are good conductors of heat. (e) They are good conductors of electricity. 2. Mention five physical properties of non metals. Answer: Five physical characteristics of nonmetals are: (a) They are brittle; i.e., they are not flexible. (b) They are not sonorous. (c) They have no lustre (except graphite and iodine). (d) They are bad conductors of heat. (e) They are bad conductors of electricity. 3. Name a non metal which When redhot reacts with steam.What happen as a result of the reaction Answer: Carbon is a nonmetal. When it is redhot, it reacts with steam. This reaction forms a gaseous mixture of carbon monoxide and hydrogen, called water gas. The following reaction occurs: C (red hot) + O2 --------------> (water gas) 4. Mention three uses of metals, giving reason? Answer : Three uses of metals are: (a) Iron is used for making tools, machines and agricultural equipment because it is very strong, tough and cheap. (b) Aluminium is malleable so Aluminium foil is used to pack food and medicines. (c) Mercury is a good conductor of heat and does not stick to glass. Thus, it is used in thermometers. 5. Describe the uses of carbon? Answer: Uses of carbon: (a) Graphite is used in making pencils. (b) Diamond is used as a gem and for cutting glass and rocks. (c) Graphite is a good conductor of electricity. Thus, it is used as an electrode. C. Answer these Questions 1. How would you use the activity series while studying the displacement of hydrogen from acids by metal? Answer: Activity series is an ordered arrangement of metals and hydrogen (nonmetal) with respect to their activities. The reactivity of a metal decreases top to bottom in the activity series. For example : Metals that are placed above hydrogen in the activity series can displace hydrogen from dilute hydrochloric acid and sulphuric acid. Metals that are placed below hydrogen in the activity series cannot displace hydrogen from acids. 2 NA + 2HCl ------------------> 2NaCl + H2 Cu + HCl ------------------> No reaction 2. Describe an activity to show that the lead of pencil is good conductors of electricity but sulpher doesnot. Answer: Following activity shows that the lead of a pencil conducts electricity and sulphur does not. Take a pencil and sharpen both its ends. Connect the ends of the pencil to an electric circuit, formed by connecting a bulb to the cell. The bulb glows when pencil is connected to it. This shows that the lead of a pencil conducts electricity. This occurs because lead contains graphite, which is a good conductor of electricity. Now, replace the pencil with a piece of sulphur. The bulb will stop glowing. This shows that sulphur does not conduct electricity. D. Complete sentence: 1. Graphite and iodine are lustrous nonmetals. 2. All metals are good conductors of heat and electricity. 3. The socalled lead (black part) of a pencil is made of clay and graphite. 4. Though a metal, mercury is a liquid at ordinary temperatures. 5. Argon is used for filling electrical bulbs. 6. Liquid nitrogen is used to preserve donated organs. E.Choose the correct options 1. (d) bromine Bromine is a nonmetal that is liquid at ordinary temperatures. 2. (d) All of these All the given options are solid nonme 3. Answer : (a) silver Silver is the best conductor of electricity. 4. (d) all of these All the mentioned metals are placed above copper in the reactivity series and can replace copper from a solution of copper (II) sulphate 5. (d) neon Neon is used in advertisement signs. F. Match the coloumn Answer : Substance Property 1 Property 2 1. Neon A. Metal p. Ductile B, s B. Nonmetal q. Malleable 2. Graphite C. Solid r. Kills germs B, C, u D. Liquid s.Nonreactive 3. Iodine t. Cuts glass B, C, r u. Conducts electricity 4. Aluminium A, C, p, q, u 5. Diamond B, C, t 6. Mercury A, D, s Next Class Basic science physics chapters A must before exam to get A++  1. Name the term for transport of food from leaves to other parts of plants. Ans.Translocation 2. Which pancreatic enzyme is effective in digesting proteins? Ans.Trypsin 3. Which enzyme is present in saliva breaks down starch? Ans.Salivary amylase. 4. Name the organelle in which photosynthesis occurs. Ans.Chloroplast. 5. Differentiate autotrophs and heterotrophs. Ans.The organism which prepare their own food are called autotrophs for eg: green plants . The organisms which depend on the food prepared by other organisms are called heterotrophs for eg: animals 6. Differentiate between Holozoic nutrition and saprophytic nutrition. Ans: Holozoic nutrition In this nutrition organisms derive their food by consuming complex organic materials by the process of ingestion and then converting the complex molecules into simpler ones to obtain nutrients. Eg. animals and human beings. Saprophytic nutrition In this type of nutrition organism derive their food from the dead and decaying materials. Eg.fungi 7. (a) How is fat digested in our body? ( b) Where does this takes place? Ans. (a) fats are emulsified by bile salts. The emulsified fats are acted upon by pancreatic and intestinal lipase to form fatty acids and glycerol. ( b)digestion of fats occurs in small intestine. 8. What is the function of epiglottis in man? Ans: Epiglottis is a triangular shaped flap of cartilage present at the glottis. It prevents the food from entering the windpipe while swallowing 9. When we breathe out, why does the air passage not collapse ? 2014 Ans: Trachea does not collapse because there are rings of muscles around our trachea and are called cartilage which prevents it from collapsing. 10. Herbivores have longer small intestine while carnivores have shorter small intestine. Give reason. 2014 Ans: Herbivores eats grass which contains a lot of cellulose and to digest this cellulose small intestine is longer in them whereas in case of carnivores small intestine is shorter because they eat meat which can be digested easily. 11. Plants absorb water from the soil. Explain how does the water reach the tree top? 2014 Ans: The solution of mineral and water that move from root to leaves is called Sap. The two forces push and pull help to pump up the sap. The absorption of water by root produces a push. Force of pull is produced by transpiration in which plant loose water through the stomata in leaves    You must read and memorize to excel in exam     Q. What is chemistry?
Ans: Chemistry is a branch of science in which we study the composition, structure, properties and change of matter. Q. What is matter? Ans: Anything that is around us having some mass and occupies space is called matter. Q. What are the different classifications of matter? Ans: Matters are classified into element, compound and mixture. Q. What is element? Ans: Elements are the purest form of substance (matter) made up of same kinds of particles. The smallest unit of element is an atom. eg. Hydrogen, oxygen, carbon. Q. What is compound? Ans: Compound is the combination of two or more elements in ix ratio by weight. For H2O is compound in which hydrogen and oxygen are combined in 1: 8 by weight. Properties of compound are different from the constituent elements. Compound are of three type aids bases and salt Molecules are the smallest unit of compound. Q. What is mixture? Ans: A material obtained by mixing two or more element or compound in any ratio is called mixture. There are two types of mixture: a. Homogenous: A mixture having uniform composition throughout is known as Homogenous mixture. The particles of mixture cannot seen by necked eyes. eg. Milk, soda water, alloy b. Heterogenous: A mixture does not have uniform composition throughout is known as Homogenous mixture. The particles of mixture can be seen by necked eyes. eg. Air, salt in water, gun powder. Q. What is symbol? Ans: The short hand or brief representation of the name of element is called symbol. J J Berzelius given the method of writing symbols of elements using English alphabets. Q. what does a symbol represent? Ans: A symbol represent (a) Name of the element (b) one atom of elements (c) one gram atomic weight of the element Q. In what way symbol of elopement have been derived? Ans: The symbol of elopement have been derived in three ways a. Using first letter of English name for example : Hydrogen – H, Oxygen – O, Carbon – C, sulphur - S b. Using first letter along with one more letter of English name Eg. Chlorine – Cl, Helium – He, Manganese - Mn c. One or two letter of Latin name for examples: Sodium - (Natrum ) Na, Gold( Aurum) – Au, Silver (Argentinum) - Ag Q. What is Formula? Ans: The short hand representation of the name of compound is called formulae. A formula gives the idea of the name of the element and no. of element present in compounds.eg. H2, O2, Cl2, Br2, O3,P4,H2O Q. What is valancy ? Ans: The combining capacity of an element with other element to form molecules of a compound is called valency of an element. For example In H2O , The valency of Hydrogen is 1 and valancy of oxygen is two. In NH3 Valancy of nitrogen is 3 and In Al2O3 valancy of Al is 3. In CH4 valency of carbon is 4 Some element has variable valancy like Cu – 1 and 2, iron – 2 and 3, Tin – 2 and 4 , Phosphorous- 3 and 5 and sulphur- 2 ,4 and 6 Q. What are radicals? Ans: A single or groups of atoms having a positive and negative charge. eg: Na+, Cl- A radicals having only one atom is called simple radicals eg: Na+, Cl- A radicals having more than one type of atom is called simple radicals eg: NO3-1, NH4+1 The positive radicals are called basic radical and negative radicals are known as acidic radicals. Q. What is chemical reaction and equation? Ans: A chemical reaction is the process in which atoms present in substances rearrange to give new chemical combinations. The substances take part in a chemical reaction are called the reactants, and the new substances formed are called the products. Representation of chemical reaction using symbols of substances is called chemical equation. CH4 + 2 O2 → CO2 + 2 H2O 2 H2(g) + O2(g) → 2 H2O(g) 2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(l) Q. What are the observations seen in chemical reaction? Ans: indications of a chemical reaction can include: Evolution of heat or light, Production of gas, Formation of precipitate or change in state and colour. Q. Why we need to balance a chemical equation? Ans: Since there is no loss or gain of atoms in a chemical reaction so, To make the equal number of atoms in reactant and product we need to balance a chemical reactions. For more study visit Class VII  Class 08 Basic sciences Crop production
A. Answer these questions 1. What are agriculture? Answer : Agriculture is the science that deals with crop production for human use 2. Name two plantation crops? Answer : Rubber and coffee are examples of plantation crops. 3. Mention the major activities involved in growing a crop Answer : Following activities are involved in growing a crop: (a) Ploughing (b) Levelling (c) Manuring 4. What are weeds? Answer: Weeds are unwanted plants that grow along with the crops and compete with the crops for water, minerals and sunlight. 5. Mention three way of removing weeds from crop field. Answer: Following ways are used to remove weeds: (a) Manually (b) By using tools like trowel, hoe or rake (c) Spraying weedicides 6. What are pets? Answer: Pests are the organisms that attack the crops and damage them. Examples: Rodents or insects 7. Why are animal proteins are considered to be better than plant proteins? Answer: Animal proteins are better than plant proteins because they contain certain essential amino acids that plant proteins lack. Also, animal proteins can be more easily digested by us. 8. When are kharif crops are shown? Answer: Kharif crops are sown during the monsoon (from June to September). 9. What is animal husbandry? Answer: Animal husbandry is the practice of rearing of animals for food and other purposes 10. What are the two things that we have to controlled in a cold storage? Answer: Temperature and humidity are the two things that have to be controlled in a cold storage. B. Answer these questions 1. (a) Why crop rotation is considered as good agriculture practice? (b) Why are pulses are alternated with crops like wheat or paddy? Answer: (a) Crop rotation is a good agricultural practice because it maintains the nutrition of the soil. In this method, different crops are grown in succession in the same field. (b) Pulses are alternated with crops like wheat or paddy because these crops consume lot of nitrogen from the soil. The roots of pulses form an association with nitrogen fixing bacteria and convert the atmospheric nitrogen into ammonia, which is used by plants. When the plants die, large amount of nitrogen fixed by them remains in the soil and becomes available for the next crop. 2. (a) What are green manure? (b) How is compost prepared? Answer: (a) Green manure refers to the plants that are grown to produce manure. Examples of green manure include black gram, cluster bean and cowpea. (b) Compost is formed when manure is produced in the presence of microorganisms that act on the waste matter covered in pits or in open. Waste that is decomposed by microorganisms includes crop residue, animal dung and sludge. 3. (a) What do you understands by biological control of weeds and pests? (b) In what way is biological control is better than the use of weedicides and pesticides? Answer: (a) Biological control of weeds and pests refers to a method in which biological enemies of the weeds and pests like fungi, bacteria and insect are introduced in the field. This organism kills the weeds and pests by feeding upon or causing diseases. The other way of biological control is to develop disease resistant varieties of crops. (b) Biological control is better than the use of weedicides and pesticides as it does not damage crops. it has no environmental impact. It is also less expensive than chemical control. 4. What are root nodules? How they are helpful? Answer: Root nodules are the swellings that are found in the roots of leguminous plants. These nodules are formed by nitrogen fixing bacteria that live in leguminous roots. Root nodules help in converting atmospheric nitrogen into ammonia that is used by plants to make protein. This process is known as nitrogen fixation. 5. What is surface irrigation? Mention two mode of irrigation under this catogary? Answer: Surface irrigation refers to the different ways of irrigation in which water is allowed to run over the field. There are two ways of surface irrigation: (a) Furrow irrigation: Here, crop is planted on ridges and water is allowed to run through furrows between the ridges. (b) Basin irrigation: Here, water is contained in the whole field by making bunds all around it C. Answer these questions 1. Write briefly about the advantages and disadvantages of using chemical fertilizers? Answer: Advantages of using chemical fertilisers: (a) They provide specific nutrients that are not present in the soil. (b) They are easy to store and transport as they are compact. (c) They are water soluble; therefore, they are readily absorbed by plants. Disadvantages of using chemical fertilisers: (a) They make the soil more prone to erosion as they do not provide humus. (b) They can harm soil fertility by making it more acidic or alkaline. (c) They may cause eutrophication by mixing in water bodies. 2. What is nitrogen cycle? Answer: Nitrogen cycle: The continuous circulation of nitrogen between plants and atmosphere is called Nitrogen cycle. Explanation: Nitrogen present in the air is converted into ammonia or other compounds of nitrogen that can be used by the plants through certain nitrogen fixing bacteria present in the soil . This is called nitrogen fixation. These compounds are used by plants to make proteins from where they pass on to the animals, who consume these plants. When plants and animals die, the nitrogen compounds present in their bodies are converted into ammonia by microorganisms. This process is called ammonification. The ammonia is ultimately converted into nitrites and a nitrate by bacteria is called nitrification. Nitrogen is returned to the atmosphere by the bacteria that break down nitrogen compounds to get energy and release nitrogen. This process id called de nitrification. 3. Write briefly about sources of irrigation in India? Answer: Sources of irrigation in India: (a) Dams: Dams are constructed across rivers. Water is carried through them by canals to different parts that are deprived of water. Permanent and inundation canals are used to carry water to different fields. (b) Wells: They are used to tap groundwater. Electrical tube wells are now used to pump water from these wells for irrigation. (c) Groundwater: Over 50 per cent of water used in irrigation is groundwater. In many parts of north India, groundwater is used as an additional source of irrigation. D. Answer: 1. The practice of growing fruit, vegetables, flowers and ornamental plants are horticulture. 2. The process of using the vapours of a chemical to disinfect a place and get rid of pests is called fumigation. 3. A buffer stock of grains is maintained to provide food during emergencies. 4. A mutually beneficial association between two organisms is called symbiosis. 5. Seeds have to be sown at the right depth and with adequate spaces in between. 6. Biological control is a method of controlling weeds or pests by using their natural enemies. 7. A fallow field is one that is left uncultivated for one or more seasons. E. 1. (c) Winnowing 2. (d) can harvest, thresh and winnow 3. (a) 4. (b) nitrogen fixation 5. (a) drip irrigation 6. (d) between 0°C and 4°C 7. (a) Rice F. 1. (a) pea Roots of peas contain nodules with nitrogenfixing bacteria. 2. (d) jute Jute is not a rabi crop 3. (a) and (c) Jowar and bajra are millets that require very little water. 4. (b) winnowing Winnowing is done after the crops are gathered. 5. (c) cowpea Cowpea is an example of green manure. For more rest of lessons visit: Basic Science for class 8 |
