Frank Solutions for Chapter 6 Water Class 9 Chemistry ICSE

1. The molecular formula of water is H2O. What information do you get from this formula about the composition of water?

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 element of which it is made up of.


2. Write two characteristic properties of water which make it a good solvent.

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.


3. From the list of materials, viz., calcium, magnesium and sodium write down the name of one which is least reactive with cold water but will decompose hot water. Give the equation for the reaction.

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


4. (a) Give any five physical properties of water.

(b) If a salt is dissolved in water, state its effect on

(i) Boiling point of water

(ii) Freezing point of water

(iii) Density of water

Answer

(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 behaviour: 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.

(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.


5. State any one significance of the high specific heat of water.

Answer

Due to its high specific heat capacity, water is used in cooling systems for e.g., as coolant in motor car radiators.


6. Give three examples, with balanced equation to show the catalytic nature of water.

Answer

In many chemical reactions water acts as a catalyst.

(a) Hydrogen and oxygen when sparkled together combines only if moisture is present.

(b) Yellow phosphorus burns in air in the presence of moisture.
(c) Water also helps in rusting process.

Fe + O2 + H2O → Fe2O3 .xH2O


7. Name the category or class of compound formed when water reacts with:

(a) Metal oxides

(b) Non–metallic oxides

Answer

(a) Water reacts with metals oxides: Corresponding hydroxides are formed which act as alkalis.

(b) Water reacts with non-metals oxides: Corresponding acids are formed.


8. Give balanced equations for the action of water of the following:

(a) Calcium oxide

(b) Sulphur trioxide

(c) Phosphorous pentaoxide

(d) Sulphur dioxide

(e) Sodium

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


9. Give two tests to confirm that given colourless, odourless liquid is water.

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.


10. (a) Water contains dissolved gases. What is the principle underlying the removal of the dissolved gases from water?

(b) Name two gases that are dissolved in water and state the significance of each to aquatic organisms.

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.

(b) Importance of dissolved gases in water:

  1. Oxygen gas is dissolved in water which is used by aquatic organism to respire.
  2. 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.


11. State one important use of water:

(a) In the plant body

(b) In the human body.

Answer

(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 developed of plants.

(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.


12. Water exists in all the three states. Discuss.

Answer

Normally water exists in liquid state but when heated under normal pressure (760 mm Hg) water boils at 100°C and changes rapidly into gaseous state (steam). Water freezes at 0°C and changes into solid state (ice).


13. Give two tests to prove that the given liquid is water.

Answer

We can test the presence of water by performing these two sets:

1. On adding few drops of water to white anhydrous copper sulphate, the latter turns blue. (CuSO4.5H2O)

2. On adding water to blue cobalt chloride the latter turns pink due to the formation of CoCl2.6H2O


14. Density of water varies with temperature. What are its consequences?

Answer

Density of water is 0.997 gcm-3 at 4°C. The density of water decreases when the temperature increases above 4°C 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.


15. How is aquatic life benefited by the fact that water has a maximum density at 4°C?

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.


16. Describe an experiment to show that tap water contains dissolved salts.

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.

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.


17. State the visible changes and give equations for reactions when:

(a) A piece of Na is added to cold water.

(b) A piece of Ca is added to cold water.

(c) A coil of Mg is heated in steam.

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.

(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.

(c) A coil of Mg is heated in steam:

It bums 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.


18. Name two:

(a) liquids,

(b) solids, and

(c) gases, which dissolve in water.

Answer

(a) Liquids: alcohols, acids

(b) Solids: Sugar, urea

(c) Gases: Oxygen, carbon dioxide


19. Name the form of natural water which is the purest form.

Answer

Rain water is the purest form of natural water.


20. How does temperature affect the solubility of (a) a solid in a liquid (b) a gas in a liquid?

Answer

(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 nitrite (KNO3) and sodium nitrate (NaNO3).

(b) A gas in a liquid : Solubility decreases with rise in temperature. Thus, gases dissolves readily in cold water than in hot water.


21. What is the advantage of solubility curves ?

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.

Variation in the solubility of some solids with changing temperature.


22. Give reasons:

(a) Ice floats on water.

(b) A saturated solution becomes unsaturated when heated.

(c) Hot water contains less dissolved air than cold water.

Answer

(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.

(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.

(c) Solubility of gases decreases with increase in temperature hence hot water contains less dissolved air than cold water.


23. Define the following:

(a) Solute

(b) Solvent

(c) Solution

Answer

(a) Solute: The substance which dissolves in a medium to produce a solution is called the solute.

(b) Solvent : It is the medium in which the solute dissolves.

(c) Solution: A homogenous mixture that has uniform composition throughout the volume of the mixture. It is the medium in which the solute dissolves.


24. What happens when :

(a) A saturated solution is warmed.

(b) A hot saturated solution is cooled.

(c) Some more of the solvent is added to a saturated solution.

Answer

(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.

(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 that it can hold at room temperature.

(c) When the quantity of solvent increases, greater amount of solute can be dissolved.


25. Why does Glauber’s salt (Na2SO4.10H2O) show anomalous solubility in water?

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.


26. Distinguish : a solution, a mixture and a compound.

Answer

27. Give three factors on which the solubility of a solid depends.

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.


28. Define Henry's Law?

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.


29. The following is a series of metallic elements placed in order of reactivity. X. Y, and Z are imaginary elements (you are not asked to suggest what elements they might be)

Answer the following questions about X Y and Z:

(i) Which of the metals X, Y or Z will be found free in nature?

(ii) Out of X and Z, which metal will combine with oxygen readily and with ease?

(iii) Write down the symbol of one of the metals (X, Y or Z) whose oxide is a strong base.

(iv) Write down the symbol of one of the metals (X, Y or Z) whose hydroxide is coloured.

(v) Write down the symbol of one of the metals (X, Y or Z) whose oxide cannot be reduced by hydrogen or carbon to give the metal.

(vi) Explain, what you would expect to see if a piece of Y was dipped into a solution of copper nitrate for a few minutes and withdrawn and examined.

Answer

(i) Z is least reactive so it will be found free in nature.

(ii) X is more reactive so it will react with oxygen more readily and with ease.

(iii) Z

(iv) Y is iron whose oxide is brown coloured Fe2O3.

(iv) X

(v) 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.


30. What test will you carry out to find out whether a given solutions is saturated or unsaturated or supersaturated ?

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 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 crystals 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 crystals starts it up and it will deposit its excess solute onto the crystals making the crystals bigger.


31. In which of the following substances can there be:

(a) Increase in weight

(b) Decrease in weight

(c) No change in weight, when exposed to air ?

  1. Sodium chloride
  2. Iron
  3. Conc. Sulphuric acid
  4. Sodium carbonate crystals

Answer

(a) Increase in weight : When sodium chloride when exposed in air it gains moisture and its weight increases.

(b) Decreases 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.

(c) No effect on weight : When conc. Sulphuric acid when exposed to air does not react and no change occurs in weight.


32. Define water pollution.

Answer

Water pollution may be defined as the contamination of water by foreign substances which make it harmful for heath of animals or plants or aquatic life, make it unfit for drinking and for domestic, industrial and agriculture use.


33. What are the major water pollutants ?

Answer

Major pollutants of water are :

  1. Ground water pollutants: Septic tanks, industry by products like pesticides, fertilizers, tanneries, mining wastes.
  2. Surface water pollutants: No. of gases present in atmosphere like SO2, CO2, H2S, NOx, CO, etc. pollute surface water.
  3. Lake water pollutant : Organic wastes from hills, toxic effluents from urban areas, industrial effluents, dumping of huge amounts of sediments etc.,
  4. River water pollutants: Industrial discharge, sewage discharge, detergents, discharges from drug, paper and textile industries
  5. Marine water pollutant : Major pollutant is oil spills.


34. Discuss the various sources of water pollution.

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:

(a) Discharge of Household detergents: detergents used as a cleaning agent produce foam and pollute water. They do not undergo bio-degradation.

(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.

(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.


35. How water is treated for safe effluents ?

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.


36. Name two water born disease.

Answer

Two water born diseases are:

  1. Gastroenteritis
  2. Bacterial dysentery

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