Categories
Chemistry 10th Class Chemistry

Chapter 15 WATER – 10th Class Chemistry Notes

Question#1) Write a detailed note on the importance of water?

Answer: Significance of water: Throughout history, importance and significance of water has been recognized by mankind. Its importance is because of two reasons. Firstly, it is an essential and major component of each and every living cell. For example, human body consists of about 70% water. Secondly, it provides an environment for animals and plants that live in water. So, all living organisms owe their life because of water. We use water in daily life for drinking, cooking and washing purposes. Some other uses are production of electricity, watering plants; to cool things such as car engines, etc.

Question#2: What factors are responsible for quality of water?

Answer: Quality of drinking water has remained a major factor in determining human health and welfare since ages. Since World War II, there has been a rapid production and use of synthetic chemicals. Many of these chemicals (fertilizers and pesticides run off from agriculture lands and industrial discharge from industrial units) have polluted water supplies. Besides this, there is also a threat to groundwater from waste chemical dumps and landfills.

Currently, waterborne toxic chemicals pose the greatest threat to the supplies of water especially in urban areas. Use of this water is causing waterborne diseases. So use of polluted water is a concern of every citizen. Therefore, understanding the sources and effects of water pollution is essential for controlling this alarming threat.

Question#3: Write a note on occurrence of water?

Answer: Occurrence of Water: The oceans contain about 97% of world’s water. The rest of the water is in the form of glaciers, ice caps, groundwater and inland water (rivers, lakes, streams). It is also present in atmosphere in the form of water vapours. Sea water is unfit for drinking and agricultural purposes due to high percentage of dissolved salts. Only 0.2% of the total water on the Earth is potable, i.e. fit for drinking purposes. Occurrence of water is shown in the table below:

Oceans    97%

Glaciers and ice caps   2.1%

Ground water    0.6

Inland water     0.2

Earth living organisms and other manufactured products   0.003%

Properties Of Water:

Water is composed of two elements: oxygen and hydrogen. One atom of oxygen combines with two atoms of hydrogen to form one molecule of water. Pure water is a clear, colourless, odourless and tasteless liquid with following properties:

  1. It is neutral to litmus.
  2. Its freezing point is 0°C and boiling point is 100 °C at sea level.
  3. Its maximum density is 1 gcm-3 at 4°C.
  4. It is excellent solvent for ionic as well as molecular compounds.

5.High heat capacity of water: The heat capacity of a defined system is the amount of heat (usually expressed in calories, kilocalories, or joules) needed to raise the system’s temperature by one degree (usually expressed in Celsius or Kelvin).  It has unusually high heat capacity about 4.2 Jg-1K-1, which is about six times greater than that of rocks. This specific property of water is responsible for keeping the Earth’s temperature within limits. Otherwise, day time temperature would have been too high to bear and night time temperature would have been too low to freeze everything.

6.High surface tention of water: The tension of the surface of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area is called surface tention. Water has high surface tension. This unique property of water is responsible for its high capillary action. Capillary action is the process by which water rises up from the roots of plants to leaves. This process is vital for the survival of the land plants.

 

Question: Explain the reasons, why is water considered a universal solvent.

 

 

 

 

Question: How polarity of water molecule plays its role to dissolve the substances?

 

Answer: WATER AS SOLVENT

Water is the universal solvent because it can dissolve almost all the minerals. Its ability to dissolve substances is because of two unique properties of water:

(i) Polarity of water molecule;

(ii) Exceptional hydrogen bonding ability.

 

(i) Polar nature of water

Water molecule has polar structure, i.e. one end of the molecule is partially positive while the other end is partially negative because of electronegativity difference between oxygen and hydrogen atoms.

 

All other polar substances are soluble in water, because the positive end of the substance is attracted by the negative end (Oẟ-) of the water and negative end of the substance is attracted by the positive end (Hẟ+) of the water. The electrostatic attractions among the ions are overcome by the ion-dipole forces of attraction between ion and water molecules. In this way, positive and negative ions of the compounds are pulled apart. Ultimately, these oppositely charged ions are surrounded by water molecules, thus separated and kept in solution. For example, most of the salts like NaCl, KCl, Na2SO4 , etc. are soluble in water.

 

Dissolving process of a polar substance in water.

 

 

 

 

 

On the other hand, many covalent substances like benzene, ether, octane, etc., which do not have polar ends or bonds are not attracted by water molecules. Therefore, non-polar compounds do not dissolve in water.

 

(ii) Extensive hydrogen bonding ability

Water molecule is composed of oxygen and hydrogen atoms. Because of two O—H bonds and two lone pairs, one H2O molecule can form hydrogen bonding with four other H2O molecules, which are arranged tetrahedrally around the H2O molecule. This unique behaviour of water enables it to dissolve many polar non-ionic compounds having hydroxyl group (-OH), like alcohols, organic acids, glucose, sugar, etc. by forming hydrogen bonds with them.

 

 

 

 

SOFT AND HARD WATER

Soft water

Soft water is that which produces good lather with soap.

Hard water

Hard water is that which does not produce lather with soap.

Causes of hardness in water.

The rain water while coming down absorbs carbon dioxide from the atmosphere. The water mixed with carbon dioxide, when passes through the beds of the soil, converts insoluble carbonates of calcium and magnesium into soluble bicarbonates. It may also dissolve chlorides and sulphates of calcium and magnesium. These salts make the water hard.

Thus, rain water dissolves many salts of divalent cations like Mg2+, Ca2+, and anions like Cl , SO42- , HCO3- and CO32- for example, gypsum (CaSO4 .2H2O) and limestone (CaCO3 ). These salts make the water hard. Gypsum is sparingly soluble in water, while limestone is insoluble in water. However, in the presence of carbon dioxide small quantity of lime stone is soluble in water according to the above chemical reaction.

 

Types of Hardness of Water

Hardness is of two types:

(i) Temporary hardness is because of presence of bicarbonates of calcium and magnesium.

(ii) Permanent hardness is because of presence of sulphates and chlorides of calcium and magnesium.

Methods of Removing Hardness

The removal of Mg2+ and Ca2+ ions which are responsible for the hardness is called water softening.

 

(i) Removal of temporary hardness

(a) By boiling:

Temporary hardness of water is easily removed by boiling water. On boiling, calcium bicarbonate Ca(HCO3)2 decomposes to produce insoluble calcium carbonate, which precipitates out of the solution.

(b) Clark’s method

A chemical method to remove temporary hardness is by the addition of slaked lime Ca(OH)2 . A calculated amount of lime water is added to temporary hard water.

Thus, once the magnesium and calcium ions precipitate out water becomes soft.

 

 

 

Question: Explain the methods of removing permanent hardness.

 

 

Give some disadvantages of hard water.

 

 

(ii) Removal of permanent hardness

Permanent hardness can only be removed by using chemicals. Calcium (Ca2+) and magnesium (Mg2+) are removed as insoluble salts by adding washing soda (Na2CO3 ) or sodium zeolite.

(a) By using washing soda: The addition of washing soda removes the calcium and magnesium ions as the insoluble calcium and magnesium carbonates, respectively.

(b) Using Sodium Zeolite (an ion Exchanger):  Sodium zeolite is a naturally occurring resin of sodium aluminium silicate NaAl(SiO3)2 , which can also be prepared artificially. It is used for softening of water at domestic as well as on industrial scale. When water is passed through resin, sodium ions of the resin are exchanged with the unwanted calcium and magnesium ions of the hard water as shown in figure.

 

 

Ion exchange for removal of hard water ions

When resin is fully used up it can be regenerated by flushing it with concentrated solution of NaCl. The reverse process takes place because of high concentration of sodium ions.

Disadvantages of Hard Water:

(i) Hard water consumes large amount of soap in washing purposes.

(ii) Drinking hard water causes stomach disorders.

(iii) Hard water is unfit for use in steam engines, boilers and turbines because insoluble calcium and magnesium salts deposit inside. They are called scales. They are bad conductors of heat and hence more fuel is used. Insoluble calcium and magnesium sulphates not only reduce the efficiency of the engine but also cause the boiler to burst.

 

Hard water hampers the cleaning action of soap.

Soap is the sodium salt of a long chain carboxylic acid (fatty acid).

Hard water contains salts of magnesium and calcium. These ions react with the soap molecule to form an insoluble precipitate of calcium and magnesium salts of fatty acids called scum. As a result, a large amount of soap is wasted in scum formation. Thus, it reduces the efficiency of soap.

 

 

 

 

 

 

 

 

 

 

What is water pollution? Describe the effects of using polluted water.

Question: Explain the water pollution because of industrial waste?

 

 

 

Justify the statement: household water is the reason of water pollution.

 

 

Explain agricultural effluents are fatal for aquatic life.

 

 

 

WATER POLLUTION

Water pollution is a contamination of water bodies (e.g. lakes, rivers, oceans and ground water). Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds.

 

Industrial Effluents

Industrial units are installed to produce the desired substances (chemicals, cloth, leather goods, paper, plastic items, petrochemicals and rubber items) on commercial scale to meet the needs of the society. But unfortunately all the industrial units discharge their wastes (chemicals and solid materials) either to open ground or to water channels. This is called industrial effluent. The industrial effluent may be highly toxic organic chemicals, inorganic salts, heavy metals, mineral acids, oil and greases, etc. On the other hand, water used as cleaning agent in industries is directly discharged out. This water contains all kinds of toxic chemicals and detergents.

When these effluents and used water enter lakes, streams, rivers or oceans, they either get dissolved or float suspended in water. Even they get deposited on the bed. This results in the pollution of water, i.e.

  1. They deteriorate the quality of water.
  2. They reduce the quantity of dissolved oxygen which ultimately affects aquatic life and ecosystem.
  3. They can also seep down and affect the groundwater deposits.They contaminate the water deposits. When this water is used by human beings, it causes serious diseases like cancer and gastro. This polluted water damages soil, crops, plants and animals.
  4. Heavy metals like cadmium, lead and mercury are toxic and health hazards for human beings. Acute cadmium poisoning causes high blood pressure, kidney damage and destruction of red blood cells. Acute lead poisoning causes dysfunction of kidney, liver, brain, central nervous system and reproductive system. Mercury poisoning causes neurological damage.

 

Domestic Effluents

Use of detergents is increasing day by day for cleaning purposes in houses and industries. It is because, detergents have strong cleaning action than that of soap even in hard water. They can work even in acidic solutions. But they have a major disadvantage over the soaps, as some of the detergents are non-biodegradable (cannot be decomposed by microorganisms like bacteria). When household water containing these detergents is discharged in streams, ponds, lakes and rivers, it causes water pollution.

The detergent remains in the water for a long time and makes the water unfit for aquatic life. The phosphate salts present in detergents cause rapid growth of algae in water bodies, which floats over the surface of water. These plants ultimately die and decay. Decaying plants being biodegradable consume oxygen gas present in water. Thus, depletion of oxygen gas results in death of aquatic life.

Domestic sewage contains a wide variety of dissolved and suspended impurities. They include food and vegetable waste, garbage, cans, bottles, chemical soaps, washing powder, etc. It also contains disease causing microbes. All these substances add to water pollution.

 

 

Agricultural Effluents

Water pollution due to agricultural waste is because of use of fertilizers and pesticides. Fertilizers are used to make up the deficiency of nitrogen, phosphorus, etc. of the soil because of intensive cultivation of crops in the recent years.

On the other hand, pesticides are used either directly to kill or control the growth of pests. Pests may be weeds, herbs, insects, fungi, viruses, etc. They all damage crops and transmit diseases both to human beings and animals.

 

Agricultural effluents have dual effects:

(i) Intensive cultivation of crops causes these chemicals from fertilizers and pesticides to seep into the groundwater commonly called leaching process. The high nitrate contents in groundwater is mainly because of irrigation run-off from agricultural fields.

(ii) Run-off from the agricultural land (where fertilizers and pesticides have been used) enters into ponds, streams or rivers. This water contains nitrate (NO3-) and phosphate (PO43-) salts. These substances results in a rapid growth of algae, floating over the surface of water. They prevent the sunlight and air (oxygen) to reach upto aquatic life. When algae dies, bacteria consume oxygen of the water for decomposition of algae. As a result oxygen depletes in the water. Aquatic animals feel suffocation and ultimately die due to insufficient supply of oxygen.

Effects of Water Pollution

Water pollution has the following effects:

  1. It is hazardous to human health. Drinking polluted water can cause cholera, typhoid and diarrhea.
  2. The use of polluted water is not only devastating for people but also for animals and birds.
  3. It causes rapid growth of algae. Death and decomposition of algae causes deficiency of oxygen in water that affects organism living in water.
  4. It is damaging aquatic life, thus breaking a link in food chain.
  5. It reduces the aesthetic quality of lakes and rivers.
  6. It is unfit for cleaning or washing purposes.

 

 

 

 

 

 

Question: Explain five important waterborne diseases. How can these be prevented?

 

 

 

 

WATERBORNE INFECTIOUS DISEASES

Diseases that spread because of drinking polluted water or eating food prepared with polluted water are called waterborne infectious diseases. Water pollution may be due to toxins or microorganisms. Toxins are arsenic, mercury, lead and many organic chemicals. Microorganisms are viruses, bacteria, protozoa and worms.

Lack of proper sanitation facilities is the main cause of rapidly spreading waterborne diseases. A few common diseases are mentioned here:

 

(i) Diarrheal diseases

Intestinal diseases, such as cholera, that may cause dangerous dehydration. Diarrhea may be caused by viruses, bacteria or parasites.

 

(ii) Dysentery

Dysentery is an intestinal disease which is typically caused by certain bacteria or parasites. It is characterized by severe diarrhea that may be accompanied by blood or mucous.

 

(iii) Cholera

Cholera is an acute infection caused by the bacteria Vibrios cholerae, which may be found in water contaminated by human feaces. Cholera causes severe diarrhea and can be fatal.

 

(iv) Cryptosporidium

Waterborne microorganism (protozoa) that causes gastrointestinal illness (cryptosporidiosis) including diarrhea and vomiting. These tiny pathogens are found in surface water sources like reservoirs, lakes and rivers.

 

(v) Fluorosis

Fluorosis is a disease caused by the consumption of excess fluoride. Fluorosis can cause bones and teeth damage.

 

(vi) Hepatitis

It is liver inflammation commonly caused by one of five viruses called hepatitis A, B, C, D, and E. Hepatitis A and E can be transmitted by contaminated water.

 

(vii) Hookworm

Hookworm is a parasitic worm that infects the small intestine. Severe cases can result in anemia and stunted growth in children. Hookworm larvae enter the body through the skin, often via the feet. Spread by poor sanitary conditions, hookworms infect about one billion people worldwide per annum.

 

(viii) Jaundice

Jaundice is caused by an excess of bile pigments in the blood. Liver ceases to function and eyes turn yellow. Patient feels weakness and fatigue.

 

(ix) Typhoid

A dangerous bacterial disease often spread by contaminated water or by food prepared with contaminated water.

 

Prevention of waterborne diseases

Waterborne diseases can be prevented by taking the following measures:

(i) Provision of safe water: Drinking water must be properly treated and purified.

(ii) Disposal of sewage: There must be adequate sanitary disposal of sewage. Any type of waste must not be thrown or discharged directly in water supplies or reservoirs.

(iii) Control of toxic chemicals: Chemical contamination can cause acute illness, but often toxic contaminants are slow poisons and carcinogens.There must be a strict control over the use of pesticides and other chemicals.

 

 

 

Chemistry of swimming pool cleanliness

Swimming pools are cleaned by chlorination process. It is the addition of chlorine solution in swimming pools. Chlorine kills bacteria and other microorganisms. Cl2 itself does not kill rather it dissociate in water to form hypochlorous acid (HOCI) and hydrochloric acid

 

HOCI further ionizes to produce hypochlorite and proton

 

 

Both the products HOCI and OCl kill bacteria and microorganisms

 

 

Key points:

  • Water is an excellent solvent, has high specific heat capacity, high surface tension and high capillary action.
  • Water is universal solvent because of its polarity and hydrogen bonding ability.
  • Soft water produces lather with soap.
  • Hard water does not produce lather with soap.
  • Hardness is of two types: temporary and permanent.
  • Temporary hardness is because of bicarbonates of calcium and magnesium. This hardness can be removed by boiling or by addition of slaked lime (Ca(OH)2) in water.
  • Permanent hardness is because of presence of carbonates and sulphates of calcium and magnesium. This hardness can be removed by treating water with washing soda and sodium zeolite.
  • Used water is called waste water or sewage.
  • Water pollution is caused by affecting water quality by pollutants.
  • Industrial effluents are one of the main causes of water pollution. It includes high toxic organic chemicals, inorganic salts, heavy metals, mineral acids, oil and greases, etc.
  • Household water in the sewage from toilets, baths, kitchens, etc. consists of detergents used for cleaning purposes. Detergent being non-biodegradable causes rapid growth of aquatic plants. When these plants die and decay, they consume O2 present in the water. Thus, aquatic life is badly affected because of scarcity of O2.
  • Agricultural effluents consist of fertilizers and pesticides. These substances provide nitrate and phosphate ions for rapid growth of aquatic plants. When these plants die and decay, their decomposition process consumes O2 of water. Thus, depletion of O2 causes damage to the aquatic life.
  • Waterborne diseases are those diseases that spread because of drinking polluted water. These diseases spread because of lack of proper sanitation arrangements. These diseases can be prevented by using safe water, properly disposing sewage and controlled use of toxic chemicals

 

 

SKILLS:

Quality of Water

Good quality water is colourless, odourless and tasteless. Hardness of water can be checked by washing. Soft water produces lather with water. Pure water has least conductivity.

Boiling point of water

Water boils at 100°C.

 

Write a note on the treatment of sewage water.

 

 

 

 

 

 

Distillation of impure water

Impure water can be purified by simple distillation apparatus as shown in figure. Distillation process involves boiling of a liquid and then condensing the vapours.

Impure water is taken in a distillation flask. It is boiled. Water vapours rise and enter the condenser. The vapours condense while passing through condenser. Thus, they are changed back into pure water, which is called distillate (distilled water). The distillate is collected in a beaker. The impurities remains behind in the distillation flask.

Short Questions

  1. How water rises in plants?

Answer: Water rises in plants through capillary action. It is the process by which water rises up from the roots of plants to leaves. The tension of the surface of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area is called surface tention. Water has high surface tension. This unique property of water is responsible for its high capillary action which carries water to leaves and shoots in land plants.

 

  1. Which forces are responsible for dissolving polar substances in water?

Answer: Polar nature of water and extensive hydrogen bonding is responsible for dissolving polar substances in water.

  1. Why non-polar compounds are insoluble in water?

Answer: According to general principle of solubility, Non polar substances are only soluble in non-polar solvents and polar substances are soluble on polar. As water is a polar solvent so non-polar compounds are insoluble in water and are only soluble in non-polar solvents like alcohol and benzene.

  1. How water dissolves sugar and alcohol?

Answer: Water molecule is composed of oxygen and hydrogen atoms. Because of two O—H bonds and two lone pairs, one H2O molecule can form hydrogen bonding with four other H2O molecules, which are arranged tetrahedrally around the H2O molecule. This unique behaviour of water enables it to dissolve many polar non-ionic compounds having hydroxyl group (-OH), like alcohols, organic acids, glucose, sugar, etc. by forming hydrogen bonds with them.

  1. How limestone dissolves in water?

Answer: Limestone is insoluble in water. However, in the presence of carbon dioxide small quantity of lime stone is soluble in water according to the following chemical reaction.

 

  1. Differentiate between soft and hard water.

Answer: Soft water: Soft water is that which produces good lather with soap.

Hard water: Hard water is that which does not produce lather with soap.

 

  1. What are the causes of hardness in water?

Answer: The rain water while coming down absorbs carbon dioxide from the atmosphere. The water mixed with carbon dioxide, when passes through the beds of the soil, converts insoluble carbonates of calcium and magnesium into soluble bicarbonates. It may also dissolve chlorides and sulphates of calcium and magnesium. These salts make the water hard.

  1. What are the effects of temporary hardness in water?

Answer: Disadvantages of Hard Water:

(i) Hard water consumes large amount of soap in washing purposes.

(ii) Drinking hard water causes stomach disorders.

(iii) Hard water is unfit for use in steam engines, boilers and turbines because insoluble calcium and magnesium salts deposit inside. They are called scales. They are bad conductors of heat and hence more fuel is used. Insoluble calcium and magnesium sulphates not only reduce the efficiency of the engine but also cause the boiler to burst.

  1. Mention the disadvantages of detergents.

Answer: The detergent remains in the water for a long time and makes the water unfit for aquatic life. The phosphate salts present in detergents causes’ rapid growth of algae in water bodies, which floats over the surface of water. These plants ultimately die and decay. Decaying plants being biodegradable consume oxygen gas present in water. Thus, depletion of oxygen gas results in death of aquatic life.

  1. What is difference between biodegradable and non-biodegradable substances?

Answer: ·  Biodegradable is referred to all those things that can be easily decomposed by natural agents like Micro-organisms, water, oxygen, ultraviolet rays of the sun, acid rains, microorganisms, etc. On the other hand non-biodegradable substances are never broken down or decomposed by environmental factors.

  • Biodegradable substances include food waste like vegetable and fruit peels, dead plants and animals, chicken, egg shells, paper materials, garden waste etc. Non-biodegradable things consist of  plastics, polystyrene, metals, plastic and aluminum cans, toxic chemicals, paints, tyres, etc.
  1. How detergents make the water unfit for aquatic life?

Answer: The detergent remains in the water for a long time and makes the water unfit for aquatic life. The phosphate salts present in detergents causes’ rapid growth of algae in water bodies, which floats over the surface of water. These plants ultimately die and decay. Decaying plants being biodegradable consume oxygen gas present in water. Thus, depletion of oxygen gas results in death of aquatic life.

  1. Why are pesticides used?

Answer: Pesticides are used to kill pests which are harmful organisms. Pests destroy crops and also responsible for various diseases in animals.

  1. What are the reasons of waterborne diseases?

Answer: Diseases that spread because of drinking polluted water or eating food prepared with polluted water are called waterborne infectious diseases. Lack of proper sanitation facilities is the main cause of rapidly spreading waterborne diseases. Water pollution may be due to toxins or microorganisms. Toxins are arsenic, mercury, lead and many organic chemicals. Microorganisms are viruses, bacteria, protozoa and worms.

  1. How waterborne diseases can be prevented?

Answer: Prevention of waterborne diseases

Waterborne diseases can be prevented by taking the following measures:

(i) Provision of safe water: Drinking water must be properly treated and purified.

(ii) Disposal of sewage: There must be adequate sanitary disposal of sewage. Any type of waste must not be thrown or discharged directly in water supplies or reservoirs.

(iii) Control of toxic chemicals: Chemical contamination can cause acute illness, but often toxic contaminants are slow poisons and carcinogens.There must be a strict control over the use of pesticides and other chemicals.

 

 

Extensive Questions

  1. How polarity of water molecule plays its role to dissolve the substances?

Answer: See in Above Questions.

  1. Explain the methods of removing permanent hardness.

Answer: See in above questions.

  1. Explain the water pollution because of industrial waste.

Answer: See in above questions.

  1. Justify the statement: household water is the reason of water pollution.

Answer: See in above questions

  1. Explain agricultural effluents are fatal for aquatic life.

Answer: See in above questions.

  1. Explain five important waterborne diseases. How can these be prevented?

Answer: See in above questions.

  1. Give some disadvantages of hard water.
  2. What is water pollution? Describe the effects of using polluted water.

Answer: See in above questions.

  1. Explain the reasons, why is water considered a universal solvent.

Answer: See in above questions.

  1. Write a note on the treatment of sewage water.

Answer: Liquid Waste (Sewage/Wastewater) Treatment 

Wastewater (liquid waste) from flushing the toilet, bathing, washing sinks and general cleaning goes down the drain and into a pipe, which joins a larger sewer pipe under the road. The larger pipe also joins a major pipe that leads to the treatment center.

STAGE ONE: SCREENING
Screening is the first stage of the wastewater treatment process. Screening removes large objects like, diapers, nappies, sanitary items, cotton buds, face wipes and even broken bottles, bottle tops, plastics and rags that may block or damage equipment.
Special equipment is also used to remove grit that gets washed into the sewer.

STAGE TWO: PRIMARY TREATMENT
This involves the separation of organic solid matter (or human waste) from the wastewater. This is done by putting the wastewater into large settlement tanks for the solids to sink to the bottom of the tank. The settled solids are called ‘sludge’. At the bottom of these circular tanks, large scrappers continuously scrape the floor of the tank and push the sludge towards the center where it is pumped away for further treatment. The rest of the water is then moved to the Secondary treatment.

STAGE THREE: SECONDARY TREATMENT
The water, at this stage, is put into large rectangular tanks. These are called aeration lanes. Air is pumped into the water to encourage bacteria to break down the tiny bits of sludge that escaped the sludge scrapping process.

STAGE FOUR: FINAL TREATMENT
Next, the ‘almost’ treated wastewater is passed through a settlement tank. Here, more sludge is formed at the bottom of the tank from the settling of the bacterial action. Again, the sludge is scraped and collected for treatment. The water at this stage is almost free from harmful substances and chemicals. The water is allowed to flow over a wall where it is filtered through a bed of sand to remove any additional particles.

The filtered water is then released into the river.

“OR”

Distillation of impure water

Impure water can be purified by simple distillation apparatus as shown in figure. Distillation process involves boiling of a liquid and then condensing the vapours.

Impure water is taken in a distillation flask. It is boiled. Water vapours rise and enter the condenser. The vapours condense while passing through condenser. Thus, they are changed back into pure water, which is called distillate (distilled water). The distillate is collected in a beaker. The impurities remains behind in the distillation flask.

 

Test yourself 15.1

  1. What is capillary action ?

Answer: Capillary action is the process by which water rises up from the roots of plants to leaves. This process is vital for the survival of the land plants.

 

  1. Point out two properties of water that make it an excellent solvent.

Answer: Water is the universal solvent because it can dissolve almost all the minerals. Its ability to dissolve substances is because of two unique properties of water:

(i) Polarity of water molecule;

(ii) Exceptional hydrogen bonding ability.

 

Question iii. Why the water molecule is polar?

Answer: Water has a polar nature. The one end of water molecule has a partial positive while the other has partial negative charge. It is due to the electronegativity difference between hydrogen and oxygen.

  1. Explain why nonionic polar compounds are soluble in water?

Answer: Water molecule is composed of oxygen and hydrogen atoms. Because of two O—H bonds and two lone pairs, one H2O molecule can form hydrogen bonding with four other H2O molecules, which are arranged tetrahedrally around the H2O molecule. This unique behaviour of water enables it to dissolve many polar non-ionic compounds having hydroxyl group (-OH), like alcohols, organic acids, glucose, sugar, etc. by forming hydrogen bonds with them.

Test yourself 15.2

  1. Which salts are responsible for hardness of water?

Answer: (i) Temporary hardness is because of presence of bicarbonates of calcium and magnesium. (ii) Permanent hardness is because of presence of sulphates and chlorides of calcium and magnesium.

 

  1. Explain the chemistry of removing the temporary hardness by boiling water.

Answer: Temporary hardness of water is easily removed by boiling water. On boiling, calcium bicarbonate Ca(HCO3)2 decomposes to produce insoluble calcium carbonate, which precipitates out of the solution.

 

 

iii. What is the principle of removing permanent hardness of water?

Answer: Permanent hardness can only be removed by using chemicals. Calcium (Ca2+) and magnesium (Mg2+) are removed as insoluble salts by adding washing soda (Na2CO3 ) or sodium zeolite.

 

 

  1. How addition of Na2C03 removes permanent hardness of water?

Answer: By using washing soda: The addition of washing soda removes the calcium and magnesium ions as the insoluble calcium and magnesium carbonates, respectively.

 

  1. How sodium zeolite softens water?

Answer: Using Sodium Zeolite (an ion Exchanger):  Sodium zeolite is a naturally occurring resin of sodium aluminium silicate NaAl(SiO3)2 , which can also be prepared artificially. It is used for softening of water at domestic as well as on industrial scale. When water is passed through resin, sodium ions of the resin are exchanged with the unwanted calcium and magnesium ions of the hard water.

  1. What do you mean by boiler scales? How are they removed?

Answer: Hard water is unfit for use in steam engines, boilers and turbines because insoluble calcium and magnesium salts deposit inside. They are called scales. They are bad conductors of heat and hence more fuel is used. Insoluble calcium and magnesium sulphates not only reduce the efficiency of the engine but also cause the boiler to burst.

Before use the hard water is made soften by the following methods.

1) Boiling. 2) Clark’s method. 3) By using washing soda. 4) By using sodium zeolite.

When the scale is formed on the surface of boilers then it will be remove by using vinegar solution.

Test yourself 15.3.

  1. What is an industrial waste?

Answer: The waste materials discharge by the industrial units is called industrial wastes. These include highly toxic chemicals, salts, heavy metals, mineral acids, oils, greases and detergents.

 

  1. How water used as a cleaning agent in industries causes pollution?

Answer: water used as cleaning agent in industries is directly discharged out. This water contains all kinds of toxic chemicals and detergents. When these effluents and used water enter lakes, streams, rivers or oceans, they either get dissolved or float suspended in water. Even they get deposited on the bed. This results in the pollution of water.

 

iii. Why use of detergents is increasing day by day?

Answer: Detergents have strong cleaning action so there role in industries and homes increasing day by day.

 

  1. How decaying plants consume oxygen?

Answer: Nitrate (NO3-) and Phosphate (PO43-) salts in pesticides results in a rapid growth of algae, floating over the surface of water. They prevent the sunlight and air (oxygen) to reach upto aquatic life. When algae dies, bacteria consume oxygen of the water for decomposition of algae. As a result oxygen depletes in the water. Aquatic animals feel suffocation and ultimately die due to insufficient supply of oxygen.

  1. What is function of fertilizers?

Answer: Fertilizers are the used to increase the fertility of the soil.

  1. How pesticides cause water pollution?

Answer: Run-off from the agricultural land (where fertilizers and pesticides have been used) enters into ponds, streams or rivers. This water contains nitrate (NO3-) and phosphate (PO43-) salts. These substances results in a rapid growth of algae, floating over the surface of water. They prevent the sunlight and air (oxygen) to reach upto aquatic life. When algae dies, bacteria consume oxygen of the water for decomposition of algae. As a result oxygen depletes in the water. Aquatic animals feel suffocation and ultimately die due to insufficient supply of oxygen.

Test yourself 15.4.

  1. Define water borne diseases

Answer: Diseases that spread because of drinking polluted water or eating food prepared with polluted water are called waterborne infectious diseases.

 

  1. What is dysentery?

Answer: Dysentery is an intestinal disease which is typically caused by certain bacteria or parasites. It is characterized by severe diarrhea that may be accompanied by blood or mucous.

 

iii. Which bacteria causes the cholera?

Answer: Cholera is an acute infection caused by the bacteria Vibrios cholerae, which may be found in water contaminated by human feaces.

 

 

  1. What do you mean by fluorosis?

Answer: Fluorosis is a disease caused by the consumption of excess fluoride. Fluorosis can cause bones and teeth damage.

  1. What is hepatitis?

Answer: It is liver inflammation commonly caused by one of five viruses called hepatitis A, B, C, D, and E. Hepatitis A and E can be transmitted by contaminated water.

Leave a Reply

Your email address will not be published. Required fields are marked *