Environmental chemistry: water: Chemistry unit 14 notes (FBISE class 11)

Earth's water

Water is an extremely important ingredient of the environment. It makes up the hydrosphere, and life and related processes are impossible without it. Water is the basis of metabolic, agricultural and industrial activities.

The world's total water is classified into two major categories: saltish (oceanic) water and fresh water. The book's diagram shows how the water is shared out.

Distribution of the world's water, from the book's diagram
Level	Store of water	Share
Total water	Oceans (saltish water)	97%
Total water	Freshwater	3%
Freshwater	Ice caps and glaciers	70%
Freshwater	Groundwater	29%
Freshwater	Easily accessible freshwater	1%
Easily accessible freshwater	Lakes	52%
Easily accessible freshwater	Soil moisture	38%
Easily accessible freshwater	Rivers	1%
Easily accessible freshwater	Water vapour	1%
Easily accessible freshwater	Water in living things	1%

Worked example: how much water is easily accessible?

Out of every

100000 L

of water on the Earth, find (a) the volume of fresh water, (b) the volume of easily accessible fresh water, and (c) the volume of this accessible water that lies in lakes. Use the shares in the book's table.

Fresh water is $3%$ of all water (the other $97%$ is saltish water in oceans).
$V_{fresh} = 100000 \times 0.03 = 3000 L$
Easily accessible fresh water is $1%$ of the fresh water, not of all water. The other shares of fresh water are ice caps and glaciers ( $70%$ ) and groundwater ( $29%$ ).
$V_{acc} = 3000 \times 0.01 = 30 L$
Lakes hold $52%$ of the easily accessible fresh water, so use $30 L$ as the base.
$V_{lakes} = 30 \times 0.52 = 15.6 L$
Check the meaning. Only $0.03%$ of all water is easily accessible, which is why saving fresh water matters.
$\frac{30}{100 000} \times 100 = 0.03%$

Each step takes a small share of the one above it. The highlighted box is the water that people can reach easily.

3000 L, 30 L, 15.6 L

Point and nonpoint sources

There are two types of sources of water pollution: point sources and nonpoint sources. The difference is whether the place where the pollutant enters the water is known.

Point source: A source of water pollution whose outlet or location is specified, so it is known where the pollutants enter the water. Examples are effluents from industry, water treatment plants and oil spills from the petroleum industry. These sources can be identified and stopped easily.
Nonpoint source: A source that is widespread, with no specified location or origin. Pollutants reach the water by seepage or runoff from many places. Examples are pesticides, fertilisers, construction, agriculture, and roadside or residential effluents such as detergents, cooking oils and their derivatives.

Feature	Point source	Nonpoint source
Location	Specified (a known outlet)	Not specified (spread widely)
How pollutants arrive	Directly from an outlet	By seepage or runoff
Examples	Industrial effluents, water treatment plants, oil spills from petroleum industry	Pesticides, fertilisers, construction, agriculture, residential effluents
Control	Can be identified and stopped easily	Harder to identify and stop

14.1Sources of water pollutionOften examined

The book's diagram sets the nonpoint sources (left) against the point sources (right) and links each source to the pollutants it releases. The table lists those links.

Sources and pollutants, from the book's diagram
Source	Type	Pollutants shown
Farms	Nonpoint	Fertilisers, herbicides and pesticides
Residential and urban streets	Nonpoint	Salt on winter roads, oil, grease and chemicals from urban runoff
Construction sites and deforested or overgrazed land	Nonpoint	Eroded soil
Abandoned mines	Nonpoint (also a point source)	Acid drainage
Animal feeding and sewage treatment plants	Point	Nutrients, waste and bacteria
Landfills and disposal sites	Point	Industrial waste and toxic chemicals
Oil tankers	Point	Oil spills

Agricultural based pollutants

Pesticides can pose serious hazards to aquatic life. Runoff from treated fields or gardens carries pesticides into water bodies, where they harm fish, insects and other organisms directly and disturb the food chain. Some pesticides are designed to persist, which increases their environmental impact.

Fertilisers, pesticides and herbicides mixed with rainwater quickly reach nearby water bodies and make them highly polluted. Agricultural slush from harvested fields also adds to the pollution.

Oil spillage

Oil cargo ships, oil carriers and offshore exploration of oil reservoirs sometimes leak oil accidentally. Crude oil on the surface forms a black sheet over the water. This stops aeration (the exchange of gases with the air) and the penetration of light into the water, which badly affects aquatic plants.

Sewage and wastewater

Sewage water treatment cleans sewage well before it is released into a water body. If it is not done properly, untreated water carries bacteria and other pathogens into the water body, causing serious diseases.

Mining activities

Mining for minerals and ores produces a lot of grit. Rain washes the grit into nearby water bodies, which makes them shallow and dirty. The water becomes unfit for fisheries and general use, and the water body loses its aesthetic value.

Industrial pollution

Industrial effluents laden with heavy metals (such as lead, mercury and cadmium) contaminate water sources. This affects aquatic ecosystems and then human health.
Heavy metals are persistent pollutants. They accumulate in sediments and aquatic organisms, causing bioaccumulation and biomagnification along the food chain. This can cause long-term ecological damage and harm people who rely on contaminated water for drinking or agriculture.
Released industrial effluents containing heavy metals can damage soil quality, disrupting plant growth and farm productivity. They can also leach into groundwater and add to the risk to the human population.

14.2Health effects of water pollutantsOften examined

Contaminated water and poor sanitation are linked to the spread of diseases such as cholera, diarrhoea, dysentery, hepatitis A, typhoid and polio. Water and sanitation services that are absent, inadequate or badly managed expose people to diseases that could be prevented.

Heavy metals such as cadmium, mercury, arsenic, lead and copper harm health. They can cause neurotoxicity (damage to the nervous system) and nephrotoxicity (damage to the kidneys), which can lead to mental problems and kidney failure. Bioaccumulation of heavy metals can lead to severe problems such as cancer and syndromes.

Effects of water pollution

Destruction of marine and land ecosystems.
Eutrophication, with effects on food and the food chain.
Effects on human health and diseases, and on the water cycle.
Effects on the groundwater system and impact on agriculture.
Shortage of water for drinking and other purposes.

14.3Environmental problems caused by water pollution

Water pollution has far-reaching effects on the environment.

Aquatic life

Polluted water cannot support fisheries, and fish cannot survive in it. Aluminium ions can leach from the soil into water bodies and form gelatinous aluminium hydroxide, $Al (OH) X_{3}$ . This clogs the gills of fish and suffocates them. It happens when surfactants, heavy metals and pesticides are present in the polluted water.

Eutrophication

Excessive use of fertilisers in fields lets nutrients drain into water bodies with the rain, along with other agricultural runoff. This causes an algal bloom, a dense growth of algae that covers the water, and the water body is eventually lost from the surface. The book notes that many lakes and ponds have been lost this way.

Contaminated drinking water

Surface water and groundwater become unfit for drinking when bacteria, other pathogens and agricultural waste contaminate them. This causes waterborne diseases.

Destruction of habitats

Natural habitats such as coral reefs and wetlands can be destroyed by water pollution. These habitats support whole ecosystems, mark the shoreline and protect the land from floods. Measures that reduce water pollution help to save these ecosystems and the environment as a whole.

Worked example: why a pond is lost after heavy use of fertiliser

Farmers around a pond use a large amount of fertiliser. After some years the pond is covered with green algae and then disappears. (a) Explain the sequence of events. (b) Is the fertiliser a point source or a nonpoint source? Give a reason.

Rain washes the extra fertiliser from the fields. The nutrients drain into the pond with the other agricultural runoff.
The extra nutrients make algae grow very fast. This dense growth is an algal bloom, and the process is called eutrophication.
The algae cover the water. Eventually the water body is lost from the surface, as the book says happened to many lakes and ponds.
Classify the source. The fertiliser reaches the pond by runoff from many fields, with no single outlet, so it is a nonpoint source. This also makes it harder to identify and stop than a factory pipe, which would be a point source.

Eutrophication of a pond: the highlighted algal bloom is the stage that covers the water.

eutrophication; the fertiliser is a nonpoint source

Worked example: aluminium ions and fish gills

Aluminium ions leach from soil into a stream and form gelatinous

Al (OH) X_{3}

that clogs fish gills. Calculate the mass of

Al (OH) X_{3}

formed when

2.7 mg

of

Al X^{3 +}

reacts completely with hydroxide ions. (

Al = 27

,

O = 16

,

H = 1

.)

Given

$m (Al X^{3 +}) = 2.7 mg = 2.7 \times 1 0^{- 3} g$
$M (Al (OH) X_{3}) = 78 g mo l^{- 1}$

To find

mass of $Al (OH) X_{3}$

Write the balanced ionic equation. Three hydroxide ions are needed to cancel the charge $3 +$ of one aluminium ion, and the mole ratio $Al X^{3 +} : Al (OH) X_{3}$ is $1 : 1$ .
$Al X^{3 +} + 3 OH X^{-} Al (OH) X_{3}$
Convert the mass of aluminium ions to moles.
$n (Al X^{3 +}) = \frac{2.7 \times 1 0 ^{- 3}}{27} = 1.0 \times 1 0^{- 4} mol$
By the $1 : 1$ ratio, the same number of moles of $Al (OH) X_{3}$ forms. Its molar mass is $27 + 3 (16 + 1) = 78 g mo l^{- 1}$ .
$m = 1.0 \times 1 0^{- 4} \times 78 = 7.8 \times 1 0^{- 3} g$
Check. The hydroxide ions used are $3 \times 1 0^{- 4} mol \times 17 = 5.1 mg$ , and $2.7 + 5.1 = 7.8 mg$ .

m (Al (OH) X_{3}) = 7.8 mg

14.4.1Raw water treatmentOften examined

Raw water is water obtained from groundwater, a river or a lake. Municipal supplies use it for drinking after it is cleaned. Raw water is contaminated with grit and pathogens, so it must be treated before use.

Order of treatment in a municipal plant

Screening: remove the first, large debris such as sticks, leaves, pebbles and soil.
Coagulation and flocculation: add the coagulant so that fine particles clump into flocs, which trap the impurities.
Sedimentation: let the water stand for a long time so that the flocs and solids settle out as sludge.
Filtration: pass the water through filters to remove the remaining impurities, pathogens and fine particles.
Disinfection: kill the pathogens that remain.

Screening

This is the first step in cleaning the raw water. It removes debris such as sticks, leaves, pebbles and soil.

Coagulation and flocculation

For this purpose, alum stone is used. The book says it contains ferric sulphate and ammonium sulphate. It forms flocs (small clumps) that coagulate with the particles and gather impurities, which are then removed.

Sedimentation

The raw water is allowed to stand in large tanks for 12 hours or more. The solid particles with a slightly greater mass settle to the bottom, forming sludge, which is removed.

Filtration

Water may be passed through different filters, such as activated charcoal and many layers of sand and gravel. Impurities, pathogens and fine particles are removed this way.

Killing the pathogens (disinfection)

Microorganisms such as bacteria, viruses and parasites must be killed so that they do not cause diseases. The methods are chlorination, ozonation and UV treatment.

Other treatment methods include pH adjustment, membrane filtration, ion exchange treatment, aeration (to remove smell) and desalination.

Raw water treatment at a municipal plant: groundwater is dosed, coagulated, filtered and chlorinated before it is stored. Simplified from the book's plant diagram.

Worked example: what if a step of raw water treatment is left out?

A treatment plant skips (a) screening, (b) coagulation and flocculation, or (c) disinfection. State what goes wrong in each case.

Recall the order: screening, coagulation and flocculation, sedimentation, filtration, disinfection. Each step removes a different kind of impurity, so no step can replace another.
(a) Without screening, sticks, leaves, pebbles and soil enter the plant. The later tanks and filters are overloaded with large debris.
(b) Without the coagulant, the fine particles do not clump into flocs. They are too light to settle in the sedimentation tank, so they pass on to the filters.
(c) Filtration removes many impurities and pathogens, but the book lists a separate step to kill the microorganisms (bacteria, viruses and parasites) that must not cause disease: chlorination, ozonation or UV treatment. Without it, some of these can stay in the supply and cause waterborne diseases.

each step removes its own kind of impurity; disinfection kills the remaining pathogens

14.4.2Waste water treatmentOften examined

Wastewater must be cleaned before it is thrown into water bodies, because it can severely contaminate them and make them unable to support aquatic life. Waste water treatment is carried out in stages.

Stages of waste water treatment
Stage	Method	What it removes or does
Primary	Screening and sedimentation	Physically removes sand, gravel and large particles
Secondary	Biological processes (activated sludge, trickling filters)	Breaks down the organic matter in wastewater
Tertiary	Advanced chemical and physical processes	Removes nutrients such as nitrates and phosphorus compounds, which cause eutrophication
Disinfection	Chlorination, ozonation, ultraviolet radiation	Kills bacteria and other pathogenic organisms

Filtration

Techniques such as sand or membrane filtration remove fine particles and impurities from wastewater.

Advanced oxidation processes (AOPs)

These are powerful oxidising methods in which strong oxidising agents break down persistent organic compounds. These compounds are usually present in contaminated water from industries.

Constructed wetlands

Natural or artificial wetlands can treat wastewater by letting natural processes purify the water.

Waste water treatment in stages: primary treatment removes solids, secondary treatment uses aeration and activated sludge, and the effluent is disinfected before discharge. Simplified from the book's diagram (credited there to Encyclopaedia Britannica).

14.5Laws and regulations related to water pollution and conservation

Legislation is made globally, and nationally, to control water pollution and conserve water resources.

Kyoto protocol (worldwide)

The protocol was agreed in Kyoto, Japan. According to the book it includes measures for water pollution worldwide, including reducing emissions from deforestation and forest degradation that can affect water resources.

Clean Water Act (CWA), USA

The Clean Water Act is the primary federal law in the United States on water pollution. Its objective is to restore the biological, physical and chemical integrity of United States waters. It recognises the responsibilities of states and people for funding and public awareness, sets national standards for drinking water quality and regulates public water systems so that they supply safe, clean drinking water.

Water pollution control act (China)

This law sets water standards, monitoring procedures and funding, permission policies for industrial units, and penalties for violations.

Laws and regulations for the prevention of water pollution in Pakistan

There are many laws and regulations in Pakistan to prevent water pollution and to conserve the water reservoirs of the country.

Pakistan Environmental Protection Act 1997: provides a legal framework for the conservation, protection and improvement of the environment, including air, water and soil. It empowers the Environmental Protection Agency (EPA) to control and regulate water pollution.
National Environmental Quality Standards (NEQS): ensure the quality of various pollutants present in water bodies, according to WHO and EPA standards in Pakistan. The book calls NEQS an agency, but they are a set of quality standards.
Pakistan Water Act 1998: a regulatory act that governs the development, conservation and management of water resources in Pakistan. It promotes sustainable use of water and reaffirms the right of all citizens to equal and affordable access to clean drinking water and sanitation.

14.6Impact of human activities on water resources

Water resources are badly affected by human activities such as deforestation, industrialisation, agricultural development, mining, construction and transportation.

Agricultural impact

Agriculture is the main user of freshwater, which creates strain on local water supplies. Water used in farming takes a long time to return to the water cycle.

Pesticides and herbicides are made of dangerous, poisonous chemicals, and some pesticide molecules are carcinogenic. Fertilisers cause eutrophication and algal bloom, which reduces the aesthetic value of the water body and can diminish the whole water body.

Poor land management and extra tilling cause soil erosion. The soil fills the bed of water bodies, making them shallow and less able to hold water.

Extra pumping from tube wells for farming lowers the groundwater and causes a shortage of water in aquifers. The aquifer is replenished by rainfall, which hardly makes up the deficit.

Converting natural landscape into farmland harms the natural ecosystem. Improved technology, such as drip irrigation, saves time and water, and sustainable farming methods help to protect water resources.

Industrial impact

Industrial units create pollution by releasing heavy metals, smoke, carbon particles, pigments, and organic and inorganic chemicals such as acids, alkalis, oxidising agents and reducing agents. These make water unfit for drinking and for aesthetic use, and unfit for some aquatic species.

Industries use a lot of water for condensation, cooling and as a solvent. Where water is used as a coolant, the demand is very large, and this strains the local water supply.

Industries can release heat into a river or stream, which raises the water temperature. As the temperature rises, the oxygen solubility of water decreases, so species such as trout cannot survive in hot water.

Large dams and canals can change the pathway of water and disrupt the aquatic ecosystem over a large region. Industrialisation can destroy natural habitats and small ecosystems, extractive industries can dislodge groundwater sources, and mining can destroy streams in a local area.

To reduce these impacts, environmental regulations, sustainable water management and wastewater treatment technologies are essential. Industries are increasingly adopting eco-friendly practices.

14.7Conservation and management strategies for protecting and preserving water resourcesOften examined

Some steps are crucial for the conservation and management of water resources, so that they are protected and preserved for the future. The book lists ten steps.

Smart use of water: promote efficient water use in homes, industries and agriculture through technologies such as drip irrigation, low-flow fixtures and water-efficient appliances.
Wastewater treatment: install treatment plants on a small scale, and purify used water for reuse. This saves freshwater.
Rainwater harvesting: collect rainwater for non-potable uses such as irrigation and toilet flushing. This reduces the demand for freshwater.
Watershed protection: restore and maintain natural watersheds for a steady supply of water. This includes erosion control and reforestation of barren lands.
Water recycling: promote the reuse of wastewater for irrigation and industrial processes.
Legislation and regulation: enforce laws to prevent pollution, control water withdrawals and ensure the sustainable management of water reservoirs.
Public awareness: educate people about the importance of water, its conservation, and methods to reduce water pollution.
Desalination: cities located near the oceans can use desalinated water as a source of freshwater, reducing the burden on freshwater supplies.
Research and technology: governments should invest in research into innovative technology for saving water and sustainable water management practices.
International cooperation: countries should collaborate with neighbouring regions and other countries on trans-boundary issues, to ensure sustainable and equitable water use.

When these strategies are implemented effectively, they can help to safeguard water resources for current and future generations.

14.8Properties of water related to pollution

Water is an excellent solvent because of its properties: polarity, a high dielectric constant, the ability to form hydrogen bonds, ion-dipole forces, high surface tension, and greater density in the liquid state.

Water dissolves a lot of substances, so it is called the universal solvent. This is why it is rarely found in pure form.

Rainwater and carbonates

Rainwater contains dissolved carbon dioxide, which makes it suitable for dissolving carbonate rocks. A small amount of carbonic acid is formed, and it converts insoluble carbonates into soluble bicarbonates. The book gives these equilibria:

H X_{2} O + CO X_{2} H X_{2} CO X_{3}

Carbon dioxide dissolves in water

H X_{2} CO X_{3} H X^{+} + HCO X_{3} X^{-}

Carbonic acid partly ionises

Organic compounds and microbes

Many organic compounds dissolve in water because of hydrogen bonding, including alcohols, amines and carboxylic acids.

Water has a high surface tension, so most microbes can walk over it. This makes water a suitable habitat for microorganisms that cause diseases.

Surfactants and heat capacity

Water can dissolve surfactants and soaps, which can leach metal ions out of the minerals in runoff. Water also has a very high heat capacity, so it absorbs a lot of heat from industries. This makes water unfit for different aquatic species.

Pollution can alter these properties, which harms aquatic ecosystems, human health and the environment. Monitoring water quality involves checking changes in these properties and their consequences for aquatic ecosystems and human populations.

Key terms

point source: A source of water pollution whose outlet or location is specified, so the place where pollutants enter the water is known.
nonpoint source: A widespread source of pollution with no specified location, where pollutants reach the water by seepage or runoff.
effluent: Liquid waste released into a water body, for example from industry or a treatment plant.
oil spill: The accidental leakage of crude oil into water, which forms a sheet over the surface.
heavy metals: Metals such as lead, mercury, cadmium, arsenic and copper that are toxic and persist in the environment.
persistent pollutant: A pollutant that does not break down easily and stays in the environment for a long time.
bioaccumulation: The build-up of a substance in the tissues of an organism over time.
biomagnification: The increase in concentration of a substance at each higher level of the food chain.
eutrophication: Excess nutrients in water that cause algal bloom and eventually destroy the water body.
algal bloom: A dense growth of algae on the surface of water, which follows excess fertilisers.
pathogen: A microorganism, such as a bacterium, virus or parasite, that can cause disease.
surfactant: A substance such as soap or detergent that lowers the surface tension of water and can dissolve in it.
raw water: Water obtained from groundwater, a river or a lake, which must be cleaned before use.
screening: The first step in raw water treatment, which removes large debris such as sticks, leaves and pebbles.
coagulation: Adding a coagulant so that fine particles clump together.
flocculation: The formation of small clumps (flocs) that trap impurities so they can be removed.
sedimentation: Letting water stand so that solid particles settle to the bottom as sludge.
sludge: The solid material that settles out during water or waste water treatment.
filtration: Passing water through filters such as charcoal, sand, gravel or membranes to remove impurities.
disinfection: Killing or removing pathogens in water, for example by chlorination, ozonation or UV treatment.
chlorination: Disinfection of water by adding chlorine or a chlorine compound such as sodium hypochlorite.
ozonation: Disinfection of water by treating it with ozone.
desalination: Removing salt from sea water to make it fit to use as fresh water.
waste water treatment: The cleaning of used water (sewage or industrial effluent) before it is released into a water body.
primary treatment: The first stage of waste water treatment, which physically removes sand, gravel and large particles by screening and sedimentation.
secondary treatment: The stage of waste water treatment in which biological processes break down organic matter.
tertiary treatment: Advanced treatment that removes nutrients such as nitrates and phosphorus compounds.
activated sludge: Sludge rich in microorganisms that is returned to the aeration tank to break down organic matter.
advanced oxidation processes: Powerful oxidising methods that break down persistent organic compounds in contaminated water.
constructed wetland: A natural or artificial wetland used to purify wastewater by natural processes.
universal solvent: Water, because it dissolves a large number of compounds.
hydrogen bonding: The attraction between a hydrogen atom bonded to an electronegative atom and a lone pair on another such atom, which lets water dissolve many substances.
aquifer: An underground layer of rock or soil that holds groundwater.
watershed: An area of land that drains water into a river, lake or groundwater.
rainwater harvesting: Collecting rainwater for non-potable uses such as irrigation and toilet flushing.
water recycling: Reusing treated wastewater for irrigation and industrial processes.

Multiple choice questionsp. 282

The book prints no answer key for these multiple choice questions, so each key is worked out from the unit text. Part (i) is not printed in the book.

ii

How much of the total water of the planet is present in the oceans?

A90%
B80%
C97%Answer
D55%

Why

The oceans and seas hold $97%$ of the planet's total water, and this water is saltish (Key points, page 281; Earth's water diagram, page 273). The other three figures do not match the book's share of the oceans.

iii

What type of pollutants can cause eutrophication?

AAgriculturalAnswer
BIndustrial
COil spillage
DDetergents

Why

Eutrophication is caused by nutrients, mainly nitrates and phosphates, that wash off treated fields with the rain and feed algal blooms. The book names fertilizers, pesticides and herbicides, which are agricultural pollutants, as the compounds that cause eutrophication (pages 273 and 281).

Oil spillage blocks aeration and light, industrial effluent mainly carries heavy metals and other chemicals, and detergents are listed only as residential or roadside effluents. None of these is the nutrient source the book describes.

Note. Detergents can carry phosphates in practice, but the book does not link detergents with eutrophication. Its answer is the agricultural group (option a).

iv

In which method of the following are the small particles coagulated into big particles and then separated?

AFiltration
BCoagulationAnswer
CSedimentation
DChlorination

Why

In coagulation, alum (ferric sulphate and ammonium sulphate) is added to raw water. It forms flocs that gather the fine particles and impurities into larger clumps, which are then removed (page 277). The question describes this joining of small particles into big ones.

Filtration only passes water through sand, gravel or charcoal, and sedimentation lets heavy particles settle but does not join them. Chlorination kills pathogens and removes no particles at all.

Note. Sedimentation is the separation step that follows coagulation, so it is the tempting option c. The question asks for the method in which the particles are joined, which is coagulation (option b).

v

Which pollution can cause cancer?

AAgricultural
BOil spills
CHeat
DHeavy metalsAnswer

Why

The book says that bioaccumulation of heavy metals, which persist in sediments and organisms and move up the food chain, can lead to severe problems such as cancer and syndromes (pages 274 and 277). Heavy metals are therefore the pollutant that the book links directly to cancer.

Oil spills block aeration and light, and heat lowers the oxygen solubility of water. Neither is linked with cancer in the unit.

Note. Option a is tempting because the book says some pesticide molecules are carcinogenic (page 279). Option d is the pollutant the book names directly with cancer, so it is the intended answer.

vi

Disinfection of water is done by:

AOzonation
BUV radiation
CChlorination
DAll of aboveAnswer

Why

Disinfection kills bacteria, viruses and parasites. The book lists chlorination, ozonation and ultraviolet treatment as the disinfection methods (pages 276 and 278). Each of options a, b and c is therefore a correct method, so the complete answer is d.

vii

World sources of freshwater are \underline{\hspace{2em}} day by day.

AIncreasing
BDecreasingAnswer
CNot changing
DFluctuating

Why

The book describes freshwater sources as under growing strain. Extra pumping from tube wells causes a shortage of groundwater, replenishment depends on rainfall which hardly recovers the deficiency, and pollution makes water unfit to use (page 279 and section 14.6).

Options a and c are not supported by the book, which describes growing shortage, and option d describes a fluctuating supply rather than a steady loss.

Note. The book prints no key for this exercise. The answer is worked out from the unit text.

Short questionsp. 282

23 marks

Explain the distribution of water on the global level.

Solution

Almost all of the planet's water is saltish. About $97%$ of the total water is in the oceans and seas, and the remaining $3%$ is freshwater. Most of that freshwater is not easily used: nearly three quarters of it is frozen in ice caps and glaciers, and about $29%$ is groundwater.

Distribution of the world's water, from the book's Earth's water diagram
Part of the water	Share	Where it is
Oceans and seas	$97%$ of total water	saltish water
Ice caps and glaciers	about $70%$ of freshwater (about $2%$ of total)	frozen freshwater
Groundwater	about $29%$ of freshwater	underground aquifers
Easily accessible freshwater	about $1%$ of freshwater (about $0.03%$ of total)	lakes, rivers, soil moisture, water vapour and living things

Only about $0.03%$ of the planet's water is fresh and easily accessible for drinking, farming and industry. This small share is the reason why the supply of freshwater is under strain, and why its pollution matters.

Note. The labelled shares of the easily accessible freshwater in the book's diagram (lakes 52%, soil moisture 38%, rivers 1%, water vapour 1%, water in living things 1%) add to 93%, so one small share is not labelled. The key points say 'only one percent fresh water', which is 1% of the freshwater, not of the total water. The solution uses the diagram's own figures.

43 marks

Why does water dissolve many substances in it and is called universal solvent?

Solution

Water is called the universal solvent because it dissolves a very wide range of substances, which is why it is rarely found in pure form. The reasons lie in its properties:

Polarity: the water molecule is polar, so its oxygen end and hydrogen ends attract ions and polar molecules.
High dielectric constant: it weakens the attraction between oppositely charged ions, so they separate and stay in solution.
Ion-dipole forces and hydrogen bonding: water binds ions and forms hydrogen bonds with alcohols, amines and carboxylic acids, so many organic compounds dissolve too.

Rainwater shows the solvent power clearly. Carbon dioxide from the air dissolves in it to form carbonic acid, which converts insoluble carbonates into soluble bicarbonates:

H X_{2} O + CO X_{2} H X_{2} CO X_{3} H X_{2} CO X_{3} H X^{+} + HCO X_{3} X^{-}

Long questionsp. 282

36 marks

How raw water treatment is done for municipal water supply?

Solution

Raw water and why it is treated

Raw water is the water taken from groundwater, a river or a lake. It supplies drinking and municipal water to a nearby city, but it is contaminated with grit and pathogens, so it must be cleaned before use. The treatment is carried out in a fixed sequence of steps.

The steps

Screening: the first step. Debris such as sticks, leaves, pebbles and soil is removed from the water.
Coagulation and flocculation: alum, a stone containing ferric sulphate $Fe X_{2} (SO X_{4}) X_{3}$ and ammonium sulphate $(NH X_{4}) X_{2} SO X_{4}$ , is added. It forms flocs that collect the fine particles and impurities. The diagram shows sulphuric acid added to lower the pH and a polymer added to help the flocs form.
Sedimentation: the water stands in large tanks for 12 hours or more. The solid particles, which are slightly heavier, settle to the bottom as sludge, and the sludge is removed.
Filtration: the water passes through filters such as activated charcoal and many layers of sand and gravel. Impurities, pathogens and fine particles are removed. The backwash water from the filters is dewatered in a lamella clarifier and a filter press.
Killing the pathogens (disinfection): bacteria, viruses and parasites are killed by chlorination, ozonation or ultraviolet treatment. The diagram shows sodium hypochlorite, $NaOCl$ , added for chlorination.
Other methods where needed: pH adjustment (the diagram adds lime to raise the pH to 7.8), membrane filtration, ion exchange, aeration to remove smell, and desalination.
Storage: booster pumps convey the clean water to the storage tank.

Raw water treatment as the book sequences it. Each step removes a different kind of impurity.

56 marks

Evaluate the strain on fresh water availability by agriculture and industry.

Solution

Introduction

Freshwater is only about $3%$ of the planet's water, and only a small part of it is easily accessible (page 281). Agriculture and industry both draw on this supply, so each adds to the strain. The effects are judged below through water use, groundwater and pollution.

Agriculture

Largest user: agriculture is the biggest user of freshwater, so it creates strain on local water supplies. Water used for irrigation also takes a long time to return to the water cycle.
Falling groundwater: extra pumping from tube wells creates a shortage of groundwater in the aquifers. Replenishment depends on rainfall, which hardly recovers the deficiency.
Pollution of what remains: fertilizers, pesticides and herbicides run off into water bodies, cause eutrophication and algal blooms, and some pesticide molecules are carcinogenic.
Soil erosion: poor land management and extra tilling let soil fill the beds of water bodies, making them shallow and less able to hold water.
Ecosystem damage: converting natural landscape into fields harms the ecosystem. Drip irrigation and other sustainable methods can save both time and water.

Industry

Heavy water use: industries use large volumes of water for condensation, cooling and as a solvent. Where water is used as a coolant the demand is very large, and it strains the local supply.
Pollution: heavy metals, smoke, carbon particles, acids, alkalis and other chemicals make water unfit for drinking and for some aquatic species.
Heat and oxygen: hot effluent raises the temperature of nearby rivers and streams. Warmer water holds less oxygen, so species such as trout cannot survive.
Changed water courses and groundwater: large dams and canals change the pathway of water, extractive industries can dislodge groundwater sources, and mining can destroy streams in a local area.

Comparison

Factor	Agriculture	Industry
Water use	Biggest user of freshwater; the water returns to the cycle only slowly	Large volumes for condensation, cooling and as a solvent
Groundwater	Extra pumping from tube wells causes a shortage in aquifers	Extractive industries and mining can dislodge groundwater sources
Pollution	Pesticides, herbicides and fertilizers; soil erosion fills water bodies	Heavy metals, acids, alkalis and other chemicals; hot effluent
Ecosystem	Natural landscape converted into fields	Dams, canals and habitat loss change the water pathways

Evaluation

Agriculture puts the greater strain on the quantity of freshwater, because it is the largest user. Industry adds to the strain both through its water use and by returning polluted or hot water, so the water available for use falls in quantity and quality at the same time. The strain is most serious where both sectors draw on the same local source.

The book's remedies reduce this strain: sustainable agricultural methods and drip irrigation, environmental regulations, wastewater treatment and eco-friendly industrial practices (page 279).

62025 Annual I, Set B, Q.2 (vii)6 marks

Suggest some measures to ensure the preservation of water reservoirs.

Solution

Why measures are needed

Reservoirs such as dams, lakes and aquifers are threatened by pollution from agriculture and industry, by overuse and by the loss of natural watersheds (sections 14.3 and 14.6). The book's conservation strategies and the laws in section 14.5 address these threats together.

Conservation and management measures

Smart use of water: promote efficient water use in homes, industry and agriculture through drip irrigation, low-flow fixtures and water-efficient appliances.
Wastewater treatment: install waste water treatment plants on a small scale, and purify used water so that it can be reused, which saves freshwater.
Rainwater harvesting: collect rainwater for non-potable uses such as irrigation and toilet flushing, which reduces the demand on freshwater.
Watershed protection: restore and maintain natural watersheds for a steady supply of water, including erosion control and reforestation of barren land.
Water recycling: promote the reuse of wastewater for irrigation and industrial processes.
Legislation and regulation: enforce laws that prevent pollution, control water withdrawals and ensure sustainable management of reservoirs. Pakistan's Environmental Protection Act 1997, the National Environmental Quality Standards and the Pakistan Water Act 1998 are examples.
Public awareness: educate people about the importance of water, its conservation and the methods of reducing pollution.
Desalination: cities near the ocean can use desalinated water as a source of freshwater, which reduces the burden on freshwater supplies.
Research and technology: governments should invest in research into technology that saves water and into sustainable water management practices.
International cooperation: countries should work with neighbouring regions on trans-boundary issues so that water is used sustainably and equitably.

Measures at the source of pollution

Managing pesticides and fertilizers: proper pesticide use and management, and sustainable farming methods, reduce run-off into water bodies.
Treating effluent before release: industrial effluents and sewage must be cleaned before they are thrown into water bodies, because untreated waste makes them non-life-supporting.

Implemented together, these strategies protect water reservoirs for present and future generations.

76 marks

Evaluate the impact of pollutants on ecosystem.

Solution

Main pollutants and their sources

Pollutants reach water from point sources, such as industrial effluent, water treatment plants and oil spills, and from non-point sources, such as the run-off of pesticides, fertilizers and herbicides, roadside and residential effluents, and mining drainage (section 14.1).

Effects on aquatic life

Oil spills: a crude oil film stops aeration and light penetration into the water, which badly affects aquatic plants.
Aluminium hydroxide: aluminium ions leached from the soil form gelatinous aluminium hydroxide, which clogs the gills of fish and suffocates them. This happens where surfactants, heavy metals and pesticides are present.
Mining girt: the grit washed out of mines makes the water shallow and filthy, unfit for fisheries and general use, and the water body loses its aesthetic value.

Eutrophication

Excessive fertilizer drains into water bodies with the rain. It causes algal blooms, and the water bodies are eventually lost from the surface. The book notes that this process is responsible for the extinction of many lakes and ponds.

Heavy metals and bioaccumulation

Heavy metals such as cadmium, mercury, arsenic, lead and copper are persistent. They accumulate in sediments and aquatic organisms, a process called bioaccumulation, and become concentrated along the food chain (biomagnification). This causes long-term ecological damage, and in humans it can cause neurotoxicity, nephrotoxicity and cancer.

Destruction of habitats

Pollution destroys natural habitats such as coral reefs and wetlands. These habitats mark the shoreline and protect the land from floods, so their loss damages the ecosystem as a whole.

Summary of the effects

Pollutant	Main effect on the ecosystem
Pesticides and herbicides	Harm fish, insects and other organisms, disturb the food chain, and some molecules are carcinogenic
Fertilizers (nutrients)	Eutrophication and algal blooms that lead to the extinction of lakes and ponds
Oil spills	Block aeration and light, so aquatic plants cannot survive
Heavy metals	Accumulate in organisms and along the food chain; cause neurotoxicity and cancer
Sewage and pathogens	Spread diseases such as cholera, typhoid and hepatitis A through water
Mining girt	Makes water shallow and filthy, and unfit for fisheries

Overall, pollutants damage the ecosystem at every level, from plants and fish to the food chain, and finally to humans who depend on the same water.

FBISE 2025 questions

Questions from the FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2025 Annual I, Set B, Q.2 (vii)3 marks

Write down any three measures that ensure the preservation of water resources.

Solution

Water is a limited resource, and the book lists steps for conserving and managing it. Any three of the following measures ensure the preservation of water resources.

Smart use of water. Use water efficiently at home, in industry and in agriculture, through drip irrigation, low-flow fixtures and water-efficient appliances.
Rainwater harvesting. Collect rainwater for uses such as irrigation and toilet flushing, so the demand for fresh water falls.
Wastewater treatment and recycling. Clean used water in treatment plants and reuse it for irrigation and industrial processes, which saves freshwater and stops pollution of water bodies.
Watershed protection. Restore and maintain natural watersheds, with erosion control and reforestation of barren land, for a steady supply of water.
Legislation and public awareness. Enforce laws to prevent pollution and control water withdrawals, and educate people about saving water.

Other measures in the book are desalination for coastal cities, research into water-saving technology, and international cooperation on trans-boundary water.

2025 Annual I, Set D, Q.2 (viii)1+1+1 = 3 marks

Write down three methods by which un-usable water can be converted into clean water.

Solution

Un-usable (raw or polluted) water is made clean by treatment. Three methods from the book are as follows.

1. Coagulation, sedimentation and filtration (1 mark)

A coagulant (alum stone, named in the book as a ferric sulphate and ammonium sulphate compound) is added, and the fine particles clump into flocs that trap the impurities. In large tanks the water stands for $12$ hours or more, and the flocs settle as sludge. The water is then passed through filters (activated charcoal, layers of sand and gravel) that remove the remaining fine particles and pathogens.

2. Disinfection (1 mark)

The remaining microorganisms (bacteria, viruses and parasites) are killed by chlorination, ozonation or ultraviolet (UV) treatment, so that the water does not cause disease.

3. Desalination or waste water treatment (1 mark)

Sea water is made fit for use by desalination, which removes the dissolved salts. Used (waste) water is cleaned in stages: primary (screening and sedimentation), secondary (biological processes such as activated sludge) and tertiary (removal of nutrients such as nitrates and phosphates), followed by disinfection.

Exam focus

Write measures that ensure the preservation of water resources (reservoirs). (short)
Asked in 2025 (Set B Q.2 (vii)) and book long question 6.
Go to answer
Describe how un-usable water is made clean: raw water treatment (screening, coagulation, sedimentation, filtration, disinfection) and waste water treatment. (long)
Asked in 2025 (Set D Q.2 (viii): three methods) and book long question 3 and MCQs 4 and 6.
Go to answer
Why is water called the universal solvent, and how do its properties relate to pollution? (short)
Book short question 4; not asked in the 2025 or 2026 papers.
Go to answer
Explain the distribution of water on the planet and the sources of fresh water. (short)
Book short question 2 and MCQs 2 and 7; not asked in the 2025 or 2026 papers.
Go to answer
Describe point and nonpoint sources of water pollution and eutrophication. (short)
Book exercise MCQ 3 and long question 7; not asked in the 2025 or 2026 papers.
Go to answer
Evaluate the strain on fresh water by agriculture and industry. (long)
Book long question 5; not asked in the 2025 or 2026 papers.
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