T1-NCERT-VII-Science

Chapter 1

Nutrition in Plants 

carbohydrates, proteins, fats, vitamins and minerals are components of food. These components of food are necessary for our body and are called nutrients. 

Carbon dioxide from air is taken in through the tiny pores present on the surface of the leaves. These pores are surrounded by ‘guard cells’. Such pores are called stomata 

The leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of the sunlight. This energy is used to synthesise (prepare) food from carbon dioxide and water. 

Since the synthesis of food occurs in the presence of sunlight, it is called photosynthesis (Photo: light; synthesis : to combine). So we find that chlorophyll, sunlight, carbon dioxide and water are necessary to carry out the process of photosynthesis. 

You often see slimy, green patches in ponds or in other stagnant water bodies. These are generally formed by the growth of organisms called algae. Can you guess why algae are green in colour? They contain chlorophyll which gives them the green colour. Algae can also prepare their own food by photosynthesis. 

SAPROTROPHS 

cotton-like threads spread on the piece of bread. 

These organisms are called fungi. They have a different mode of nutrition. They secrete digestive juices on the dead and decaying matter and convert it into a solution. Then they absorb the nutrients from it. This mode of nutrition in which organisms take in nutrients in solution form from dead and decaying matter is called saprotrophic nutrition. Plants which use saprotrophic mode of nutrition are called saprotrophs. 

Fungi also grow on pickles, leather, clothes and other articles that are left in hot and humid weather for long time. 

Some organisms live together and share shelter and nutrients. This is called symbiotic relationship. For example, certain fungi live in the roots of trees. The tree provides nutrients to the fungus and, in return, receives help from it to take up water and nutrients from the soil. This association is very important for the tree. 

In organisms called lichens, a chlorophyll-containing partner, which is an alga, and a fungus live together. 

fungus provides shelter, water and minerals to the alga and, in return, the alga provides food which it prepares by photosynthesis. 

though nitrogen gas is available in plenty in the air, plants cannot use it in the manner they can use carbon dioxide. They need nitrogen in a soluble form. The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a soluble form. But Rhizobium cannot make its own food. So it lives in the roots of gram, peas, moong, beans and other legumes and provides them with nitrogen. Most of the pulses (dals) are obtained from leguminous plants. In return, the plants provide food and shelter to the bacteria. 

symbiotic relationship 

Chapter 2

Normally bacteria are present in our mouth but they are not harmful
to us. However, if we do not clean our teeth and mouth after eating, (a) many harmful bacteria also begin to live and grow in it. These bacteria break down the sugars present from the leftover food and release acids (see Chapter 5 to know what an acid is). The acids gradually damage the teeth (Fig. 2.5). This is called tooth decay. If it is not (b) treated in time, it causes severe toothache and in extreme cases results in tooth loss. Chocolates, sweets, soft drinks and other sugar products are the major culprits of tooth decay. 

Therefore, one should clean the teeth with a brush or datun and dental floss (a special strong thread which is moved between two teeth to take out trapped food particles) at least twice a day and rinse the mouth after every meal. Also, one should not put dirty fingers or any unwashed object in the mouth. 

glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released. The food that remains undigested and unabsorbed enters into the large intestine. 

The grass is rich in cellulose, a type of carbohydrate. Many animals, including humans, cannot digest cellulose. 

• The grazing animals like cows, buffaloes and deer are known as ruminants. They quickly ingest, swallow their leafy food and store it in the rumen. Later, the food returns to the mouth and the animal chews it peacefully.
  

Chapter 4

The people living in the coastal areas experience an interesting phenomenon. During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze. To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea. At night it is exactly the reverse (Fig. 4.11). The water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze. 

The normal temperature of the human body is 37°C.

In solids, generally, the heat is transferred by conduction. In liquids and gases the heat is transferred by convection. No medium is required for transfer of heat by radiation.

Chapter 5

Soil treatment 

Excessive use of chemical fertilisers makes the soil acidic. Plants do not grow well when the soil is either too acidic or too basic. When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralises the basic nature of the soil. 

Factory wastes 

The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances. 

Chapter 6

For rusting, the presence of both 

oxygen and water (or water vapour) is essential. 

In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster. 

This process of depositing a layer of zinc on iron is called galvanisation. The iron pipes we use in our homes to carry water are galvanised to prevent rusting. 

Chapter 7

Some migratory birds travel as much as 15000 km to escape the extreme climatic conditions at home. Generally they fly high where the wind flow is helpful and the cold conditions allow them to disperse the heat generated by their flight muscles. But how these birds travel to the same place year after year is still a mystery. It seems that these birds have a built–in sense of direction and know in which direction to travel. Some birds probably use landmarks to guide them. Many birds may be guided by the sun during the day and stars at night. There is some evidence that birds may use the magnetic field of the earth to find direction. And it is not only birds that migrate; mammals, many types of fish and insects are also known to migrate seasonally in search of more hospitable climates. 

The tropical rainforests 

The tropical region has generally a hot climate because of its location around the equator. 

Tropical rainforests are found in Western Ghats and Assam in India, Southeast Asia, Central America and Central Africa. Because of continuous warmth and rain, this region supports wide variety of plants and animals. The major types of animals living in the rainforests are monkeys, apes, gorillas, tigers, elephants, leopards, lizards, snakes, birds and insects. 

The lion-tailed macaque (also called Beard ape) lives in the rainforests of Western Ghats (Fig. 7.10). Its most outstanding feature is the silver-white mane, which surrounds the head from the cheeks down to its chin. It is a good climber and spends a major part of its life on the tree. It feeds mainly on fruits. It also eats seeds, young leaves, stems, flowers and buds. This beard ape also searches for insects under the bark of the trees. Since it is able to get sufficient food on the trees, it rarely comes down on the ground. 

• All the changes in the weather are driven by the sun.
  
•  Migration is another means to escape the harsh, cold conditions.
  
•  Animals in the tropical rainforests are adapted such that they eat different kinds of food to overcome the competition for food and shelter.
  

Chapter 8

increased wind speed is, indeed, accompanied by a reduced air pressure. 

WIND CURRENTS ARE GENERATED DUE TO UNEVEN HEATING ON THE EARTH 

1-Uneven heating between the equator and the poles 

regions close to the equator get maximum heat from the Sun. The air in these regions gets warm. The warm air rises, and the cooler air from the regions in the 0 – 30 degrees latitude belt 

on either side of the equator moves in. These winds blow from the north and the south towards the equator. At the poles, the air is colder than that at latitudes about 60 degrees. The warm air at these latitudes rises up and the cold wind from the polar regions rushes in, to take its place. In this way, wind circulation is set up from the poles to the warmer latitudes, as shown in 

The winds would have flown in the north-south direction from north to south, or from south to north. A change in direction is however, caused by the rotation of the earth. 

2-Uneven heating of land and water 

In summer, near the equator the land warms up faster and most of the time the temperature of the land is higher than that of water in the oceans. The air over the land gets heated and rises. This causes the winds to flow from the oceans towards the land. These are monsoon winds.In winter, the direction of the wind flow gets reversed; it flows from the land to the ocean (Fig. 8.10). 

The winds from the oceans carry water and bring rain. It is a part of the water cycle. 

The monsoon winds carry water and it rains. 

THUNDERSTORMS AND CYCLONES 

Thunderstorms develop in hot, humid tropical areas like India very frequently. The rising temperatures produce strong upward rising winds. These winds carry water droplets upwards, where they freeze, and fall down again. The swift movement of the falling water droplets along with the rising air create lightning and sound. It is this event that we call a thunderstorm. 

Structure of a cyclone 

The centre of a cyclone is a calm area. It is called the eye of the storm. A large cyclone is a violently rotating mass of air in the atmosphere, 10 to 15 km high. The diameter of the eye varies from 10 to 30 km (Fig. 8.11). It is a region free of clouds and has light winds. Around this calm and clear eye (Fig. 8.12), there is a cloud region of about 150 km in size. In this region there are high-speed winds (150–250 km/h) and thick clouds with heavy rain. Away from this region the wind speed gradually decreases. 

Before cloud formation, water takes up heat from the atmosphere to change into vapour. When water vapour changes back to liquid form as raindrops, this heat is released to the atmosphere. The heat released to the atmosphere warms the air around. The air tends to rise and causes a drop in pressure. More air rushes to the centre of the storm. This cycle is repeated. The chain of events ends with the formation of a very low-pressure system with very high-speed winds revolving around it. It is this weather condition that we call a cyclone. Factors like wind speed, wind direction, temperature and humidity contribute to the development of cyclones. 

A cyclone is known by different names in different parts of the world. It is called a ‘hurricane’ in the American continent. In Philippines and Japan it is called a ‘typhoon’ 

Tornadoes

In our country they are not very frequent. A tornado is a dark funnel shaped cloud that reaches from the sky to the ground (Fig. 8.16). Most of the tornadoes are weak. A violent tornado can travel at speeds of about 300 km/h. Tornadoes may form within cyclones. 

The whole coastline of India is vulnerable to cyclones, particularly the east coast. The west coast of India is less vulnerable to cyclonic storms both in terms of intensity and frequency of the cyclones. 

We have learnt that all storms are low- pressure systems. Wind speed plays an important role in the formation of storms. It is, therefore, important to measure the wind speed. The instrument that measures the wind speed is called an anemometer. 

Thanks to satellites and radars, a Cyclone alert or Cyclone watch is issued 48 hours in advance of any 

expected storm and a Cyclone warning is issued 24 hrs in advance. The message is broadcast every hour or half hour when a cyclone is nearer the coast. Several national and international organisations cooperate to monitor the cyclone-related disasters. 

As warm air rises, air pressure at that place is reduced and the cooler air moves to that place.

Chapter 9

The rotting dead matter in the soil is
called humus.
You probably know that the soil is
formed by the breaking down of rocks by the action of wind, water and climate. This process is called weathering. The nature of any soil depends upon the rocks from which it has been formed and the type of vegetation that grows in it.
A vertical section through different layers of the soil is called the soil profile.

The mixture of rock particles and humus is called the soil. 

The soil is classified on the basis of the proportion of particles of various sizes. If soil contains greater proportion of big particles it is called sandy soil. If the proportion of fine particles is relatively higher, then it is called clayey soil. If the amount of large and fine particles is about the same, then the soil is called loamy. Thus, the soil can be classified as sandy, clayey and loamy. 

So clay soils have little air. But they are heavy as they hold more water than the sandy soils. 

The best topsoil for growing plants is loam. Loamy soil is a mixture of sand, clay and another type of soil particle known as silt. Silt occurs as a deposit in river beds. The size of the silt particles is between those of sand and clay. The loamy soil also has humus in it. It has the right water holding capacity for the growth of plants. 

• Soil is affected by wind, rainfall, temperature, light and humidity. These are some important climatic factors which affect the soil profile and bring changes in the soil structure.
  

Clayey and loamy soils are both suitable for growing cereals like wheat, and gram. Such soils are good at 

retaining water. For paddy, soils rich in clay and organic matter and having a good capacity to retain water are ideal. For lentils (masoor) and other pulses, loamy soils, which drain water easily, are required. For cotton, sandy- loam or loam, which drain water easily and can hold plenty of air, are more suitable. 

Crops such as wheat are grown in the fine clayey soils, because they are rich in humus and are very fertile. 

Soil erosion 

The removal of land surface by water, wind or ice is known as erosion. Plant roots firmly bind the soil. In the absence of plants, soil becomes loose. So it can be moved by wind and flowing water. Erosion of soil is more severe in areas of little or no surface vegetation, such as desert or bare lands. So, cutting of trees and deforestation should be prevented and effort should be made to increase the green areas 

Chapter 10

The process of breakdown of food in the cell with the release of energy is called cellular respiration. Cellular respiration takes place in the cells of all organisms. 

In the cell, the food (glucose) is broken down into carbon dioxide and water using oxygen. When breakdown of glucose occurs with the use of oxygen it is called aerobic respiration. Food can also be broken down, without using oxygen. This is called anaerobic respiration. Breakdown of food releases energy. 

Glucose With the use of oxygen→ carbon dioxide + water + energy 

there are some organisms such as yeast that can survive in the absence of air. They are called anaerobes. They get energy through anaerobic respiration. In the absence of oxygen, glucose breaks down into alcohol and carbon dioxide, as given below: 

Glucose Without the use of oxygen → alcohol + carbon dioxide + energy 

Yeasts are single-celled organisms. They respire anaerobically and during this process yield alcohol. They are, therefore, used to make wine and beer. 

Our muscle cells can also respire anaerobically, but only for a short time, when there is a temporary deficiency of oxygen. During heavy exercise, fast running (Fig. 10.1), cycling, walking for many hours or heavy weight lifting, the demand for energy is high. But the supply of oxygen to produce the energy is limited. Then anaerobic respiration takes places in the muscle cells to fulfil the demand of energy: 

Glucose In the absence of oxygen→ lactic acid + energy 

(in muscle)

Have you ever wondered why you get muscle cramps after heavy exercise? The cramps occur when muscle cells respire anaerobically. The partial breakdown of glucose produces lactic acid. The accumulation of lactic acid causes muscle cramps. We get relief from cramps after a hot water bath or a massage. Can you guess why it is so? Hot water bath or massage improves circulation of blood. As a result, the supply of oxygen to the muscle cells increases. The increase in the supply of oxygen results in the complete breakdown of lactic acid into carbon dioxide and water 

Breathing means taking in air rich in oxygen and giving out air rich in carbon dioxide with the help of respiratory organs. 

A breath means one inhalation plus one exhalation. 

you must have realised that whenever a person needs extra energy, he/she breathes faster. As a result more oxygen is supplied to our cells. It speeds up the breakdown of food and more energy is released. Does this explain why do we feel hungry after a physical activity? 

On an average, an adult human being at rest breathes in and out 15ñ18 times in a minute. During heavy exercise, the breathing rate can increase upto 25 times per minute. 

Chapter 11

The fluid part of the blood is called plasma. 

One type of cells are the red blood cells (RBC) which contain a red pigment called haemoglobin. Haemoglobin binds with oxygen and transports it to all the parts of the body and ultimately to all the cells. It will be difficult to provide oxygen efficiently to all the cells of the body without haemoglobin. The presence of haemoglobin makes blood appear red. 

The blood also has white blood cells (WBC) which fight against germs that may enter our body. 

The clot is formed because of the presence of another type of cells in the blood, called platelets 

two types of blood vessels, arteries and veins are present in the body. 

Arteries carry oxygen-rich blood from the heart to all parts of the body. Since the blood flow is rapid and at a high pressure, the arteries have thick elastic walls. 

Why do you think there is throbbing? This throbbing is called the pulse and it is due to the blood flowing in the arteries. Count the number of pulse beats in one minute. 

A resting person, usually has a pulse rate between 72 and 80 beats per minute. 

Veins are the vessels which carry carbon dioxide-rich blood from all parts of the body back to the heart. The veins have thin walls. There are valves present in veins which allow blood to flow only towards the heart. 

arteries divide into smaller vessels. On reaching the tissues, they divide further into extremely thin tubes called capillaries. The capillaries join up to form veins which empty into the heart. 

Heart 

The heart is an organ which beats continuously to act as a pump for the transport of blood, which carries other substances with it. 

TRANSPORT OF SUBSTANCES IN PLANTS 

Plants absorb water and minerals by the roots. The roots have root hair. 

The root hair increase the surface area of the root for the absorption of water and mineral nutrients dissolved in water. The root hair is in contact with the water present between the soil particles 

Well, Boojho is right. Plants have pipe-like vessels to transport water and nutrients from the soil. The vessels are made of special cells, forming the vascular tissue. A tissue is a group of cells that perform specialised function in an organism. The vascular tissue for the transport of water and nutrients in the plant is called the xylem 

The xylem forms a continuous network of channels that connects roots to the leaves through the stem and branches and thus transports water to the entire plant 

You know that leaves synthesise food. The food has to be transported to all parts of the plant. This is done by the vascular tissue called the phloem. Thus, xylem and phloem transport substances in plants. 

Transpiration 

Plants absorb mineral nutrients and water from the soil. Not all the water absorbed is utilised by the plant. The water evaporates through the stomata present on the surface of the leaves by the process of transpiration. The evaporation of water from leaves generates a suction pull which can pull water to great heights in the tall trees. Transpiration also cools the plant. 

Chapter 12

Asexual reproduction 

In asexual reproduction new plants are obtained without production of seeds or spores. 

Vegetative propagation 

It is a type of asexual reproduction in which new plants are produced from roots, stems, leaves and buds. Since reproduction is through the vegetative parts of the plant, it is known as vegetative propagation. 

Budding 

tiny organisms like yeast can be seen only under a microscope. These grow and multiply every few hours if sufficient nutrients are made available to them. Remember that yeast is a single-celled organism. 

Fragmentation 

You might have seen slimy green patches in ponds, or in other stagnant water bodies. These are the algae. When water and nutrients are available algae grow and multiply rapidly by fragmentation. An alga breaks up into two or more fragments. These fragments or pieces grow into new individuals (Fig. 12.6). This process continues and they cover a large area in a short period of time. 

Spore formation 

In Chapter 1 you learnt that the fungi on a bread piece grow from spores which are present in the air.When spores are released they keep floating in the air. As they are very light they can cover long distances. 

The spores are asexual reproductive bodies. Each spore is covered by a hard protective coat to withstand unfavourable conditions such as high temperature and low humidity. So they can survive for a long time. Under favourable conditions, a spore germinates and develops into a new individual. Plants such as moss and ferns (Fig. 12.8) also reproduce by means of spores. 

SEXUAL REPRODUCTION 

You have learnt earlier the structure of a flower. You know that the flowers are the reproductive parts of a plant. The stamens are the male reproductive part and the pistil is the female reproductive part 

The flowers which contain either only the pistil or only the stamens are called unisexual flowers. The flowers which contain both stamens and pistil are called bisexual flowers. Corn, papaya and cucumber produce unisexual flowers, whereas mustard, rose and petunia have bisexual flowers. 

Both the male and the female unisexual flowers may be present in the same plant or in different plants. 

The transfer of pollen from the anther to the stigma of a flower is called pollination. If the pollen lands on the stigma of the same flower it is called self-pollination. When the pollen of a flower lands on the stigma of another flower of the same plant, or that of a different plant of the same kind, it is called cross-pollination.

SEED DISPERSAL 

In nature same kind of plants grow at different places. This happens because seeds are dispersed to different places. 

What do you think will happen if all seeds of a plant were to fall at the same place and grow there? There would be severe competition for sunlight, water, minerals and space. As a result the seeds would not grow into healthy plants. Plants benefit by seed dispersal. It prevents competition between the plant and its own seedlings for sunlight, water and minerals. It also enables the plants to invade new habitats for wider distribution. 

The pulse of a normal healthy adult at rest beats about 72 times in a minute that is about 12 times in 10 seconds. This rate may be slightly higher for children 

 A nanosecond is one billionth of a second. Clocks that measure such small time intervals are used for scientific research. The time measuring devices used in sports can measure time intervals that are one tenth or one hundredth of a second. 

However, before buying bulbs or tubes, or CFLs, look for the ISI mark of the Bureau of Indian Standards. In fact, before buying any electrical appliance, look for this mark. The ISI mark ensures that the appliance is safe and wastage of energy is minimum. 

One reason for excessive currents in electrical circuits is the direct touching of wires. This may happen if the insulation on the wires has come off due to wear and tear. This may cause a short circuit. Another reason for excessive current can be the connection of many devices to a single socket. This may cause overload in the circuit. 

There is a maximum limit on the current which can safely flow through a circuit. If by accident the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires 

Miniature circuit breakers (MCBs) are increasingly being used in place of fuses. These are switches which automatically turn off when current in a circuit exceeds the safe limit. You turn them on and the circuit is once again complete. Look for ISI mark on MCBs also. 

So, when electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current. In fact, an electric current can be used to make magnets. 

The coil in the above activity behaves like a magnet when electric current flows through it. When the electric current is switched off, the coil generally loses its magnetism. Such coils are called electromagnets. The electromagnets can be made very strong and can lift very heavy loads. Do you remember the crane about which you read in Chapter 13 of Class VI? The end of such a crane has a strong electromagnet attached to it. The electromagnets are also used to separate magnetic material from the junk. Doctors use tiny electromagnets to take out small pieces of magnetic material that have accidentally fallen in the eye. 

• Whenanelectriccurrentflowsthroughawire,thewiregetsheated.Itis the heating effect of current. This effect has many applications.
  

Chapter 15

REFLECTION OF LIGHT 

One way to change the direction of light is to let it fall on a shiny surface. For example, a shining stainless steel plate or a shining steel spoon can change the direction of light. The surface of water can also act like a mirror and change the path of light. Have you ever seen the reflection of trees or buildings in water 

An image formed by a plane mirror is erect and of the same size as the object. 

If the reflecting surface of a spherical mirror is concave, it is called a concave mirror. If the reflecting surface is convex, then it is a convex mirror 

concave and convex mirrors called spherical mirrors

Take a concave mirror. Hold it facing the sun. Try to get the light reflected by the mirror on a sheet of paper. Adjust the distance of the paper until you get a sharp bright spot on it (Fig. 15.14). Hold the mirror and the sheet of paper steady for a few minutes. Does the paper start burning? 

This bright spot is, in fact, the image of the sun. Notice that this image is formed on a screen. An image formed on a screen is called a real image. Recollect that in Activity 15.2 the image formed by a plane mirror could not be obtained on a screen. Such an image is called a virtual image. 

Concave mirrors are used for many purposes. You might have seen doctors using concave mirrors for examining eyes, ears, nose and throat. Concave mirrors are also used by dentists to see an enlarged image of the teeth (Fig. 15.17). The reflectors of torches, headlights of cars and scooters are concave in shape 

Can you now recognise the mirrors used as side mirrors in scooters? These are convex mirrors. Convex mirrors can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them 

IMAGES FORMED BY LENSES 

You might have seen a magnifying glass. It is used to read very small print (Fig. 15.21). You might have also used it to observe the body parts of a cockroach or an earthworm. The magnifying glass is actually a type of a lens. 

Those lenses which feel thicker in the middle than at the edges are convex lenses 

It is dangerous to look through a lens at the sun or a bright light. You should also be careful not to focus sunlight with a convex lens on any part of your body. 

Take a convex lens or magnifying glass. Put it in the path of sunrays. Place a sheet of paper as shown (Fig. 15.23). Adjust the distance between the lens and the paper till you get a bright spot on the paper. Hold the lens and the paper in this position for a few minutes. Does the paper begin to burn 

Chapter 16

22 March is celebrated as the world water day 

not to waste water. No wonder we celebrate water day every year to attract the attention of everybody towards the importance of conserving water. 

Year 2003 was observed as the International Year of Freshwater to make people 

aware of this dwindling natural resource. 

The rainwater and water from other sources such as rivers and ponds seeps through the soil and fills the empty spaces and cracks deep below the ground. The process of seeping of water into the ground is called infiltration. The groundwater thus gets recharged by this process. At places the groundwater is stored between layers of hard rock below the water table. This is known as an aquifer. Water in the aquifers can be usually pumped out with the help of tube wells or handpumps. 

on the one hand we are consuming more groundwater, and on the other we are allowing lesser water to seep into the ground. This results in depletion of water table. 

We have at many places in India an age old practice of water storage and water recharge like the bawris. Bawri was the traditional way of collecting water. 

(Fig. 16.9). Drip irrigation is a technique of watering plants by making use of narrow tubings which deliver water directly at the base of the plant. 

Paheli wondered what mushroom and other micro-organisms eat. Prof Ahmad replied that they feed upon the dead plant and animal tissues and convert them into a dark coloured substance called humus. 

The micro-organisms which convert the dead plants and animals to humus are known as decomposers. 

Chapter 18

Realising the urgency of the situation on the World Water Day, on 22 March 2005, the General Assembly of the United Nations proclaimed the period 2005 – 2015 as the International Decade for action on “Water for life”. All efforts made during this decade aim to reduce by half the number of people who do not have access to safe drinking water. 

Cleaning of water is a process of removing pollutants before it enters a water body or is reused. This process of wastewater treatment is commonly known as “Sewage Treatment”. It takes place in several stages. 

Sewage is a liquid waste. Most of it is water, which has dissolved and suspended impurities. These impurities are called contaminants. 

Mahatma Gandhi said: 

“No one need to wait for anyone else to adopt a humane and enlightened course of action.””

You must realise your responsibility in maintaining the water sources in a healthy state. Adopting good sanitation practices should be our way of life.