NCERT CLASS 8^THMATERIALS: METALS AND NON-METALS

1. Which of the following can be beaten into thin sheets?

(a) Zinc

(b) Phosphorus

(c) Sulphur

(d) Oxygen

Answer: (a) Zinc

2. Which of the following statements is correct?

(a) All metals are ductile.

(b) All non-metals are ductile.

(c) Generally, metals are ductile.

(d) Some non-metals are ductile.

Answer: (c) Generally, metals are ductile

3. Fill in the blanks :

(a) Phosphorus is very ………….non-metal.

Answer: Reactive

(b) Metals are …………..conductors of heat and …………..

Answer: Good, temperature

(c) Iron is …………..reactive than copper.

Answer: More

(d) Metals react with acids to produce …………..gas.

Answer: hydrogen

4. Mark ‘T’ if the statement is true and ‘F’ if it is false.

(a) Generally, non-metals react with acids. (F )

(b) Sodium is a very reactive metal. ( T)

(c) Copper displaces zinc from zinc sulphate solution. (F )

(d) Coal can be drawn into wires. ( F)

5. Some properties are listed in the following Table. Distinguish between metals and non-metals on the basis of these properties.

Answer:

Properties	Metals	Non-metals
Appearance	Lustrous	Dull
Hardness	Hard	Brittle
Malleability	Malleable	Non-malleable(Brittle)
Ductility	Ductile	Non-ductile(Brittle)
Heat conduction	Conductor	Non-conductor
Conduction of electricity	Conductor	Non-conductor

6. Give reasons for the following :

(a) Aluminium foils are used to wrap food items.

Answer: Aluminium is one of the least reactive metals, so it does not react with food items and does not alter the taste. Moreover, being a metal; aluminium is highly malleable and can be made into very thin foils which are perfect for wrapping food.

(b) Immersion rods for heating liquids are made up of metallic substances.

Answer: Immersion rods for heating liquids are made up of metallic substances because metals are good conductors of heat and electricity. The immersion rod needs electric supply to get heated and; in turn; to heat liquids.

(c) Copper cannot displace zinc from its salt solution.

Answer: Copper is less reactive than zinc. This is the reason, copper cannot displace zinc from its salt solution.

(d) Sodium and potassium are stored in kerosene.

Answer: Sodium and potassium are highly reactive metals. If kept in open, they readily react with oxygen in the atmosphere. The reaction is so quick and that sodium and potassium easily catch fire when exposed to air. To prevent accidental fire, they are stored in kerosene.

7. Can you store lemon pickle in an aluminium utensil? Explain.

Answer: Aluminium is a metal and most of the metals react with acid to produce hydrogen gas. Lemon pickle contains acid which would react with aluminium. Hence, lemon pickle should not be stored in an aluminium pickle.

8. In the following Table some substances are given in Column I. In Column II some uses are given. Match the items in column I with those in Column II.

Question Column I Column II Gold Thermometers Iron Electric wire Aluminium Wrapping food Carbon Jewellery copper Machinery Mercury Fuel

Answer Column I Column II Gold Jewellery Iron Machinery Aluminium Wrapping food Carbon Fuel copper Electric Wire Mercury Thermometers

9. What happens when

(a) Dilute sulphuric acid is poured on a copper plate?

(b) Iron nails are placed in copper sulphate solution?

Write word equations of the reactions involved.

Answer: (a) When dilute sulphuric acid is poured on a copper plate, bubbles appear on the surface of plate. This happens because sulphuric acid reacts with copper to produce hydrogen gas. This can be shown by following equation:

Sulphuric acid + Copper → Copper sulphate + Hydrogen gas

(b) When iron nails are placed in copper sulphate solution, the blue colour of copper sulphate solution fades and turns into light green. This happens because iron displaces copper from copper sulphate solution. This can be shown by following equation:

Copper sulphate + Iron → Iron sulphate + Copper

10. Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.

(a) How will she find the nature of the gas?

Answer: For this, the evolved gas should be passed into a test tube which is filled with lime water. If the lime water turns milky, it shows that the evolved gas is carbon dioxide.

(b) Write down word equations of all the reactions taking place in this process.

Answer: Carbon + Oxygen → Carbon dioxide + Heat

11. One day Reeta went to a jeweller’s shop with her mother. Her mother gave old gold jewellery to the goldsmith to polish. Next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason for the loss in weight?

Answer: The goldsmith use a mixture of acids to clean jewellery. Gold usually doesn’t react with acids. But the mixture which is used by goldsmith is Aqua regia which can even dissolve gold. Due to this, some gold is lost during the polishing process. This explains the loss in weight.

NCERT CLASS 8^THCOAL AND PETROLEUM

Question: 1. What are the advantages of using CNG and LPG as fuels?

Answer: Following are the advantages of using CNG and LPG as fuels:

(a) They are more efficient

(b) They can be easily transported; either in cylinders or through pipelines

(c) They are less polluting than other fossil fuels

Question: 2. Name the petroleum product used for surfacing of roads.

Answer: Bitumen

Question:3. Describe how coal is formed from dead vegetation. What is this process called?

Answer: Dead remains of plants got buried under the earth millions of years ago. Due to intense heat and pressure inside the earth they got converted into coal. The process of conversion of dead remains of plants into coal is called carbonization.

Question: 4. Fill in the blanks :

(a) Fossil fuels are …………, ………… and ……………..

Answer: Coal, Petroleum and Natural gas

(b) Process of separation of different constituents from petroleum is called ……………..

Answer: Refining

(c) Least polluting fuel for vehicle is ………………

Answer: CNG

Question: 5. Tick True/False against the following statements :

(a) Fossil fuels can be made in the laboratory. (F)

(b) CNG is more polluting fuel than petrol. (F)

(c) Coke is almost pure form of carbon. (T)

(d) Coal tar is a mixture of various substances. (T)

(e) Kerosene is not a fossil fuel. (F)

Question: 6. Explain why fossil fuels are exhaustible natural resources.

Answer: It takes millions of years for the formation of fossil fuels. The rate at which we are using them is very fast compared to the rate of their formation. Due to this, fossil fuels are exhaustible in nature.

Question: 7. Describe characteristics and uses of coke.

Answer: Coke is a type of coal. It is a tough, porous and black substance. It is almost pure form of carbon. Coke burns much cleaner than coal and provides more energy for comparable mass. It is used in manufacture of steel and in extraction of many minerals.

Question: 8. Explain the process of formation of petroleum.

Answer: The organisms which lived in the sea got buried at the bottom of oceans. Over a period of time, their dead bodies were covered with sediments. Intense pressure and heat under the earth’s layers transformed these organisms into petroleum.

Question: 9. The following Table shows the total power shortage in India from 1991– 1997. Show the data in the form of a graph. Plot shortage percentage for the years on the Y-axis and the year on the X-axis.

Answer:

NCERT CLASS 8^THCOMBUSTION AND FLAME

Question: 1. List conditions under which combustion can take place.

Answer: Conditions under which combustion can take place are as follows:

(a) Presence of oxygen

(b) Presence of an inflammable substance

(c) Ignition temperature

Question: 2. Fill in the blanks:

(a) Burning of wood and coal causes …………….of air.

Answer: Pollution

(b) A liquid fuel, used in homes is …………….

Answer: Kerosene

(c) Fuel must be heated to its ………… …………….before it starts burning.

Answer: Ignition temperature

(d) Fire produced by oil cannot be controlled by ……………...

Answer: Water

Question: 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer: CNG is a cleaner fuel; compared to petrol and diesel. CNG produces lesser amount of harmful gases on burning. Due to this, CNG is now being used in automobiles in many cities; which has helped in reducing pollution in our cities.

Question: 4. Compare LPG and wood as fuels.

Answer: Wood has traditionally been used as kitchen fuel and is still predominantly being used in rural areas. Burning of wood creates many air pollutants which can result in respiratory problems. Moreover, incomplete oxidation during burning of wood creates carbon monoxide which is a poisonous gas. LPG is much better because it burns without giving smoke. It produces lesser amount of air pollutants. Complete oxidation during burning of LPG does not lead to carbon monoxide production.

Question: 5. Give reasons:

(a) Water is not used to control fires involving electrical equipment.

Answer: Pure water is a bad conductor of electricity but normal water contains many salts and hence is a good conductor of electricity. Trying to douse fires; involving electrical equipment; with water can result in electric shock. Due to this, water is not used to control fires involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

Answer: LPG is a better domestic fuel than wood because of several reasons. Unlike wood, LPG burns without smoke. This makes the life of housewives more comfortable and they do not have to worry about blackening of pots and pans. Moreover, use of LPG as domestic fuel also rules out the chances of getting respiratory disorders which may happen when someone uses wood as kitchen fuel. Storage and transportation of LPG is easier compared to that of wood.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer: The ignition temperature of paper is lower compared to that of aluminium. When paper is wrapped around an aluminium pipe; the ignition temperature increases. That is why paper itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Question: 6. Make a labelled diagram of a candle flame.

Answer:

Question: 7. Name the unit in which the calorific value of a fuel is expressed.

Answer: The calorific value of a fuel is expressed in kilo joule per kg (kJ/kg).

Question: 8. Explain how CO2 is able to control fires.

Answer: Carbon dioxide is heavier than oxygen. Due to this, carbon dioxide forms a blanket around the burning material and makes a barrier between the burning material and oxygen. As oxygen supply is cutoff, it helps in extinguishing the fire.

Question: 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer: Green leaves contain moisture and as a result, the ignition temperature of green leaves is much higher than that of dry leaves. Due to this, it is difficult to burn a heap of green leaves but dry leaves catch fire easily.

Question: 10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer: The outermost zone of a flame is the hottest zone. Due to this, goldsmith uses the outermost zone of a flame for melting gold and silver.

Question: 11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

Answer: Total mass of fuel = 4.5 kg

Heat produced by burning the given mass of fuel = 180,000 kJ.

We know that calorific value of fuel

Hence, the calorific value of the given fuel = 40,000 kJ/kg

12. Can the process of rusting be called combustion? Discuss.

Answer: Although the process of rusting also involves oxidation but it cannot be termed as combustion. The reason for this is that combustion is defined as a process in which oxidation is accompanied by heat and heat is not produced during rusting.

13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

Answer: Since Abida has kept the beaker in the luminous zone of the flame, the beaker will take more time to get heated. On the other hand, Ramesh has kept the beaker in the non-luminous zone of the flame so his beaker will be heated in shorter time.

NCERT CLASS 8^THSYNTHETIC FIBRES AND PLATSTICS

Question: 1. Explain why some fibres are called synthetic.

Answer: Some fibres are made in factories and hence they are called synthetic.

Question: 2. Rayon is different from synthetic fibres because

(a) It has a silk like appearance

(b) It is obtained from wood pulp

(c) Its fibres can also be woven like those of natural fibres

Answer: (b) It is obtained from wood pulp

Question: 3. Fill in the blanks with appropriate words:

(a) Synthetic fibres are also called ______ or _______ fibres.

Answer: man-made or artificial

(b) Synthetic fibres are synthesised from raw material called ________.

Answer: petrochemicals

(c) Like synthetic fibres, plastic is also a __________ .

Answer: polymer

Question: 4. Give examples which indicate that nylon fibres are very strong.

Answer: Nylon is used in making ropes which are used for heavy-duty purposes; like mountaineering and in parachute. These examples show that nylon fibres are very strong.

Question: 5. Explain why plastic containers are favoured for storing food.

Answer: Plastic is a non-reactive and non-corroding material. So, plastic containers do not change the flavour or quality of food. Due to this, plastic containers are favoured for storing food.

Question: 6. Explain the difference between the thermoplastic and thermosetting plastics.

Answer: The shape of thermoplastic can be easily changed by heating again, while the shape of thermosetting plastic cannot be altered once it has been mould into a particular shape. Thermoplastic is used in making toys, combs, containers, etc. Thermosetting plastic is used for making those items which need to withstand high temperature; such as handles of cooking utensils.

Question: 7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

Answer: Saucepan handles have to withstand high temperature during cooking and thermosetting plastic is ideal for that.

(b) Electric plugs/switches/plug boards

Answer: Electric plugs/switches/plug-boards need to be durable and should withstand continuous use over many years. Moreover, in case of some heating due electric current; they should not melt. Hence, these devices are made from thermosetting plastic.

Question: 8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’:

(Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches)

Answer:

• Can be recycled: Plastic toys, carry bags, ball point pens, plastic bowls, , plastic covering on electrical wires, plastic chairs
• Cannot be recycled: Telephone instruments, cooker handles, electrical switches

Question: 9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Answer: Cotton has better absorbent property than synthetic fibre. During summer, cotton cloth can absorb sweat. The sweat would give cooling effect to the body, due to evaporation. Thus a cotton shirt would be much more comfortable than a shirt made from synthetic fibre. Hence, Rana should buy cotton shirts for summer.

Question: 10. Give examples to show that plastics are noncorrosive in nature.

Answer: Pickles usually come packed in plastic sachets or glass bottles. Pickles contain acids, yet no harm is done to the plastic sachet. Sanitary cleaners and some acids; which are used as bathroom cleaner are sold in plastic bottles. These examples show non-corrosive nature of plastic.

Question: 11. Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.

Answer: The handle and bristles of a tooth brush should be made of different materials. The material which is used for making toothbrush handle should be hard and flexible; because the handle has to withstand lot of stress and has to reach every corner of mouth. The material which is used for making bristles should be soft, flexible yet durable; because the bristles should not damage the tooth or gum but should last longer.

Question: 12. ‘Avoid plastics as far as possible’. Comment on this advice.

Answer: Plastic has gained popularity because of its versatile nature. But because of its unmatched popularity; in terms of variety of usage; it has turned into a curse as well. We are creating too much of plastic waste around us. Plastic is non-biodegradable and many forms of plastic cannot be recycled. This is creating a burden on the environment around us. Hence, the statement, “Avoid plastics as far as possible” is a correct statement.

Question: 13. Match the terms of column I correctly with the phrases given in column II.

Question: 14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.

Answer: Forests provide many important raw materials from which we make various useful items. Discovery of synthetic fibres has resulted in many things; which were earlier made from wood; are now being made from plastic. For example; plastic chairs have replaced wooden chairs in a big way. Similarly, wooden cartons and crates have been replaced by plastic cartons and crates. The way plastic has replaced wood from many items, it is definitely minimizing the need to cut forests. Thus, it can be said that manufacturing synthetic fibres is actually helping conservation of forests.

Question: 15. Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer: For this, take a discarded plastic toy, a torch which is in working order. Switch the torch on to check that it is in working condition. From the plastic toy, cut a small piece about the size of a coin. Now insert this coin between two cells of the torch. Then tighten the lid of the torch. Now, switch on the torch. You will notice that the bulb does not glow. This happens because the plastic coin; between two cells; does not allow current to pass between two cells. This shows that thermoplastic is a poor conductor of electricity.