Frank Brothers Solutions for ICSE Class 9 Chemistry Chapter 6 Water

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Question 1:
Answer: The molecular formula of water is H₂O, this shows that water is not an element but it is a compound made up of two elements hydrogen and oxygen combined in a fixed ratio, i.e., 1:8 by mass. This also shows that the properties of water is totally different from those of hydrogen and oxygen as compounds have completely different properties than the elements of which it is made up of.
Even though hydrogen is a flammable gas and oxygen supports burning, they join together to form water which actually puts out fires. This shows that when atoms form a compound, they lose their old identity and take on a brand new one.
Teacher's Tip: Remember the 1:8 ratio by thinking of one part Hydrogen and eight parts Oxygen sticking together like glue.
Exam Tip: Always mention both the elements and the fixed ratio by mass to get a perfect score.

Question 2:
Answer: Two characteristics properties responsible for making water a good solvent are:
1. Water molecule can form hydrogen bonds with the molecules of other compounds.
2. Water molecule is polar in nature.
Being polar means the water molecule acts like a tiny magnet with positive and negative ends that pull other substances apart. Because of this magnetic pull, water can dissolve more substances than almost any other liquid on Earth.
Teacher's Tip: Think of water as a "Social Butterfly" molecule that loves to hold hands and bond with everyone!
Exam Tip: Use the term "universal solvent" whenever you talk about water's ability to dissolve things.

Question 3:
Answer: Magnesium is least reactive with cold water but with hot water it burns with an intense white light, liberating hydrogen gas and a white ash, i.e., magnesium oxide
Mg + H₂O → MgO + H₂ ↑
Magnesium hot water Magnesium oxide Hydrogen
Action of steam on magnesium
When magnesium reacts with steam, it produces a lot of energy which we see as a bright flash. The white ash left behind is the new compound formed from the magnesium and oxygen.
Teacher's Tip: Remember that steam is just very energetic hot water that makes the metal react faster.
Exam Tip: Always draw an upward arrow next to H₂ to show that it is a gas escaping.

Question 4:
Answer: 1. (a) Physical properties of water:
1. Pure water is colourless, transparent, odourless and tasteless liquid.
2. Boiling and Freezing points: At normal atmospheric pressure water boils at 100°C and freezes at 0°C.
3. Pure water being a covalent compound is a very poor conductor of heat and electricity. On addition of electrolytes it becomes a good conductor of heat and electricity.
4. Solvent properties: Water is a remarkable solvent dissolves many substances forming aqueous solutions because of its high dielectric constant.
5. Anomalous behavior: On cooling water contracts in volume, as do other liquids, but at 4°C, it starts expanding, and continues to do till the temperature reaches 0°C, the point at which it converts to ice.
2. (b)
1. Boiling point of water: It increases with the addition of salt in water.
2. Freezing point of water: It decreases with the addition of salt in water.
3. Density of water: It increases with the addition of salt in water.
Pure water is a unique substance that behaves differently from almost every other liquid when it gets cold. These physical stats help scientists identify pure water and understand how it interacts with the environment.
Teacher's Tip: Think of pure water as a blank slate with no color, no smell, and no taste until you add something!
Exam Tip: Memorize 0°C and 100°C as the two most important temperatures for water on the Celsius scale.

Question 5:
Answer: Due to its high specific heat capacity, water is used in cooling systems for e.g., as coolant in motor car radiators.
Water is amazing at soaking up heat without getting too hot itself, which protects the engine from melting. This is why it takes a long time for a large pot of water to start boiling on the stove.
Teacher's Tip: Think of water as a heat sponge that can hold onto a lot of warmth.
Exam Tip: Use the specific term "specific heat capacity" to explain why water is a great coolant.

Question 6:
Answer: In many chemical reactions water acts as a catalyst.
a. Hydrogen and oxygen when sparked together combines only if moisture is present.
4H₂ + O₂ \xrightarrow{{Moisture}} 2H₂O
b. Yellow phosphorus burns in air in the presence of moisture.
4P + 5O₂ \xrightarrow{{Moisture}} 2P₂O₅
c. Water also helps in rusting process
Fe + O₂ + H₂O → Fe₂O₃ . xH₂O
A catalyst like water helps a reaction start or go faster without being used up itself. Without a tiny bit of water vapor, some chemicals would just sit next to each other and never react.
Teacher's Tip: Think of water as the matchmaker that helps other chemicals meet and react!
Exam Tip: Always write the condition "Moisture" above the arrow in these specific chemical equations.

Question 7:
Answer: 1. (a) Water reacts with metals oxides: corresponding hydroxides are formed which act as alkalis.
2. (b) Water reacts with non metals oxides: corresponding acids are formed.
When metal oxides meet water, they turn basic, but when non-metal oxides meet water, they turn acidic. This is why some gases in the air can cause acid rain when they mix with rainwater.
Teacher's Tip: Metal = Base, Non-metal = Acid. Just remember the letters M-B and N-A.
Exam Tip: Mention the formation of alkalis and acids specifically to show the chemical nature of the products.

Question 8:
Answer: a. Calcium oxide reacts with water:
CaO + H₂O → Ca(OH)₂
b. Sulphur trioxide reacts with water:
SO₃ + H₂O → H₂SO₄
c. Phosphorous pentoxide reacts with water:
2P₂O₅ + 6H₂O → 4H₃PO₄
d. Sulphur dioxide reacts with water:
SO₂+ H₂O → H₂SO₃
e. Sodium reacts with water:
2Na + H₂O → 2NaOH + H₂
These reactions show how water can combine with different compounds to create completely new substances like acids and bases. In every case, the water molecule breaks apart or joins with the oxide to change its properties.
Teacher's Tip: If the product has (OH), it is a base. If it starts with H, it is usually an acid.
Exam Tip: Make sure both sides of your equation have the same number of atoms to keep it balanced.

Question 9:
Answer: We can test the odourless, colourless liquid by:
1. Measuring its boiling point, if comes out to be 100°C then it confirms the liquid to be water.
2. Measuring its melting point, if comes out to be 0°C then it confirms the liquid to be water.
Every pure substance has its own unique fingerprint temperatures where it changes state. If a liquid does not boil exactly at 100°C at sea level, then it is either not water or it has impurities in it.
Teacher's Tip: Think of 0 and 100 as the magic numbers that prove a liquid is water!
Exam Tip: If you use boiling point to test water, mention that it must be at "normal atmospheric pressure" for the test to be valid.

SQuestion 10:
Answer: a. Principle underlying the removal of dissolved gases from water is the downward displacement of water. This explains that the gases dissolved in water i.e., hydrogen and oxygen are less dense then water due to which they are able to displace water downwards and gases are collected upwards.
[Diagram showing Dropping funnel, Delivery tube, Tap water, Round bottom flask, Burner, Trough, Gas collected, Air bubbles, Beehive shelf]
b. Importance of dissolved gases in water:
i. Oxygen gas is dissolved in water which is used by aquatic organism to respire.
ii. Carbon dioxide is also dissolved in water, which is consumed by aquatic plants to prepare food in the form of carbohydrates by photosynthesis. Also carbon dioxide dissolved in water reacts with calcium carbonate present in rocks to form soluble calcium bicarbonate, which is absorbed by the marine animals.
Gases like Oxygen and CO₂ are hidden inside liquid water, allowing fish to breathe and underwater plants to eat. Heating water makes these gases escape, which is why you see tiny bubbles forming before water actually starts boiling.
Teacher's Tip: Fish don't breathe the O in H₂O; they breathe the extra oxygen gas (O₂) dissolved between the water molecules!
Exam Tip: List at least two different dissolved gases and their specific uses to get full marks.

Question 11:
Answer: 1. (a) Water being the universal solvent, acts as an important mode of transport in plants, also water taken from natural source contains dissolved salts in them which is essential for the growth and development of plants
2. (b) Water constitutes 3/4th of our human body and it is necessary for the survival of human beings and being the universal solvent helps in transportation and also regulates the body temperature. It also contains dissolved salts which supply essential minerals which are necessary for our body.
Water acts like a highway system inside both plants and humans, carrying nutrients to every single cell. It also acts as a natural air conditioner that keeps our bodies from overheating on a hot day.
Teacher's Tip: Remember 3/4 - most of you is actually just water!
Exam Tip: Mention "temperature regulation" and "transportation" when discussing water's role in the human body.

Question 12:
Answer: Normally water exist in liquid state but when heated under normal pressure (760mm Hg) water boils at 100°C and changes rapidly into gaseous state (steam). Water freezes at 0°C and changes into solid state (ice).
[Phase diagram showing 1atm pressure, 0°C, 100°C, solid, liquid, and gas states. Normal States of Water]
Water is one of the few substances on Earth that we can easily see in all three states of matter - solid, liquid, and gas. These changes happen because adding or removing heat energy changes how fast the water molecules move around.
Teacher's Tip: Think of molecules as dancers - they stand still in ice, walk in liquid, and run in steam!
Exam Tip: Use the term "steam" for the gaseous state and "ice" for the solid state to be very clear.

Question 13:
Answer: We can test the presence of water by performing these two tests:
1. On adding few drops of water to white anhydrous copper sulphate, the latter turns blue. (CuSO₄ . 5H₂O)
CuSO₄ + 5H₂O → CuSO₄ . 5H₂O
Copper sulphate (anhydrous white) Copper sulphate pentahydrate (blue)
2. On adding water to blue cobalt chloride the latter turns pink due to the formation of CoCl₂ . 6H₂O
CoCl₂ + H₂O → CoCl₂ . 6H₂O
Cobalt chloride (Blue) Hydrated cobalt chloride (pink)
These color-changing chemicals act as detectors because they trap water molecules inside their crystals. This chemical change is very obvious to the eye, making it a perfect test for even tiny amounts of moisture.
Teacher's Tip: Think: Copper turns Blue (like the ocean) and Cobalt turns Pink (like a sunset)!
Exam Tip: Use the term "anhydrous" to describe the dry chemical before it touches the water.

Question 14:
Answer: Density of water is 0.997{ gcm}^{-3} at 4°C. The density of water decreases when the temperature increases above 4°C or decreases below 4°C. At 4°C, water has its maximum density and minimum volume. At any temperature above or below 4 C, the density of water decreases. This property is called anomalous expansion of water. The significance of this unique property of water is that it enables marine life to exist in the colder regions of the world, because even when water freezes on the top, it is still liquid below the ice layer.
While most things shrink when they get cold, water actually starts to expand once it drops below 4°C. This makes ice lighter than liquid water, so it floats to the top and creates a warm blanket for the fish living underneath.
Teacher's Tip: Remember 4°C as the magic temperature where water is at its heaviest!
Exam Tip: Explain the survival of marine life to get full points on the "significance" part of this question.

Question 15:
Answer: At 4°C, water has its maximum density and minimum volume. The property of anomalous expansion of water enables marine life to exist in the colder regions of the world because even when water freezes on the top, it is still liquid below the ice layer.
Because ice floats, it insulates the water below from the freezing air outside. This keeps lakes and oceans from freezing solid all the way to the bottom, protecting all the animals inside.
Teacher's Tip: If water behaved like other liquids, lakes would freeze from the bottom up and kill all the fish!
Exam Tip: Be sure to use the specific phrase "anomalous expansion" in your description.

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Question 16:
Answer: Procedure: Put some tap water on a clean watch glass and place it over a beaker containing water as shown in Fig. Boil the water in the beaker. When all the water has evaporated from the watch glass, remove it from over the burner and let it cool.
[Diagram showing Tap water, Watch glass, Steam bath, Concentric rings of solid residue, Beaker, Burner]
Observation: Hold the watch glass against the light, a no. of concentric rings of solid matter are observed. These rings are deposits of the dissolved solids left behind after evaporation.
This experiment proves that tap water is not just pure H₂O, but a mixture containing dissolved minerals. When the water turns into steam, it leaves the solid minerals behind, creating visible patterns on the glass.
Teacher's Tip: Those rings are like the dirty footprints left behind by the water after it disappears!
Exam Tip: Describe the concentric rings clearly to show you understand what the residue looks like.

Question 17:
Answer: (a) A piece of Na is added to cold water:
It melts forming silvery globule is formed which floats on the surface of water, after the completion of the reaction a colourless soapy and warm solution is left.
2Na + H₂O → 2 NaOH + H₂
Sodium Sodium hydroxide hydrogen
(b) A piece of Ca is added to cold water:
Calcium being heavier sinks in water and due to the formation calcium hydroxide in the reaction, the area around the metal turns milky.
Ca + H₂O → Ca(OH)₂ + H₂
Calcium Calcium hydroxide
(c) A coil of Mg is heated in steam:
It burns with an intense white light liberating hydrogen gas, and forms some white ash, i.e., magnesium oxide when steam is passed over heated magnesium wire.
Mg + H₂O → MgO + H₂ ↑
Magnesium steam Magnesium oxide
Different metals react with water at different speeds - some explode instantly while others need to be heated first. In every case, the metal grabs oxygen from the water and releases hydrogen gas into the air.
Teacher's Tip: Sodium (Na) is so reactive it dances on the water, while Calcium (Ca) just sinks and bubbles!
Exam Tip: Be sure to specify the state of water (cold water vs steam) for each specific metal reaction.

Question 18:
Answer: 1. (a) Liquids: alcohols, acids
2. (b) Solids: Sugar, urea
3. (c) Gases: Oxygen, carbon dioxide
Water can dissolve substances from all three states of matter, which is why it is called the universal solvent. From the sugar in your tea to the oxygen fish breathe, water is always working to mix things together.
Teacher's Tip: Water is the master mixer - it can handle solids, liquids, and gases!
Exam Tip: Always give two distinct examples for each category to show you have a broad understanding.

Question 19:
Answer: Rain water is the purest form of natural water.
When water evaporates into the sky, it leaves all the dirt and salt behind, making the falling rain very clean. However, once it starts falling, it can pick up dust or gases from the air on its way down.
Teacher's Tip: Think of rain as water that has been steam-cleaned by the sun!
Exam Tip: Specify "natural water" because distilled water in a lab is actually even purer than rain.

Question 20:
Answer: 1. (a) Solid in a liquid: Solubility decreases with rise in temperature in an exothermic process, for example, calcium sulphate(CaSO₄), sodium sulphate(NaSO₄), and in an endothermic reaction solubility increases with rise in temperature, for example potassium nitrte(KNO₃) and sodium nitrate(NaNO₃).
2. (b) A gas in a liquid: Solubility decreases with rise in temperature. Thus, gases dissolves readily in cold water than in hot water.
For most solids, hot water is better for dissolving things, like how sugar melts faster in hot tea. But for gases, the opposite is true - cold water can hold much more air than hot water can.
Teacher's Tip: Hot tea dissolves sugar better, but cold soda holds onto its bubbles (gas) longer!
Exam Tip: Clearly state that gas solubility decreases as temperature goes up.

Question 21:
Answer: The advantages of solubility curve are:
1. It helps in the determination of solubility of a solute at a particular temperature.
2. It helps in comparing the solubilities of different solutes in a solvent at a particular temperature.
3. In separation and purification of solutes.
4. It also help to know the transition temperature i.e. there is a sharp break observed.
[Solubility graph showing KNO₃, NaNO₃, NaCl, KCl, CaSO₄, K₂SO₄ with Solubility in gm per 100 gm of water vs Temperature in °C]
Variation in the solubility of some solids with changing temperature.
A solubility curve is like a growth chart for how much stuff can fit into water at different heats. It tells scientists exactly when a solution will be full and when it will start spitting out crystals.
Teacher's Tip: The steeper the line on the graph, the more that chemical loves hot water!
Exam Tip: Be prepared to read a specific value from the graph, such as the solubility of KNO₃ at 40°C.

Question 22:
Answer: 1. (a) Density of water is 0.997 gcm^-3 at 4°C. The density of water decreases when the temperature decreases below 4°C. So ice has less density of 0.92 and is lighter then that of water hence it floats on water.
2. (b) A solution is said to be saturated if at a particular temperature if no more of the solute can be dissolved in it at that temperature, when the temperature is increased more solute particles can be dissolved in that saturated solution as the solubility of most of the substances generally increases with rise in temperature and thereby making it unsaturated.
3. (c) Solubility of gases decreases with increase in temperature hence hot water contains less dissolved air than cold water.
These three facts explain why ice stays at the top of a drink and why fish prefer cold water where there is more air to breathe. A saturated solution is basically a liquid that is totally stuffed and cannot eat any more sugar or salt.
Teacher's Tip: Saturated means the water is completely FULL!
Exam Tip: Always mention that saturation is defined "at a particular temperature."

Question 23:
Answer: 1. (a) Solute: The substance which dissolves in a medium to produce a solution is called the solute.
2. (b) Solvent: It is the medium in which the solute dissolves.
3. (c) Solution: A homogeneous mixture that has a uniform composition throughout the volume of the mixture. It is the medium in which the solute dissolves.
You can think of a solution as a team made of two players: the Solute is the one being eaten and the Solvent is the one doing the eating. Together, they mix so perfectly that you cannot tell where one starts and the other ends.
Teacher's Tip: Think: Solute is small (6 letters), Solvent is big (7 letters). Small goes into big!
Exam Tip: Use the word "homogeneous" to describe the final solution to get a perfect score.

Question 24:
Answer: 1. (a) When we heat a saturated solution more solute can be dissolved in it, it as mostly solubility increases on raising the temperature and the solution then becomes an unsaturated solution.
2. (b) When a hot saturated solution is cooled slowly and is kept undisturbed the excess salt does not separates out. Thus the solution becomes a supersaturated solution which contains more solute in it than it can hold at room temperature.
3. (c) When the quantity of solvent increases, greater amount of solute can be dissolved.
Temperature is like stretching the water so more solute can fit inside. A supersaturated solution is like a magic solution that is holding more than it should, and it can turn back into crystals with just a tiny tap.
Teacher's Tip: Supersaturated solutions are very unstable - they are just waiting for a reason to turn back into solid!
Exam Tip: Be clear on the difference between "saturated" (full) and "supersaturated" (over-full).

Question 25:
Answer: The solubility curve of sodium sulphate decahydrate (Na₂SO₄ . 10H₂O) shows sudden change in direction at 32.8°C. Its solubility increases sharply upto 32.8°C and then falls suddenly. Anomalous solubility of Glauber’s salt (Na₂SO₄ . 10H₂O) in water is due to the reason that sodium decahydrate change to anhydrous sodium sulphate at 32.8°C. Due to this, a sharp increase occurs in solubility curve at 32.8°C and then falls suddenly.
[Graph showing Solubility vs Temperature with a peak at 32.8°C for Na₂SO₄ . 10H₂O and Na₂SO₄]
Fig. 6.10: Solubility curve for Na₂SO₄ . 10H₂O (Glauber’s salt)
Glauber's salt behaves like a hill on a graph - it loves heat until it reaches a certain point, then it suddenly changes its mind! This happens because at that specific temperature, the chemical actually transforms into a different version of itself.
Teacher's Tip: This broken curve is very rare - most things just go up or down, but this one does both!
Exam Tip: Memorize the specific temperature (32.8°C) for Glauber's salt as it is a key fact.

Question 26:
Answer:
Solution | Mixture | Compound
1. In it solute is not present in any fixed proportion but its composition is uniform. | 1. In it the composition is not uniform. The component may be present in any proportion. | 1. In it the constituents are combined in a fixed proportion.
2. In it the solute can be recovered by evaporating the solvent, i.e., by physical means. | 2. In it the components can be separated by ordinary physical means. | 2. In it the constituents can be separated by chemical means only.
Solutions are a middle ground between random mixtures and solid compounds. While they look uniform like a compound, they can be taken apart easily using simple methods like boiling or filtering, just like a mixture.
Teacher's Tip: Think: Solutions are uniform like a compound but separable like a mixture!
Exam Tip: Use "evaporation" as an example of a physical mean used to separate a solution.

Question 27:
Answer: Solubility of a solid depends on:
1. Size of particles: Smaller the size of particles of the solute, greater is the solubility.
2. Contact between solute and the solvent: Increased contact between the solute and the solvent increases the solubility.
3. Temperature: The variation of solubility with temperature depends on its nature i.e. whether the reaction is exothermic or endothermic.
For exothermic reactions-The solubility decreases on increasing the temperature.
For endothermic reaction-The solubility increases on increasing the temperature.
To dissolve something faster, you should crush it into powder and stir it well. This creates more surface area where the water can attack the solid and pull it into the solution.
Teacher's Tip: Crush it, Heat it, Stir it - the three steps to making things disappear in water!
Exam Tip: Be sure to list all three factors (size, contact, and temperature) for a complete answer.

Question 28:
Answer: Henry’s law states that:
At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
This law explains why soda bottles are pressurized - the extra pressure forces more bubbles to stay dissolved in the liquid. When you open the cap and the pressure drops, the gas cannot stay dissolved anymore and it rushes out as fizz.
Teacher's Tip: More Pressure = More Bubbles! Just think of a soda can.
Exam Tip: Use the phrase "directly proportional" to accurately state the relationship in Henry's Law.

Question 29:
Answer: 1. Z is least reactive so it will be found free in nature.
2. X is more reactive so it will react with oxygen more readily and with ease.
3. Z
4. Y is iron whose oxide is brown coloured Fe₂O₃.
5. X
6. The piece of Y i.e., iron is more reactive then copper so it will displace copper from copper nitrate and a brown layer of copper will be developed on the iron piece.
This section ranks different metals based on how much they love to react with other elements. Metals like gold (Z) are found pure in the ground, but reactive metals like iron (Y) are always found dressed up in an oxide coat.
Teacher's Tip: In chemistry, being reactive means you are social and always making new bonds!
Exam Tip: Identifying the brown color of iron oxide Fe₂O₃ is a common clue in chemistry puzzles.

Question 30:
Answer: Drop in, a crystal of the solute that is in the solution. If the crystal dissolves its unsaturated, if it stays the same size then its saturated and if it gets bigger than its supersaturated.
1. This is because an unsaturated solution will be able to take in more solute, so it dissolves the crystal.
2. A saturated solution will not take in more solute but will also not deposit any solute so the crystal stays the same size.
3. A supersaturated solution wants to deposit its excess solute and become saturated, it just needs something to start it along.
The crystal starts it up and it will deposit its excess solute onto the crystal making the crystal bigger.
Adding a single crystal is like asking the solution "Are you full yet?" If it's supersaturated, it will start building more solid right onto the crystal you dropped in, almost like magic.
Teacher's Tip: This is called the Seed Crystal test because you are planting a seed to see what grows!
Exam Tip: Clearly define all three outcomes (dissolves, stays same, grows) to show complete understanding.

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Question 31:
Answer: 1. (a) Increase in weight: when sodium chloride when exposed in air it gains moisture and its weight increases.
2. (b) Decrease in weight: iron when exposed to air gets rusted due to the presence of moisture in the air it gets corroded, and hence its weight is reduced.
3. (c) No effect on weight: when conc. Sulphuric acid when exposed in air does not react and no change occurs in weight.
Substances can grow heavier or lighter just by sitting in the air because they interact with water vapor or oxygen. While salt just drinks the water and gets heavy, iron actually starts to flake away as it turns into rust.
Teacher's Tip: Think of it as chemical weathering - the air is slowly changing the weight of these objects!
Exam Tip: Be sure to link the increase in weight specifically to the gain of moisture.

Question 32:
Answer: Water pollution may be defined as the contamination of water by foreign substances which make it harmful for health of animals or plants or aquatic life, make it unfit for drinking and for domestic, industrial and agriculture use.
Water pollution is not just about trash; it can be invisible chemicals or heat that change the water balance. When water is polluted, it becomes a poison instead of a source of life for everyone on Earth.
Teacher's Tip: Think of pollution as dirtying the Earth's lifeblood.
Exam Tip: Define it as making water unfit for specific uses like drinking or farming.

Question 33:
Answer: Major pollutants of water are:
Ground water pollutants: Septic tanks, industry by products like pesticides, fertilizers, tanneries, mining wastes.
Surface water pollutants: No. of gases present in atmosphere like SO₂, CO₂, H₂S, NO_x, CO, etc., pollute surface water.
Lake water pollutant: organic wastes from hills, toxic effluents from urban areas, industrial effluents, dumping of huge amounts of sediments etc.,
River water pollutants: Industrial discharge, sewage discharges, detergents, discharges from drug, paper, and textile industries
Marine water pollutant: major pollutant is oil spills.
Pollutants can enter our water from almost every part of human life - from the soap we use at home to the large factories in cities. Each type of water body, like a lake or a river, faces its own unique enemy chemicals.
Teacher's Tip: Remember: Marine pollution is usually about oil, while river pollution is usually about sewage.
Exam Tip: List pollutants based on where they are found (ground, surface, lake, river) to show you know the different categories.

Question 34:
Answer: Various sources of water pollution are:
1. Natural process: Washing away of decomposed and animal wastes into main stream of water.
2. Human activity:
1. (a) Discharge of Household detergents: detergents used as a cleaning agent produce foam and pollute water. They do not undergo bio-degradation.
2. (b) Discharge of industrial effluents: Industrial activities generate a variety of waste products which are generally discharged into water streams. The pollutants associated with the industrial effluents are organic matter, inorganic dissolved salts, suspended solids. They inhibit oxidation or organic compounds; stabilize the colloidal impurities which do not aggregate to settle down.
3. (c) Sewage: Sewage is cloudy dilute aqueous solution containing minerals and organic matter. Sewage from homes and industries contains decomposable organic matter, inorganic cations and anions, toxic metals etc. Pouring the drains and sewers in fresh water bodies causes water pollution.
Water pollution due to sewage creates the following problems:
1. Self purifying ability of water is lost and it becomes unfit for domestic purpose.
2. Self regulatory capabilities of aquatic organism is retarded.
3. Sewages produce pathogens which are diseases causing bacteria and result in water born gastro-intestinal diseases.
Nature can handle a little bit of waste on its own, but human cities produce much more than the rivers can clean. This overload of waste stops the water's natural self-cleaning system and makes it a breeding ground for germs.
Teacher's Tip: Human waste is the #1 cause of water pollution in many parts of the world.
Exam Tip: Describe at least two problems caused by sewage to get full points for that section.

Question 35:
Answer: Water treated for safe effluents involves sequential treatment:
1. Primary treatment: The primary treatment involves physico-chemical processes to reduce settle able suspended solids of the wastes water and smoothened out individual effluent flow variations. It involves physic-chemical processes such as sedimentation, aeration, adsorption, oxidation etc.
2. Secondary treatment: In this, the dissolved and colloidal organic matter present in waste is removed by biological processes involving bacteria and other micro organisms. These process may be aerobic or anaerobic.
Aerobic treatment: Purification is carried out by aerobes in the presence of molecular oxygen.
Anaerobic treatment: Purification of waste is achieved by anaerobes in complete absence of molecular oxygen.
3. Tertiary treatment: It is the final treatment for polishing the effluents from secondary treatment. By this process suspended solids are removed, bacteria are removed, and organic and inorganic solids are removed.
Cleaning dirty water is like a three-step spa for the planet. We start with physical cleaning, then use tiny bacteria to eat the leftover waste, and finally do a deep-clean to make it safe for the environment.
Teacher's Tip: Step 1 = Physical, Step 2 = Biological (using bugs!), Step 3 = Chemical polishing.
Exam Tip: Define the difference between aerobic (with air) and anaerobic (no air) treatment clearly.

Question 36:
Answer: Two water born diseases are:
1. Gastroenteritis
2. Bacterial dysentery
These painful sicknesses are caused by drinking water that has been contaminated with sewage or animal waste. Proper water treatment and boiling water before drinking are the best ways to stay safe from these germs.
Teacher's Tip: Water-born means the germs use water like a tiny boat to travel into your body!
Exam Tip: Be ready to name these specific diseases if asked about the health impacts of water pollution.