Current Affairs 6th Class

Structures and Functions of the Living Body   All animals and plants have different organs to perform various functions. Each part of the body of an animal or plant is different in structure. The organs, however, function in coordination with one another.   SPECIFIC PARTS OF PLANTS
  • Plants have two main systems: (i) the root system (ii) the shoot system
  The Root System The root system grows mainly underground. Root systems are of two types: (i) Tap root system (ii) Fibrous root system (i) Tap root system: It is a main root and grows vertically down into the soil. The tap root gives out branches. For example, pea, neem, mango. (ii) Fibrous root system: Some plants do not have main root. They have many fibre-like roots. These are called fibrous roots. These roots spread out in the soil and give firm support to the plant. For example, wheat, grass, maize and millet. Advantages of the root system: As roots grow normally underground, they fix the plant to the ground. They absorb the mineral salts and water from the ground, which are needed for the plant to grow. Roots also help hold the soil together. They save the soil from being blown off or washed away.   The Shoot System The shoot system grows above the ground. It consists of the main stem, its branches and leaves. (i)   The stem: The stem holds the plant upright. The stem is the strongest part of a tree and is known as the trunk. Most trunks are observed with bark. The bark protects the inner part of the tree. The stems-of some plants are weak. They cannot stand erect. Stems carry water from the roots to the leaves and flowers. They also carry food from the leaves to other parts of the plant. They hold the leaves in such a way that the leaves get plenty of light from the sun. (ii) The leaves: Leaves are important parts of plants. They manufacture food for the plants. They are green because they contain a green pigment. This pigment is called chlorophyll. To manufacture food, the green leaves need sunlight, air and water. The process of making food in the presence of sunlight is called photosynthesis. (iii) Flowers and fruits: In a flower, the green leaf-like parts in the outermost circle are called sepals. Towards the centre of a flower many little stalks with swollen tops are present and they called stamens. The swollen tops are called anthers. They contain a powdery substance called pollen. The stamen is the male part of a flower. In the centre of the flower, there is a flesh-shaped organ called the carpel. The carpel is the female part of a flower. The little swollen portion at the base is called the ovary. The ovary contains egg-like structures called ovules. Pollen are transferred to the carpel in a process called pollination. This is done by insects, wind and water. Eventually, the ovules of the flower turn into more...

  • Air is everywhere around us. No living being can survive without air.
  • The air covers the whole Earth. This cover of air is called the atmosphere. We live within the atmosphere. It extends over hundreds of kilometres. Up to a height of 16 km, we find clouds, rain and snow. As we go up in the atmosphere, there is less and less air. Jet planes usually fly above the clouds.
  • Air is matter. It occupies space and has mass. It has no colour and we can see through it. It fills all the space available to it.
  • Air is a mixture of several gases. Nearly fourth-fifths (4/5) of it is nitrogen. About one-fifth (1/5) of it is oxygen. Air also has a small amount of carbon dioxide, argon, helium, water vapour and dust particles.
  • At higher altitudes, air is thin and under the water, the available oxygen is less. Thus, breathing becomes difficult. Therefore, mountaineers and divers carry cylinders containing oxygen with them. Oxygen is also supplied to the patients who suffer from breathing difficulties.
  • Oxygen is used by living organisms for respiration. During respiration, oxygen breaks down the food to release energy. During this process, carbon dioxide and water vapour are produced and released.
  • Carbon dioxide along with water is used by the green plants in the presence of sunlight to make their food. This process is called photosynthesis. During this process, oxygen is released. In nature, the balance of oxygen and carbon dioxide is, thus, maintained.
  • Air is required by human beings for respiration.
  • Air is necessary for burning. A chulha, sign or kerosene stove burn only when sufficient air is available.
  • Vehicles such as bicycles, few kinds of carts, scooters, cars, trucks and aeroplanes have tyres filled with compressed air. Such tires make transport smooth and easier.
  • Balls used for playing and balloons can be used only when they are inflated with air.
  • Compressed air is used in machines for digging, mining and breaking stones. It is also used for lifting liquid substances from a mine.
  • Compressed air is used in the brake systems of trains.
  • Air helps in drying agricultural products such as grains, pulses, dry fruits and wet clothes.
  • People feel cool in summer under a fan. The fan circulates air. This helps in rapid evaporation of sweat.
  • Air helps in the movement of sail boats, gliders, parachutes and aircraft. Birds, bats and insects fly in the air.
  • Air makes a windmill move. A windmill is used to draw water from tube wells and to operate flour mills. Along the coast, windmills are used to generate electricity.

Water   Water is the most common and important substance around us. Water is essential for almost every task we perform on a daily basis as well as for agriculture and industries.
  • All animals and plants need water. The human body has about 70 percent water by weight. Similarly the elephants and plants have 80 percent and 60 per cent water by weight respectively.
  • Animals drink water from ponds, streams and rivers. Plants take in water from the soil through their roots. From the roots, it goes to different parts of the plant. The plant uses this water for its life processes. It also loses water continuously from the tiny openings in the leaves. The process is called transpiration.
  • Seeds cannot germinate without water. Water helps animals in releasing heat which maintains their body temperatures.
  • A villager in India uses about 12 litres of water every day. In cities, a person uses 50-2000 litres of water every day. With the rising living standards, the requirement of water has also increased.
  • Large amounts of water are consumed in agricultural activities. Many industries such as paper, rayon, petroleum refining, fertiliser, dye, drug and chemical industries require large quantities of water.
  • In some countries, people use water to warm their houses. It is used to keep things cool. A car radiator is filled with water to keep the engine cool.
  • The largest amount of water on the Earth is in oceans. The oceans cover more than two-thirds (2/3) surface area of the Earth. Seawater is salty and cannot be used at home and in agriculture. So, we depend upon other sources of water like springs, rivers, lakes, ponds, wells, rain, snow and underground water. Water obtained from these sources is not always fit for drinking and cooking purposes. Many impurities and germs may be present in it.
  • Various methods are used to make this impure water fit for drinking. People in cities get pure water from taps. This impure water travels a long way to reach the taps. In many cases, water is first pumped from a source, such as a river or lake, and collected in a reservoir. Then it goes to the waterworks where it is cleaned. Here, it is filtered through layers of gravel and sand. The dirt stays behind in the sand, then water is treated with some chemicals like chlorine to kill the germs. The clean water is supplied through main pipes to different parts of the city. Smaller pipes take the water to each house.
At places where tap water is not available, people draw out water from rivers, lakes, springs and wells. Water from these sources should be made fit for use by boiling, filtering and treating it with some chemicals such as potassium permanganate.    
  • Pure water is colourless, odourless, tasteless and transparent. Small quantities of dissolved salts and gases give a pleasant taste to water. Water from the wells, more...

  Motion and Measurement of Distances   Synopsis    
  • Different modes of transport are used to go from one place to another.
  • In ancient times, people used the length of a foot, the width of a finger, the distance of a step as units of measurement. This caused variation and inaccuracy and a need to develop a uniform system of measurement arose.
  •  Now, the International System of Units (S.I. unit) is followed all over the world.
  • A unit is a standardized quantity of a physical property, used as a factor to express quantities of that property.
  •  A standard unit is the measurement value which remains the same even when it is measured by anybody at any place.
  • The standard units of length are millimetre (mm), centimetre (cm), metre (m) and kilometer (km).
  • The following is the relationship between the units.
\[10\text{ }mm=1\text{ }cm\] \[100\,\,cm=1\text{ }m\] \[1000\text{ }m=1\text{ }km\]        
  • The correct technique of measuring length is the eye being directly in line with the other end of the object as shown in the adjacent figure.
  •  A thread or string and a ruler are used for measuring the length of a curved line and the circumference of a ball.
  Types of motion
  •   Motion in a straight line is called rectilinear motion.
  • A man walking and the motion of a cycle, a bus and that of a car on a straight path are all in translatory motion.
  • If the motions of points or parts of an object are along a circular path instead of a straight line path/then the motion of the body is said to be in rotatory motion.
  • The to and fro motions of an object about a fixed point is called oscillatory motion.
  • Oscillatory motion which is very fast to begin with and soon slows down and comes to rest is called vibratory motion.

Light, Shadows and Reflection   Synopsis    
  • Objects which emit light energy by themselves are called luminous bodies.
          e.g., the sun, the stars, and glow worms.
  •  The bodies which do not have light energy of their own but reflect the light energy falling on them and hence are visible to us are called non-luminous bodies.
         e.g., the moon, objects around us, books, chairs, buildings, trees, etc.  
  • The moon appears bright due to the reflection of sunlight falling on it.
  • Transparent bodies are substances through which light is propagated easily,
          e.g., glass/water/etc.  
  • Translucent bodies are substances through which light is propagated partially.
          e.g., oil, ground glass, etc.  
  • Opaque bodies are substances through which light is not propagated.
         e.g., wood, iron, etc.  
  • Light travels in straight lines.
  • Smooth surfaces like mirrors form images.
  • The shadow of an object is formed because of the rectilinear propagation of light. A shadow is the area of darkness formed on the screen, when an opaque body is placed in between the screen and a source of light
  • A pin hole camera uses the principle of rectilinear propagation of light. It produces a real image which is much smaller than the object and is inverted (upside down).
  •  The returning of a light ray passing through an optical medium into the same medium from the surface of the second medium is called the reflection of light.
  • The surfaces which reflect light are called reflecting surfaces.
  •  Images are different from shadows.

Electricity and Circuits   Synopsis  
  • Electricity is a form of energy widely used by man.
  • The simplest form of a circuit is when the two terminals of a cell are connected to the two terminals of a bulb through a switch.
  • In an electric circuit/the direction of current is taken from the positive terminal to the negative terminal of the source.
  • When a circuit is complete, current (charges) starts flowing. It stops flowing when the circuit breaks at a point.
  • A switch is used to break or complete a circuit.
  • Materials that allow electricity to pass through them are called conductors. Most of the metals and impure water are examples of conductors.
  • Materials that do not allow electricity to pass through them are called insulators. Dry wood, pure water, air and rubber are examples of insulators.
  • To represent an electric circuit certain symbols are being used. A list of few symbols are given below in the table.
Circuit components Symbols
Tap key
Fun With Magnets   Synopsis  
  • Substances that can attract iron are called magnets.
  • Substances can be divided into magnetic and non-magnetic substances.
  • Substances that are attracted by a magnet are called magnetic substances. Usually things made up of iron, nickel or cobalt are magnetic.
  • Substances that are not attracted by a magnet are called nan-magnetic substances. Paper, plastic and wood are a few examples of non-magnetic substances.
  • Magnets are divided into two groups - natural and artificial magnets. Magnetite (lodestone) is a naturally occurring magnet.
  • Artificial magnets can be of many shapes and are commonly used in different articles like electric bells, radio, etc.
  • The magnetic force or the force of attraction in a magnet is concentrated at the two ends of a magnet. These ends are called poles. They are called the north pole and the south pole.
  • Properties of magnets
         (i) There are always two poles in a magnet placed at opposite ends. This holds true even if we break or cut the magnet into      smaller pieces. Each piece           will have two poles.          (ii) When two different magnets are brought closer, like poles repel and unlike poles attract each other.          (iii) A magnet, when suspended freely, always comes to rest in a line along the north-south line of that place.  
  • The bar AB (of iron or steel) to be magnetized is stroked with a bar magnet from one end to the other using the same pole as shown in the figure. This process is repeated for about 50 times.
  • The end A from where the process starts develops the same polarity as the pole of the magnet stroking and the other end develops the opposite polarity.
  • Magnets lose their properties when they are heated, hammered or dropped and if they are not stored properly.
  • Magnetism is widely used to make articles like cassettes, computers, T.V., ATM cards, etc.

Sorting Materials into Groups   Synopsis    
  • The process of the classification of objects based on some known criteria is called grouping or sorting.
  • We have a huge variety of objects present around us. Because of this, it is very important to classify objects into groups. If we know the properties of one member of the group, it would be very easy to predict the properties of the other members too.
Some of the properties of materials are:  
  • Appearance
The various parameters governing the appearance of materials are colour/texture, roughness, shape, size, etc.  
  • Lustre
Some materials shine when light falls on them. This property is generally observed in objects that are grouped as metals, e.g. gold and silver. They are hard, strong, flexible and good conductors of heat and electricity. They can be drawn into wires (ductile) and sheets (malleable).  
  • Hardness
It is the measure of how resistant a material is to various kinds of shape changes when a force is applied. Hardness can also be measured by the ease with which a material can be compressed or stretched. Materials that can be easily stretched and compressed are called soft materials. Materials which are difficult to compress and stretch are called hard materials.  
  • Solubility
In general, solubility is defined as the ability of a substance to dissolve in water. In the process of dissolving/the substance which is being dissolved is called a solute and the substance in which the solute is dissolved is called a solvent. A mixture of solute and solvent is called a solution, e.g. if we add sugar to water, then, sugar is the solute and water is the solvent. The mixture of sugar and water is called sugar solution. Substances which are not soluble in water are called insoluble substances, e.g., sand, wood. If two liquids mix together completely then they are called misdble liquids, e.g. water and ink. If two liquids do not mix but form separate layers, then they are called immiscible liquids, e.g., water and oil.  
  • Relative density or Density with respect to water
\[\text{Relative density =}\frac{\text{Mass of substance}}{\text{Mass of water}},\,\,\,\text{Density=}\frac{Mass}{Volume}\] The density of water at \[{{4}^{o}}C\] is taken as 1. Substances having relative density of more than 1 tend to sink in water and those having less than 1 tend to float on water.  
  • Behaviour towards light
Substances can be divided into three groups depending on their behaviour towards light. Transparent materials They allow most of the light to pass through them, e.g., glass. Translucent materials They allow only some part of the light to pass through them, e.g., butter paper. Opaque materials They do not allow light to pass through them, e.g. iron sheet or wood.       more...

Separation of Substances   Synopsis      
  • Matter can be broadly divided into two major groups "pure?? and "impure"
  • Matter is made up of tiny particles called molecules. Molecules are further made of same or different kinds of atoms.
  • Molecules of a substance have all the properties of that substance.
  •  A pure substance is made up of only one type Of molecules and so it is called homogeneous substance, e.g., elements, compounds.
  • An impure substance is made up of more than one type of molecules and it is called heterogeneous substance, e.g., mixtures.
  •  A substance which is uniform in its composition and properties throughout is called a homogeneous substance.
  •  A substance which is not uniform in its composition and properties throughout is called a heterogeneous substance.
  • Methods of Separation: Some simple methods of separating substances that are mixed together are given below.         
  •  Handpicking is used for separating slightly larger sized impurities like the pieces of dirt stones and husk from wheat, rice or pulses.
  • Threshing is used to separate grain from stalks.
  • Winnowing is used to separate heavier and lighter components of a mixture by wind or by blowing air. 
  • Sieving makes the fine flour particles to pass through the holes of the sieve while the bidder impurities remain on the sieve.
  • Sedimentation settles the heavier component in a mixture after water is added to it.
  • Filtration is used to separate components of a mixture of an insoluble solid and a liquid.
  • Evaporation is the process of conversion of water into its vapour.
  • Condensation is the process of conversion of water vapour into its liquid form.
  • Saturated solution is the one in which no more of a substance can be dissolved.
  • Sometimes, a combination of methods are to be used to separate mixtures of more than two substances.
  • Substances may need to be separated from each other to remove impurities or to get two or more useful products.
  Methods of separation            
Type of mixtures   Methods of separation
(i) Solid-solid (heterogeneous)      Handpicking, winnowing, sieving, sublimation, magnetic separation
(ii)  Solid-liquid (heterogeneous) Filtration, sedimentation, decantation
(iii) Solid-liquid (homogeneous)      Evaporation, distillation, crystallisation
(iv) Liquid-liquid (heterogeneous)    Decantation, separating funnel
Changes Around Us   Synopsis   Types of changes  
  • Changes can also be classified based on the rate at which they take place.
  • Fast change: The change that takes place within a very short time.
  • Slow change: The change that occurs over a long period of time.
  • Another factor for change is uniformity with respect to time.
  • Periodic changes: Changes that repeatedly occur at fixed intervals of time e.g., change of seasons, formation of day and night.
  •  Non-periodic changes: Changes that do not repeatedly occur at fixed intervals of time are non-periodic e.g., occurrence of storms and rain.
  • Reversible change: A change in which the original substances can be retrieved. e.g., mixing of iron particles and sulphur.
  • Irreversible change: A change in which a new substance is formed and the original substances cannot be retrieved, e.g., heating a mixture of iron pieces and sulphur.
  • Exothermic reaction: A change in which heat is released e.g., burning of coal.
  • Endothermic reaction: A change in which heat is absorbed e.g., evaporation of water.
  • When substances react with each other, they require the right conditions like temperature, pressure, light, catalyst, etc. The rate and the extent of change depends on these conditions.
  • Unstable substances undergo change easily.
  • Differences between physical and chemical changes:
Physical Changes Chemical Changes
1. Change is temporary.  1. Change is permanent.
2. It can be reversed. 2.  It cannot be reversed by simple, chemical or physical means.

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