Chapter 15 · Our Environment

NCERT Exemplar Class 10 Science Chapter 15 Long Answer Questions

All 8 questions of NCERT Exemplar Class 10 Science Chapter 15 Long Answer Questions (Our Environment), in the book's order, each with its answer and a step-by-step solution.

1. Q31
   Indicate the flow ​of energy ​in an ​ecosystem. ​Why is it unidirectional? ​Justify.
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   Energy flows one way from the Sun up the trophic levels, losing about $90\%$ as heat at each step, so it cannot be recycled.

   Flow of energy: the Sun is the source. Green plants capture about $1\%$ of the sunlight falling on their leaves and convert it by photosynthesis into chemical energy, so the producers form the first trophic level ($T_1$). From them the energy passes to the herbivores ($T_2$), then to the small carnivores ($T_3$) and then to the top carnivores ($T_4$), while decomposers act on the dead matter of every level. At each transfer only about $10\%$ of the energy is stored at the next level — the ten per cent law — so $10000$ J at the producers leaves about $1000$ J at the herbivores, $100$ J at the small carnivores and $10$ J at the top carnivores. The remaining $90\%$ at each step is used up in respiration, movement and other life processes and is given off to the surroundings as heat. The flow is unidirectional because: (i) Energy enters the ecosystem at a single point, since only producers can trap solar energy; (ii) It moves only in the direction of feeding, producer to herbivore to carnivore, and never backwards; (iii) At every step a large part is converted into heat, which is dissipated into the atmosphere, and no organism can capture heat and rebuild it into food; and (iv) Therefore energy cannot be recycled the way carbon, nitrogen and water are — matter cycles, energy flows. Two consequences confirm it: food chains are short, usually of three or four links, because after four transfers only about $1$ J of the original $10000$ J is left; and the pyramid of energy is always upright. This is why every ecosystem needs a continuous fresh supply of solar energy.

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2. Q32
   What ​are decomposers? ​What will ​be the consequence of ​their absence in an ​ecosystem?
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   Decomposers are bacteria and fungi that break down dead matter; without them waste piles up, nutrients stay locked and the ecosystem collapses.

   Decomposers are saprophytic microorganisms — mainly bacteria and fungi — that feed on the dead remains of plants and animals and on their excreta, breaking the complex organic matter down by their enzymes into simple inorganic substances such as carbon dioxide, water, nitrates, phosphates and minerals, and obtaining their own energy from the process. If they were absent from an ecosystem: (i) Dead bodies, fallen leaves and excreta would not be broken down and would go on piling up, leaving no room for new organisms and making the surroundings filthy and foul; (ii) The nitrogen, phosphorus, potassium, carbon and minerals held inside those dead bodies would stay locked in them and would never return to the soil, water and air; (iii) The nitrogen, carbon and phosphorus cycles, whose return arm is decomposition, would break, so matter would no longer be recycled; (iv) The soil, no longer replenished and forming no humus, would lose its fertility, its texture and its water-holding capacity; (v) The producers, starved of raw materials, could not grow, and since every food chain rests on the producers, the herbivores would starve and then the carnivores, so the whole ecosystem would collapse; and (vi) Sewage and other biodegradable waste would remain untreated, spreading pollution, foul odour and disease. In short, decomposers are the link that returns matter to the non-living environment, and without that link an ecosystem cannot go on working.

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3. Q33
   Suggest ​any ​four ​activities ​in daily life ​which ​are eco-friendly.
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   Carry a cloth bag, segregate and compost waste, save electricity, and save water and travel wisely.

   Four eco-friendly activities for daily life (any four): (i) Carry a cloth or jute bag for shopping and refuse plastic carry bags, disposable plastic plates, cups and straws — one reusable bag replaces hundreds of polythene bags, which are non-biodegradable, choke drains, kill cattle that swallow them and give toxic fumes when burnt. (ii) Segregate household waste into biodegradable and non-biodegradable, compost the kitchen and garden waste in a pit or a vermicompost bed to make manure, and hand paper, glass, metal and plastic to a recycler — this returns the wet waste to the nutrient cycle and keeps the dry waste fit for recycling. (iii) Save electricity: switch off lights, fans and appliances when they are not needed, use LED or CFL lamps, dry clothes in the sun and use solar water heaters or cookers — most of our electricity comes from burning coal, a non-renewable fuel whose combustion pollutes the air. (iv) Save water and travel wisely: mend leaking taps, use a bucket instead of a running shower, reuse water from washing for the plants, harvest rainwater, and walk, cycle, car-pool or use public transport for short journeys — this conserves a limited resource and reduces vehicle exhaust. Other acceptable answers include planting and caring for trees, using both sides of paper, using rechargeable batteries, and not burning dry leaves or waste.

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4. Q34
   Give ​two differences ​between ​food ​chain and ​food ​web.
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   A food chain is a single linear sequence that breaks easily; a food web is a stable network of interlinked chains.

   (i) Structure and number of food sources: a food chain is a single straight sequence of organisms in which each one is eaten by the next, so every organism has only one source of food, only one consumer and one trophic level — for example Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake $\to$ Hawk. A food web is the network formed when many such chains of an ecosystem are interconnected, because most organisms eat several kinds of food and are eaten by several kinds of predator, so an organism may feed at more than one trophic level and the energy has many alternative paths. (ii) Stability: a food chain is fragile and rarely occurs alone — if any one member is removed, the chain breaks and the levels above it starve; a food web is stable, because if one prey species disappears its predators switch to an alternative food and the ecosystem continues to function. This is why food webs, and not isolated food chains, are what actually exist in nature.

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5. Q35
   Name ​the ​wastes which are ​generated ​in your house daily. What ​measures would you take ​for ​their disposal?
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   Household waste is wet, dry or hazardous; segregate it, compost the wet part, recycle the dry part and never burn or dump waste.

   Wastes generated in a house daily: (i) Biodegradable (wet) waste — vegetable and fruit peels, leftover food, tea leaves, egg shells, spoilt food, garden leaves and flowers, cow dung. (ii) Non-biodegradable (dry) waste — plastic bags, wrappers, bottles and containers, glass bottles and broken glass, metal tins, aluminium foil, thermocol, rubber, synthetic cloth, and paper and cardboard (biodegradable, but better recycled). (iii) Hazardous and sanitary waste — used batteries, fused CFL and tube lights, expired medicines, broken thermometers, empty pesticide and paint tins, e-waste, used sanitary napkins and diapers. Measures for their disposal: (1) Reduce and reuse first — carry a cloth bag, avoid disposable plastic articles and over-packed goods, refill bottles and jars, use both sides of paper. (2) Segregate at source into three separate bins — green for wet waste, blue for dry recyclable waste, and a covered container for hazardous and sanitary waste. (3) Compost the wet waste in a compost pit or vermicompost bed, or give it to the wet-waste collection; it returns as manure for the plants, and where a biogas plant is available it also gives fuel gas. (4) Wash, dry and hand over the dry waste — paper, glass, metal and plastic — to the recycler so that it is made into new articles. (5) Deposit batteries, CFLs, medicines and e-waste only at authorised collection or take-back points, and wrap and dispose of sanitary waste separately as directed. (6) Send only the inert residue that is left to a properly built landfill. Never burn waste in the open, never throw it into drains, ponds or empty plots, and never leave it uncovered, since that breeds flies, mosquitoes and rats.

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6. Q36
   Suggest ​suitable ​mechanism (s) ​for waste management ​in ​fertiliser ​industries.
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   Reduce waste at source, treat the effluent, control gaseous emissions, recover and recycle materials and manage solid waste safely, with regular monitoring.

   A fertiliser plant makes three kinds of waste — liquid effluent (ammoniacal nitrogen, urea, nitrate, phosphate, fluoride, acids, oil), waste gases (ammonia, oxides of nitrogen and sulphur, fluorides, carbon dioxide, fertiliser dust) and solid waste (phosphogypsum, sludge, spent catalyst) — and each needs its own mechanism. (i) Reduce at source: closed modern process design, recovery of process condensate, leak-proof handling, separate storm-water drains and good housekeeping, so that less waste is made at all. (ii) Effluent treatment plant, in sequence: screening and equalisation, neutralisation with lime or acid, primary settling to remove solids and oil, steam or air stripping to drive off ammonia, biological treatment by activated sludge with nitrifying and denitrifying bacteria to remove the nitrogen, secondary clarification, and tertiary polishing by sand filtration and activated carbon. (iii) Recovery and recycling: condense and return the stripped ammonia, recover urea dust from the prilling tower, reuse the treated water as cooling and process water to reach zero liquid discharge, use phosphogypsum for cement and plaster board, recover sulphur from waste gas and send spent catalyst for metal recovery. (iv) Air-pollution control: cyclone separators, bag filters and electrostatic precipitators for dust; water, acid and alkali scrubbers to absorb ammonia, sulphur dioxide, oxides of nitrogen and fluorides; and a tall stack with continuous on-line monitoring. (v) Solid-waste management: dewater the sludge and store phosphogypsum on lined, impervious platforms with leachate collection, so rain cannot carry fluoride and heavy metals into the groundwater, and send only inert residue to a secure landfill. (vi) Monitoring and protection: regular testing of effluent and emissions against the prescribed standards, a green belt of trees around the plant, worker training and an emergency plan for ammonia leaks.

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7. Q37
   What ​are ​the by-products ​of ​fertiliser industries? ​How ​do they affect the environment?
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   By-products are gases, liquid effluent and solid wastes; they cause acid rain, fluorosis, eutrophication and contamination of groundwater and soil.

   By-products of fertiliser industries: (i) Gases — ammonia, oxides of nitrogen, sulphur dioxide, carbon dioxide and carbon monoxide, fluorine compounds such as hydrogen fluoride from phosphatic plants, and fertiliser dust and particulates. (ii) Liquid effluent — ammoniacal nitrogen, urea, nitrates, phosphates, fluoride, acids and alkalis, oil and grease, suspended solids and traces of heavy metals such as cadmium from rock phosphate. (iii) Solids — phosphogypsum, sludge, spent catalyst and off-specification product. Their effects on the environment: (1) Sulphur dioxide and oxides of nitrogen dissolve in rain to form sulphuric and nitric acid, giving acid rain, which acidifies soils and lakes, damages crops, forests and aquatic life and corrodes marble buildings; oxides of nitrogen also help form photochemical smog. (2) Ammonia irritates the eyes, nose and lungs and is dangerous in a large leak; fluoride gases settle on fodder and cause fluorosis in cattle, with mottled teeth and stiff joints, and scorch the foliage of crops; dust and particulates cause respiratory disease. (3) Nitrate and phosphate in the effluent reach ponds, lakes and rivers and cause eutrophication — a heavy algal bloom grows and cuts off the light, and when it dies the decomposers consume the dissolved oxygen, so fish and other aquatic animals suffocate and the water body becomes foul. (4) Nitrate seeping into the groundwater makes drinking water unsafe and causes methaemoglobinaemia, the blue-baby disease, in infants, while fluoride causes dental and skeletal fluorosis. (5) Heaps of phosphogypsum and sludge occupy land and their leachate carries fluoride and cadmium into the soil and groundwater, from where cadmium can enter crops and the food chain. (6) Carbon dioxide adds to the greenhouse effect and nitrous oxide is both a powerful greenhouse gas and an ozone-depleting one. Almost every one of these is the product itself, escaped untreated, so recovery and effluent treatment cure the damage and save material at the same time.

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8. Q38
   Explain some harmful ​effects ​of ​agricultural practices ​on the ​environment.
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   Fertilisers cause eutrophication and unsafe nitrate; pesticides are biomagnified; over-irrigation, groundwater use, monoculture and burning residue also damage soil, water and air.

   Harmful effects of agricultural practices: (i) Excess chemical fertiliser — the part the crop cannot absorb is washed into ponds, lakes and rivers and causes eutrophication, in which an algal bloom grows, dies and is decomposed, using up the dissolved oxygen so that fish die; nitrate also leaches into the groundwater and makes drinking water unsafe, causing the blue-baby disease in infants; and continued heavy use makes the soil acidic, kills earthworms and soil microorganisms and destroys the soil structure. (ii) Pesticides and weedicides — being non-biodegradable, their residues stay in the soil and water, are absorbed by plants and pass into the food chain. Since animals cannot excrete them, their concentration increases at every higher trophic level — biomagnification. Taking a ten-fold rise at each step, producers at $0.01$ ppm give herbivores $0.1$ ppm, small carnivores $1$ ppm and top carnivores $10$ ppm, while over the same three steps the available energy falls from $10000$ J to $10$ J: the residue rises exactly where the energy falls, and man, at the top of many chains, gets the largest dose. Pesticides also kill honey bees and other pollinators, earthworms, fish, birds and the natural predators of the pest, and the surviving pests become resistant. (iii) Over-irrigation with poor drainage — waterlogging and the accumulation of salts left behind by evaporation, so the soil becomes saline or alkaline and finally barren. (iv) Excessive withdrawal of groundwater by tube wells — the water table falls, wells dry up and sea water enters coastal aquifers. (v) Monoculture of high-yielding varieties and the clearing of forest and grassland — loss of agro-biodiversity, so that a single pest or disease can destroy the whole crop, along with habitat loss and soil erosion. (vi) Burning the crop residue — smoke, particulates and greenhouse gases in the air and the destruction of the soil's organic matter and microorganisms; paddy fields and cattle also release methane and nitrogenous fertiliser releases nitrous oxide, both powerful greenhouse gases. The remedies mirror the causes: apply fertiliser by soil test and use manure and biofertilisers, use biological pest control and integrated pest management, irrigate by drip or sprinkler with proper drainage, harvest rainwater, practise crop rotation and mixed cropping, and plough the stubble back into the soil.

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