• Materials, made up of same kind of particles, are known as pure substances.
• A sample, of a given element, or a given compound, is a 'pure substance'. This is because a given element (/compound) is made up of atoms (/molecules) of the same kind.
• Different elements/compounds differ from each other in their physical and chemical properties as they are made up of atoms/molecules of different kind. 

• There are about 118 elements known at present. It is difficult to study and describe all the properties of all these elements separately. Hence, elements, showing similar properties, are grouped together and their general characteristics are studied. Such 'grouping' of elements is called classification of elements.
• The elements have been broadly classified into two categories: metals and non-metals; this classification is based on the differences in their physical and chemical properties.

• Metals exist in abundance in the earth's crust. Aluminium is the most abundant metal; it is followed by iron.
• Highly reactive metals, like sodium, potassium, aluminium and zinc, are found in nature in the form of their compounds (like oxides, sulphides and carbonates).
• Less reactive metals, like gold, silver and platinum, occur in nature in their elemental, or native state. 
• Many non-metals are found in their free state in the atmosphere. Oxygen and nitrogen are two well known examples of such non-metals.
• Several non-metals exist in nature in the form of their compounds (like oxides and nitrates).
• Sulphur exists in the free state as well as in the combined state (as sulphides and sulphates).
• Carbon exists in its native state as diamond, graphite and fullerene; it also exists in the form of its compounds like carbon dioxide, carbonates and bicarbonates.

• A mineral is a naturally occuring inorganic substance found deep under the surface of the earth.
• An ore is a mineral from which one or more metals can be extracted profitably.
  For example, aluminium is extracted from its bauxite ore and iron from its haematite ore.
• The sequence of processes, used to extract a metal, in its pure form from its ore, is called metallurgy.

1. Concentration of ore: It is the process of removal of impurities from the ore. 
2. Reduction: It is the process of 'treating the metal ore' to get the metal in its free state. 
3. Refining of metal: The metal, obtained by reduction, is generally impure. Refining is the process of purification of this impure metal.

Some Important Terms

• Alloy: A homogenous mixture of two or more metals, or a metal and a non-metal.
• Conductivity: The ability of a material to transfer heat energy, or electrical current, from one point to another.
• Ductility: The property of metals due to which they can be drawn into thin wires.
• Malleability: The property of metals due to which they can be hammered into thin sheets.
• Metalloid: Elements which show (some) properties of metals as well as non-metals
• Metallurgy: The sequence of processes used to extract a metal from its ore in its pure form.
• Mineral: A natural occuring inorganic substance found deep under the surface of earth.
• Noble metals: Those metals which are least reactive and so are not chemically affected by the substances around them. 
• Ore: It is a mineral from which one, or more, metals can be extracted.
• Reactivity: The tendency of an element to react with other substances to form compounds.
• Reactivity series: The series of metals arranged in the order of their decreasing reactivity.
• Sonorosity: The property of metals to produce sound when struck with something hard.
• Tensile strength: The property, due to which a substance can bear a lot of strain, without breaking.

1. Physical State
2. Melting point and boiling point
3. Density
4. Hardness
5. Lustre
6. Malleability
7. Ductility
8. Tensile strength
9. Conductivity
10. Sonorosity

• All metals (except mercury, which is a liquid) are solid at room temperature.
  Iron, copper, aluminium, gold and silver are some of the examples of metals.

• Non-metals (at room temperature) may exist in solid, liquid or gaseous state.
  Carbon, sulphur, phosphorus and iodine are a few examples of non-metals which exist in the solid state.
  Bromine exists as a liquid while chlorine, oxygen and nitrogen are examples of non-metals which exist in the gaseous state. 

• Metals generally have high melting as well as high boiling points.
  For example, melting point of iron is 1536°C and its boiling point is 3000°C.
  Some metals, however, have an exceptionally low melting point.
  For example, the melting point of caesium metal is only 28.7°C.
• Non-metals generally have low melting as well as low boiling points.
  For example, the melting point of sulphur is 119°C.
  However, there are exceptions in non-metals also. The melting point of diamond (form of carbon) is very high, i.e. 3723°C. 

• Metals generally have a high density. However, there are some exceptions. Sodium and potassium, which are metals, have quite a low density. Their density value is less than that of water.
• Non-metals generally have a low density.  

• Most metals are very hard. They can withstand quite high pressures without getting distorted.
  There are some exceptions. Sodium and potassium are metals but they are so soft that they can be easily cut with a knife or a razor blade. 
• Non-metals are generally not hard; they are brittle and easily break into pieces when hammered. 
  Diamond which is a non-metal is a form of carbon, is the hardest substance known. 

• Metals have a shining surface. This is known as metallic lustre.
• Non-metals generally have a dull appearance, that is, they are non-lustrous. Iodine is the only non-metal which has a natural lustre.

• Metals are malleable, that is, they can be hammered into thin sheets without breaking. 
• Non-metals are non-malleable, that is, they cannot be hammered into sheets.
  They are brittle, and break into pieces on being hammered. 

• Metals can be drawn into thin wires. This property of metals is known as ductility.
  Copper, aluminium and iron wires are commonly used for electrical fittings, making net doors and wire meshes, and so on.
• Non-metals are non-ductile, that is, they cannot be drawn into wires.

• Metals have high tensile strength due to their ductility and malleability.
• Non-metals generally do not have high tensile strength (except carbon-fibre).

1. Thermal conductivity
2. Electrical conductivity. 

• Metals are good thermal conductors. It is due to this property that metals like copper and iron are used for making cooking utensils and water boilers.

• The activity shows that metals are good conductors of electricity. It is due to this property that metal wires (generally copper wires) are used in electrical fittings.
• Non-metals are generally poor conductors of heat and electricity. Most of them are non-conductors or 'insulators'.
  However, graphite, which is a form of carbon, is a good conductor of electricity; it is used in batteries.

• Metals are said to be sonorous. It is due to this property that metals are used for making bells.
• Non-metals are non-sonorous. It is on the basis of these differences in their physical properties that one can distinguish metals from non-metals.

There are some elements which show some properties of metals as well as non-metals. Such elements are called metalloids. Silicon (Si), germanium (Ge), arsenic (As) are the well-known examples of metalloids.

Some Important Points

• Silver is the best conductor of heat and electricity, followed by copper and aluminium. Among metals, mercury is the poorest conductor.
• Gold and silver are highly malleable. Gold can be converted into a foil which is only 2.0 ×10^-5 mm thick.
• Gold is the most ductile metal. A two kilometre long wire can be drawn from only one gram of gold.
• On exposure to air for sometime, metals lose their shine. This is because metals react with various gases present in air and get coated with a thin layer of their oxide, carbonate or sulphide.

1. Reaction with Oxygen
2. Reaction with Water
3. Reaction with Acids
4. Reaction with Alkalies

2MgO (s) + H₂O (l) → 2Mg(OH)₂ (aq)

• This activity shows that metals react with oxygen to form metallic oxides which are basic in nature. Potassium (K), sodium (Na) and calcium (Ca) are highly reactive metals. They react with the oxygen present in the air, even at room temperature, to form their respective oxides.

• This activity shows that non-metals can react with oxygen to form non-metallic oxides which are acidic in nature. 

• This activity shows that some metals can react with water to liberate hydrogen gas.
• Metals differ in their reactivity towards water. For example, sodium and potassium metals reacts very vigorously with water; this reaction gives out so much heat that the hydrogen evolved catches fire. Therefore, these metals are stored under kerosene, or paraffin wax.
• Magnesium does not react with cold water but reacts on heating.
• Zinc reacts with boiling water and iron reacts with steam, indicating that it is very much less reactive.
• Metals like copper, silver, gold, platinum and mercury do not react with water at all.
• Non-metals do not react with water. Therefore, some reactive non-metals are stored in water to prevent their reaction with air.
• For example, phosphorus is kept in water to prevent its contact with air; it catches fire on reaction with air. 

• This activity shows that most metals react with dilute acids to liberate hydrogen gas and form metal salts. 
• Some metals, like copper and lead, do not react with dilute hydrochloric acid. They react with sulphuric acid and nitric acid but they do not liberate hydrogen gas.
• Metals, like gold and platinum, do not react with dilute acids.
• Non-metals generally do not react with acids.
• Some non-metals, like sulphur and phosphorus, react with hot concentrated sulphuric acid, or nitric acid, but they do not liberate hydrogen gas. 

• The tendency of an element to react with other substances to form compounds is an indicator of its reactivity. The more is the tendency of an element to form compounds, the more is its reactivity. 
• All metals do not have the same reactivity. Some are more reactive than the others, that is, they have a greater tendency to form compounds. Such metals occur in the form of their compounds in earth's crust.
• Lesser reactive metals occur in their native state, that is, elemental state.
• On the basis of experiments, involving the reaction of different metals with a particular substance, metals have been arranged in the decreasing order of their reactivity.
• The series of metals, arranged in the order of their decreasing reactivity, is called the reactivity series. 

• This happens because magnesium, being more reactive than copper, displaces it from copper sulphate.
• The magnesium sulphate formed is colourless and the copper metal, that gets deposited on the magnesium ribbon, appears as a brown coating.
• This displacement reaction can be represented by the following equation:
  

• As seen from the reactivity series, platinum is the least reactive metal. It does not react with air, water, acids, bases and most other substances. Another metal, which shows a similar behaviour, is gold. Gold and platinum are called noble metals.
• Since noble metals are least reactive, they are not chemically affected by the substances around them. Hence, they do not get tarnished, and retain their lustre for a very long time. 
• Both, these noble metals are also highly ductile and malleable; they can be drawn into extremely thin wires and can be beaten into very thin foils. It is because of these properties that gold and platinum are used for making jewellery.
• Gold can also be used for plating other metals, like copper and silver. Platinum is used in dentistry and in making scientific instruments. 
• Pure gold is very soft. Therefore, it cannot be used for making jewellery in its pure form. Hence, for making jewellery, it is often mixed with silver or copper to make it appropriately hard. 
• The purity of gold is expressed in terms of carats (or karats). The carat number gives the number of parts of gold present in 24 parts of a mixture of gold with the other metals.
  For example, 22 carat gold means that 22 parts of pure gold is present in 24 parts of a mixture of gold with copper (or silver). This implies that pure gold would be rated as: '24 carat gold'. 

• Iron is the most widely used metal. It is used for making cooking vessels, water boilers, stoves, toys, tools, pipes, agricultural implements, chains, wires, nails, bolts, electromagnets, and so on.
• Aluminium, being a very light metal, is used for making aircraft bodies. It is also used for making cooking vessels. Its thin foils are used for packaging of food stuffs and medicines.
• Copper is the most widely used metal for making electrical cables and other electrical goods. It is also used for making cooking vessels.
• Silver is used for making jewellery, decoration pieces, tableware, etc. Silver, being highly malleable, can be converted into very thin foils which are used for decorating food items. Silver is a very good conductor of electricity, but it is not commonly used for electrical fittings because it is very expensive. Silver and Gold wires are, however, used for high precision electrical contacts in computers. 

• By adding appropriate amount of other metals, or non-metals, to form the alloys, the properties of a given metal can be (significantly) modified.
• Alloys are generally stronger, harder and more resistant to corrosion than the (pure) metal itself.

1. Nitrogen, in the form of fertilisers, is essential for the growth and development of seeds and plants.
2. Phosphorus is used in matchbox industry and in fertilisers.
3. Iodine is used as an antiseptic.
4. Sulphur is used for making fire crackers, gun powder and sulphuric acid.
5. Oxygen is essential for survival of all living beings.
6. Diamond (a form of carbon) is used in making jewellery, in cutting glass and for grinding of tools.
7. Graphite (also a form of carbon) is used in batteries and in pencils.

• Iron is an essential and important component of haemoglobin; its deficiency can lead to serious complications.
• Sodium and potassium play an important role in the transmission of (electric) signals, to and from, the brain.
• Several other metals also play an important role in the human body.
• Non-metals, like carbon and phosphorus, also play an important role in human body.
• Phosphorus is present in bones and helps cells obtain energy from food. 

• When we push or pull an object, we are exerting a force. A football, at rest, has to be kicked to send it over a distance. We again say that we are exerting a force on it. We, therefore, say that we exert a force when we push, pull, kick or lift a given object. 
• In all these situations, some kind of external agency (very often a muscular effort) is involved and its effect can be noticed or felt quite easily. It follows that we need an external force to move a body from its rest position, or to stop a moving body.
• For example, we can stop a ball rolling down an inclined plane by applying a force against the direction of its motion. 
• We can thus say that force is a push or pull which comes into play when there is an interaction of one object with another object.
• For example, when we wish to change the position of a study table in our room, we have to push it. The study table does not move due to our presence alone. There has to be an interaction (push/pull) between us and the study table. The study table begins to move in the direction of the applied force. From the above activity, we can infer that a force comes into play only when at least two objects 'Interact' with each other.
• Thus an interaction of one object with another object can result in a force between the two objects.

• if the applied force acts on a body along its direction of motion, the speed of the body will increase.
• if the direction of force, on the body, is opposite to its direction of motion, the speed will decrease. (In both the above two cases, the object is supposed to be moving in a straight line).
• if the force acts at an angle to the direction of motion, it can change the speed as well as the direction of motion.

• make an object move from rest.
• change the speed of a moving object.
• change the direction of a moving object.
• bring a change in the size, or shape, of an object.
• cause two, or more, or all, of these effects. 

• We know that harder we kick a football, faster it moves. It means that: 'greater is the applied force, greater is the change in the speed of the object.'
• Consider a lighter mass (car) and a heavier mass (loaded truck) parked on a horizontal road. We all know from our experience that a much greater push (force) is needed to move the truck than the car.
• Now, suppose we apply the same force, for the same time, to both the car and the truck. The car picks up a greater speed than the truck in that time.
• We thus realise the mass of an object, and the value of the change in its speed (in a given time) are both important parameters that determine the magnitude of the force needed.

1. Try to push it, all by yourself, from one corner of the room to another corner. Can you move it?
2. Take the help of one of your friends. Ask him to push the container in the same direction in which you are pushing it.
3. Next ask him to push from the opposite direction. What do you observe? In which case does it become easier, or difficult, to move the container?

• equal and opposite forces (balanced forces) do not change the state of rest or motion of an object. They may, however, cause a change in the size and/ or shape of the object.
• unequal (unbalanced) forces may lead to
  (i) change in state of rest or motion as well as
  (ii) change in size and/or shape of an object.

• Spring balance is a simple device that can be used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force. The SI unit of force is 1 newton (1N). The force is said to be 1N if it produces an acceleration of lm/s² in a body of mass 1kg.
• The force need not always act in the direction of motion. Depending upon the situation, force may act at any angle to the direction of motion. A force, acting perpendicular to the direction of motion, does not cause any change in speed but can still cause a change in direction. 

1. Contact forces 
2. Non-Contact forces ('action at a distance' forces) 

• Muscular force: In our daily life, we push, pull or lift many things. The effort (force) is caused by the action of muscles in our body. Animals, like bullocks, horses and camels, have been used for pulling carts. In arctic regions, reindeers are made to pull the sledges that are used as passenger vehicles. In these cases, the muscles of animals apply the force. This force is called muscular force. All animals, including human beings, use muscular force for most of their activities. 
• Frictional force: Ball rolling along the ground, gradually slows down and finally comes to rest. In order to move a bicycle along a straight level road, we have to keep pedalling it all the time. This is because of the frictional force acting between the two surfaces in contact. The magnitude of the frictional force depends upon the nature of the two surfaces in contact. The direction of the frictional force is (usually) opposite to the direction of motion of the object.

1. gravitational force
2. magnetic force
3. electric force.

• Gravitational force: We know that when we throw a ball upwards, the ball goes up in the air but then falls down again. Ripe fruits, that grow on trees, fall to the ground by themselves. This happens, due to a force, we call as the gravitational force.
• Magnetic force: The magnetic property of lodestone was known to mankind since quite early times. Magnets have the well-known property of attracting objects made of iron. Likes poles of two magnets repel and unlike poles of two magnets attract each other. A magnet can exert a force on another magnet without being in contact with it. The force, exerted in this case, is known as a magnetic force.
  
• Electric force: Two charged bodies exert a force on each other. When a glass rod is rubbed with a silk cloth, the rod becomes positively charged. Similarly, when an ebonite rod is rubbed with wool, the rod acquires a negative charge. Bring this charged ebonite rod near the suspended (and charged) glass rod. You will find that the suspended glass rod moves towards the charged ebonite rod. From these observations, we observe that like charges repel and unlike charges attract. This attraction, or repulsion, between charges is due to the (non-contact) electric force between them.
  

• Careful observations and analysis reveal that the effect of a force also depends on the surface area over which the force acts.
• It is a common experience that it is difficult to carry a school bag, when it is tied to narrow thin string; it becomes easier to carry the same school bag when it is tied to 'broad straps'. This implies that if the same force (say, the weight of an object) acts over a smaller area, its effect is felt more.

1. its magnitude
2. the area over which it acts.

• Pressure = Thrust/Surface area over which it acts (contact area) 

• It is the same force that is acting on the clay in all the three cases. However, the effect produced is different because of the difference in the contact area. We, thus, realise that for a given applied force, when the contact area is less, its pressure, on the surface in contact, becomes more.

1. Buses and trucks usually have double wheels on the rear side.
2. High rise buildings and dams have a wide base.
3. Tanks and bulldozers are fitted with caterpillar tracks rather than wheels.
4. Railway tracks are laid on large sized wooden/iron sleepers.

1. It is easier to cut with a sharp knife than with a blunt knife.
2. Nails, pins and spikes have pointed ends so that they can be driven into the surface with minimum effort. 

1. It is easy to observe that a liquid exerts pressure, due to its weight, on the bottom of its container. This is much the same way as a solid does on the surface supporting it.
2. The pressure, exerted by a stationary liquid (kept in a container) at any point inside the liquid, is known as hydrostatic (liquid) pressure. 

• Liquid pressure, on the bottom of the container (due to weight of liquid column), does not depend on the area of the bottom. 
• Liquid pressure, at any point inside the liquid, depends upon the density of the liquid and the height of liquid column above that point.
• Liquids exert (an equal) pressure on all the walls of the container. 
• An external pressure, applied on a liquid in a closed container, is transmitted uniformly throughout the liquid. 

• Pour some hot water into a (good quality) plastic bottle, carefully.
• Close the lid of the bottle. Shake it for half minute.
• Now, pour out the water and (quickly) close the lid of bottle very tightly. 

• As we move upwards through the atmosphere, the height of air column, above us, would decrease. This would result in a decrease in air pressure at higher altitudes.
• In fact, when we move towards higher altitudes, breathing can become difficult. Sometimes bleeding from nose may also occur.
• Most climbers, who attempt to scale high range mountains, (like Mount Everest), need to carry oxygen cylinders with them. For this very reason, aircrafts have 'pressurised cabins'. The air pressure in these is increased to a (sufficient) value that safeguards the passengers and the crew.
• Air pressure also varies with temperature and time at a given place. We have already learnt that due to uneven heating of earth's surface, air pressure can change rapidly.
• At a hotter place, the warm air there is lighter than the cooler air in the surrounding regions. Hence, air can rush in from (the neighbouring) high pressure surrounding area to this lower pressure area. This phenomenon can result in land breezes, sea breezes, winds and storms.

1. When we drink liquid with a straw, the air pressure inside the straw decreases (due to our sucking). The air pressure, acting on the surface of liquid, then becomes greater than the pressure inside the straw. This forces the liquid to move up inside the straw. The syringe also works in a similar way. 
2. We, sometimes, use rubber suckers for installing hooks in a room. When we press the sucker the air between the air sucker and the wall gets forced out. Hence, the air, pressing on it from outside, holds it firmly against the wall. If we wish to pull the sucker off the surface, the force, applied by us, has to be large enough to overcome this effect of the atmospheric pressure.

Friction 

Cause of Friction

Factors affecting Friction

Important Points

1. static friction
2. sliding friction.

• The box still remains at rest. When an external force acts on the box, the force of friction, known as static friction, comes into play and opposes the motion of the box.
• This static friction balances the force which we exert on the box and the box remains at rest. When applied force is increased to, say '2F', static friction also increases to '2F' and again opposes the motion of the box.
• Now, increase the applied force gradually till the box just begins to slide over the horizontal surface (floor).

• Take a wooden tray/block. Place it near the one edge of a table top.
• Keep a cylindrical pencil/rod on the opposite edge of the table top (using drawing pins).
• Make sure the pencil/rod is free to rotate about its axis (between the drawing pins) as shown in the figure. (You can also use a pulley for this purpose).
• Next take the plastic lid of a jar. Make three symmetric holes in it. Put three pieces of string through the holes and tie them together.
• Next tie them to a longer string from which we can suspend the plastic lid freely. Tie the other end of the longer string to the wooden tray/block such that it passes over the pencil.
• Take some marbles (marble chips/very small marbles). Add these marbles in the plastic lid one by one until the wooden tray/block starts just sliding.
• Note down the number of marbles required. The number of marbles, put in the plastic lid, is an indicator of the magnitude of the limiting force of (static) friction.
• Once the wooden tray/block begins to slide, (gently) take out a small marble from the plastic lid.

• In the first case, when the tray/block just begins to slide, the force of friction is the limiting (or maximum) value of the force of static friction. Once the tray/block begins to slide, the friction, that exists, is sliding friction.
• When two/three (small) marbles are (gently) removed from the plastic lid, the tray/ block still keeps on sliding and moves to the other end of the table top (with almost the same speed). It shows that sliding friction is (slightly) less than static friction. 

• We now observe that, for a given pair of surfaces, static friction is (a little) more than sliding friction. Once motion gets started, the friction becomes slightly less than the maximum, or limiting, value of the force of static friction.
• This can be understood as follows:
• We can say that relative motion between two surfaces starts only when the interlockings between their irregularities (ups/downs) have been unlocked.
• Once motion starts the irregularities act just as an obstruction against their relative motion; they are not interlocked now.
• In other words, once the motion starts, the 'contact points' on one surface, do not get enough time to lock into the 'contact points' of the other surface.
• Thus, it is easier to keep on moving an object (when it is already in motion) than to start it from rest.

• We observe that it is much easier 'to roll', than 'to slide', a body over a given surface. On a railway platform, or bus stop, we often notice that small children can easily pull a suitcase fitted with wheels.
• It is because of the reduced friction, (associated with rolling), that makes it easy for the child to pull along the suitcase. It Is easy now to appreciate why labourers prefer placing logs under heavy machines/objects while moving them from one place to another place.
• Rolling friction is, thus, the force of friction that comes into play when one body rolls over the surface of another body. It is (much) less than the (force of) sliding friction. 

• The realisation, that rolling friction is much less than sliding friction, led man to invent the wheels. The wheel has been considered one of the greatest inventions in the history of mankind.
• It is much easier to cart a heavy load, put on a trolley with wheels, than to push it. Wheels are used extensively in daily lives for transportation.
• They save labour and energy to a great extent. This is because of the (very much) reduced friction associated with rolling. 

• Proper inflated tyres roll without sliding since rolling friction is less than the sliding friction. Therefore, there is less dissipation of energy against friction. Hence, proper inflated tyres save fuel.

Advantages of Friction

Disadvantages of Friction

Methods of Increasing Friction

Methods of Reducing Friction

1. Polishing

2. Lubrication

3. Ball-bearings

Fluid Friction

• We need energy for all our activities, from baking chappatis or cookies, to sending astronauts into space.
• All forms of energy, that we use everyday, are stored in different ways in various sources of energy.
• The substances which produce heat on burning in air are called fuels. 

1. Occurrence
2. Physical State
3. Availability

1. Natural Sources: Natural energy sources are those which are made available to us by nature. Solar energy, wind energy, energy from water (hydro energy) are some of such natural sources of energy.
2. Synthetic Sources: Synthetic energy sources are those that use man-made materials as sources of energy. For example, chemical energy, stored in the batteries, (used in calculators, watches, etc.) is a synthetic source of energy.

1. Solid: Firewood, charcoal, coal are examples of solid fuels.
2. Liquid: Kerosene, petrol and diesel are all liquid fuels.
3. Gas: Petroleum gas, commonly used as LPG (Liquified Petroleum Gas), and natural gas, also used as CNG (Compressed Natural Gas), are examples of gaseous fuels. 

1. Renewable: A renewable source of energy is a natural resource that can replenish itself naturally over a short period of time. Wind, sun, biomass (from plants) and hydropower (from water) are all renewable sources of energy. These are inexhaustible natural resources.
2. Non-renewable: Energy sources, which get used up and cannot be replaced, or replenished, in a short period of time are called non-renewable sources of energy. These are also called exhaustible natural resources. Fossil fuels, (like petroleum, natural gas and coal), are non-renewable sources of energy.

• Wood is a major renewable natural resource.
• The combustion of wood is currently the largest source of energy.
• Wood can be used for cooking and heating; it may occasionally be used in steam engines and steam turbines (that generate electricity).
• Combustion of wood produces heat which can be put to a variety of uses.
• However, it also produces other gases, like CO₂ (carbon dioxide) and CO (carbon monoxide), which are undesirable, irritating or dangerous. 

• Coal is found deep under the surface of the earth.
• The major coal mines in India are located in Jharkhand, Madhya Pradesh, Odisha and West Bengal.
• The largest coal mines are at Bokaro and Jharia (in Jharkhand) and Raniganj (in West Bengal).

• Coal was formed from plant remains got buried deep under the earth's crust.
• Over a large period of time, the chemical and physical properties of these plant remains were changed through geological actions that led to the creation of this solid material.
• The process, of conversion of dead plant materials into coal, is called carbonisation. 

1. peat
2. lignite
3. bituminous
4. anthracite

• Peat is a soft brown substance that is made up of 30 per cent carbon. It is the earliest stage during the formation of coal.
• Lignite has a carbon content of about 38 per cent; it is regarded as the lowest grade of coal. It has been used mainly for electric power generation.
• Bituminous coal has about 65 per cent carbon; it is used for power generation and for making another type of fuel, called coke.
• Anthracite is hard coal and contains over 90 per cent carbon. It is regarded as the highest grade of coal. 

• It is used as a domestic as well as an industrial fuel in stoves and furnaces. It gives little (or no) smoke.
• It is used for extraction of metals. 
• It can be used to make fuel gases (like water gas, CO + H₂).

• synthetic dyes
• drugs
• explosives
• perfumes
• paints
• photographic materials
• roofing materials.

• a domestic fuel
• an industrial fuel (usually only for those industries which are located near coal processing units). 

• Petroleum is found at moderate depths (500 m - 2,000 m) between two layers of impervious rocks. 
• Petroleum deposits are usually found mixed with salt water.
• Petroleum, being lighter than salt water, floats over it.
• Natural gas is found above petroleum; it is trapped between the rock cap and the petroleum layer.

• Petroleum is a mixture of various materials which are separated from one another and are used for different purposes. 
• The process, of separating different components of petroleum into their various useful forms, is called refining of petroleum. 

1. Residual Oil: It is further separated into following products:
   (i) Asphalt: It is a black and sticky solid. It is used for making roads and coating the underside of electric poles (to prevent their rusting).
   (ii) Paraffin wax: It is a white semi-solid, which is often used as grease. It is also used for making vaseline, ointments and candles.
   (iii) Lubricating oil: It is a viscous oil used for lubricating machines. 
2. Fuel Oil: It is used as a fuel in boilers.
3. Diesel: It is used to run generators and heavy vehicles (such as buses, tractors, etc.). 
4. Kerosene: It has been used as a domestic fuel as well as in lanterns.
5. Gasoline, or Petrol: It is mainly used as a fuel in cars, scooters and motor cycles. It is also used for 'dry cleaning' of clothes. Highly refined petrol (aviation fuel) is used as a fuel in aeroplanes.
6. Petroleum Gas: It is liquified under pressure and is then known as Liquified Petroleum Gas (LPG). LPG is used as a domestic fuel.

• It burns readily and has a high calorific value.
• It is used as a fuel in homes and industries.
• It is also used as a better, and greener, automobile fuel and as 'base ingredient' in the manufacture of fertilisers and chemicals.

• Burning of fossil fuels, such as coal and petroleum products, is a major source of pollution. 
• The present day strategy, to address these environmental issues, is to develop technology that uses renewable natural resources (such as biomass, water, wind and solar energy).
• India is blessed with an abundance of sunlight, water and biomass.
• Fuels, like LPG, CNG and biomass are cleaner and better fuels.
• When burnt, biomass and LPG release carbon dioxide but in relatively smaller amounts.
• When biomass crops are grown, a significant amount of carbon dioxide is consumed through photosynthesis.
• Natural gas is a cleaner fuel as it provides relatively low amounts of pollutants. It has fewer emissions than coal and petroleum products; also, it leaves (virtually) no ash particles after burning.
• For our future and present day energy needs, we need to look for alternative (cleaner) sources, such as solar energy, tidal energy, geothermal energy, hydel energy and wind energy.
• This will help to protect the environment and reduce the risk of global warming.
• Further fossil fuels are then more likely to be available for future generations. 

• The materials, which on heating in the presence of air or oxygen, catch fire easily and produce heat and light energy, are called combustible materials.
• The chemical process in which a substance burns in air, or oxygen, with the release of heat and light energy is called combustion.
• The substance that undergoes combustion is called a combustible substance or fuel. 
• A fuel may be solid, liquid or gaseous. 

• Fix a candle on a table.
• Light the candle and observe the flame.
• Now, take a glass chimney and place it as shown in the figure.
• Observe the candle flame carefully.
• Now, place a cardboard on the top of the chimney.
• Observe the flame.

• Take two paper cups.
• Half fill one of the cups with water and keep the other empty.
• Take two spirit lamps and put these two paper cups, (one empty and the other half-filled) above the flame carefully. 

• A piece of paper burns quickly when a burning matchstick is brought near it. On the other hand, a piece of wood cannot be burnt with the help of a matchstick.
• These observations tell us that different substances catch fire at different temperatures.
• The lowest temperature (minimum temperature), at which a (combustible) substance catches fire, is called its ignition temperature, or kindling temperature.

1. We can boil water in a paper cup on a spirit lamp. It is because a part of the heat energy, supplied to the paper cup, passes on to water. As a result, the ignition temperature of paper is not achieved and it does not catch fire.
2. Ignition temperature of white phosphorus is 35°C. During summer, room temperature often rises to above 35°C. White phosphorus then catches fire spontaneously. 
3. A matchstick can be lighted by striking its tip on a rough surface. The tip of the matchstick is made up of red phosphorus along with some other chemicals. On striking, friction generates enough heat to light the matchstick by making the chemical catch fire. The wood, used in the matchstick, is also of a particular kind; its ignition temperature is attained by the heat produced at the tip of the lighted matchstick. 
4. The flame of a burning candle goes off when we blow over it strongly. It is because the
   (i) air current lowers the temperature of burning wax vapours below their ignition point.
   (ii) the carbon dioxide, in our breath, acts as a fire extinguisher. Thus, the flame goes off.

1. There must be a combustible substance.
2. There must be a continuous supply of oxygen or air.
3. The temperature should be above the ignition temperature of the combustible material. 

1. Slow Combustion
2. Rapid Combustion
3. Spontaneous Combustion
4. Explosive Combustion

• Substances which have a low ignition temperature and hence, catch fire easily with a flame are called inflammable substances.
• Food is a fuel for our body. Energy is produced during the 'combustion' of food.
• In pure oxygen, a substance burns five times faster than in air.
• More than 5,000 years ago, Egyptians made matchsticks by coating small pieces of pine wood with sulphur. These caught fire easily when they were rubbed strongly with each other. As a result of such rubbing, friction raised the temperature of sulphur above its ignition temperature and made it catch fire. This, in turn, ignited the pine wood pieces. 
• Many of the forest fires are caused by human negligence. People living near the forest, may not extinguish the burning wood (etc.) after cooking, or sometimes, they may carelessly throw a burning matchstick on dry grass. Such acts of carelessness can lead to forest fires. ) 
• Sun produces an enormous amount of heat and light energy. 98 per cent of sun's mass is made of hydrogen at an extremely high temperature. At such temperatures hydrogen atoms can combine to form helium atoms and in the process release an enormous amount of energy in the form of heat and light. Energy produced by the sun in one hour is about 1020 times more than the total energy produced by burning 1000 kg of coal.

• Different crops have different requirements for their proper growth as they grow in different seasons. Based on the season in which they grow, crops are categorised as:
  (i) Kharif crops
  (ii) Rabi crops
• The main kharif crops, paddy and maize, are grown during the months from June to October. These are dependent on the western monsoon.
• Wheat, gram and barley are the main rabi crops which are grown during the months from November to April. These are not dependent on monsoon.
  

1. Preparation of soil
2. Sowing of seeds
3. Addition of manures and fertilisers (soil replenishment)
4. Irrigation (supply of water)
5. Crop protection (from weeds and pests)
6. Harvesting 
7. Storage 

• We have already studied about the phenomenon of refraction of light and its role in the working of lenses.
• We have also studied about the nature, position and relative size of images formed by different types of lenses.
• We also know that it is only when light from an object enters our eye that we are able to see that object.
• These basic ideas will help us in our study of the human eye. 

• Light enters the eye through a transparent curved (tough) front surface. This is known as the cornea of the eye.
• It is whitish in colour (the 'white' of the eye).
• Its main function is to act as a protective layer for the eye. 
• The space behind it is filled with a liquid called acqueous humor. 

• Behind the cornea is a dark coloured muscular diaphragm called the iris. 
• The iris may be pigmented and is responsible for the characteristic colour of the eye of a person.

• A small circular aperture (opening) is present in the centre of iris. The size of this aperture is variable and self-adjustable. This aperture is known as the pupil. 
• The pupil appears black as no light is reflected from it. 
• The iris regulates the amount of light entering the eye by adjusting the size of pupil. 
• In dim light, the pupil gets enlarged and thus lets more light enter the eye. In bright light, the pupil contracts. 
• It is this self-adjustment of the size of the pupil that not only protects the interior of the eye from excessive brightness but also improves its image forming ability.

• The light entering the eye is focussed by the eye lens.
• The eye lens is a convex lens made up of transparent crystalline layers. It is harder at its middle and gradually becomes softer towards its edges.
• The eye lens is held in its position by ciliary muscles. 
• These muscles help in changing the curvature and hence the focal length of the eye lens.

• The lens of the eye forms a real, inverted image of the object on the inner coat of the eye. This screen of the eye is called the retina. 
• It is a light sensitive screen.
• It is a delicate membrane having enormous number of light sensitive cells or photoreceptors.
• These light sensitive cells are of two types:
  (i) The rod-shaped cells
  (ii) The cone-shaped cells. 
• The rods and cones (sensory nerve cells) respond, respectively, to the amount of light energy and to the colours present in it.
• The rods are responsible for the vision in dim condition and the cones help us in colour vision.
• The cones get activated only in bright light conditions. 

• Take a collection of objects that look almost identical (e.g. caps of different colours but of same size). 
• First work in a bright room and separate out the caps into piles of similar colours. 
• Count the number of caps of each type. 
• Mix them together after doing the counting. 
• Now, turn off most of the lights so that the room is only very dimly lit. 
• Again try to separate out the caps into similar coloured piles. 
• Turn on the lights and look at the results. 
• Count the number of errors made by you. 
• Mix up the lot once again. 
• Dim the lights again and after waiting for 7-10 minutes, again try to separate the caps in the same dim light conditions. 
• Switch on the lights and again count the errors.

• Draw a thick broken blue line and a red circle on a white sheet of paper. The distance between the line and circle may be adjusted to be about 6-8 cm. 
• Hold the sheet of paper at an arm's length from the eye. Close your right eye. 
• Look at the red circle and slowly move your head closer to the image.

• The rod shaped cells respond to the amount of incident light energy, i.e. to different degrees of brightness and darkness. 
• In dim light, the rods are sensitive but the cones are not. 
• They (the cones) respond mainly to the colours of the incident light. 
• They become active in bright light only and then enable us to make colour perception possible. 
• The cones are generally sensitive to red, green and blue light to different extents.