What mass of potassium nitrate would be needed to produce a saturated

A student, while the testing the solubility of potassium nitrate at 313 k, is found that the solubility is 62 in 100 g of water. What masses of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 g of water at313k?

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• Answer:

  (a) Solubility of potassium nitrate at

  
        100 g of water contains potassium nitrate = 62 g                                
  
   50 g of water contains potassium nitrate
  
    Thus, 31 g potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 g of water at 313 K.                                                   (b) When a saturated solution of potassium chloride at 353 K is cooled, the solubility potassium chloride in water decreased. As a result the amount of potassium chloride which exceeds its solubility at lower temperature separates out as crystals.   (c) The maximum amount of the salt which is dissolved in 100 g of water to form a saturate solution at the given temperature is known as solubility.                        Solubility of potassium nitrate at
  
  Solubility of sodium chloride at
  
  Solubility of potassium chloride at
  
                                     Solubility of ammonium chloride at
  
    From the above data it is clear that ammonium chloride has the highest solubility at 293 K   (d) In general the solubility of a salt increases with temperature. The increase is however different for different salts. For example the solubility of potassium nitrate increase appreciably that of ammonium chloride increases slightly that of potassium chloride increases marginally while that of sodium chloride almost remains constant.         
  

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  (a) Saturated solution : A solution in which no more solute can be dissolved in a given amount of solvent at a particular temperature is called saturated solution. For example, if we dissolve 40 g sodium chloride in 100 g of water at 293 K it will form a saturated solution because the solubility of sodium chloride at 293 K is 36 g per 100 g of water.    (b) Pure substance : A substance made up of only one kind of atoms or molecules is called a pure substance. A pure substance has the same colour taste and texture at a given temperature and pressure. A pure substance also has a fixed melting and boiling point at a constant pressure. For example hydrogen gas sodium chloride water etc.         (c) Colloid : A substance is said to be a colloid if its particles size lies between 1 to 100 nm. A colloidal solution is a heterogeneous and consists of two phases, i.e., dispersed phase (colloidal particles) and dispersion medium in which colloidal particles are suspended. For example colloidal solution of sulphur or starch milk etc.                          (d) Suspension : It is a heterogeneous mixture in which the particles of the solute do not dissolve but remain suspended throughout the bulk of the solvent. The size of the suspension particles is more than 10~7 m. For example, Chalk powder in water is a suspension.  
  

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  Homogeneous mixtures : Soda water, vinegar and filtered tea. Heterogeneous mixtures : Wood and soil. Air is a homogeneous mixture of different gases. However, if some dust or other particles are also present, then air becomes heterogeneous mixture.  
  

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  The boiling point and freezing point of the given liquid comes out to be 100°C or 373 K and 0°C or 273 K respectively under one atmospheric pressure, it confirms that the given liquid is pure water.
  

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  Ice, iron, calcium oxide and mercury are pure substances since they contain particles of only one kind of matter. In contrast, milk, hydrochloric acid (hydrogen chloride gas dissolved in water), brick and air cannot be called pure substances because they consists of particles of more than one kind of matter.
  

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  A solution is a homogeneous mixture of two or more substance. In the light of this, the solution among the following are (b) sea water, (c) air and (e) soda water.
  

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  (b) milk and (d) starch solution show Tyndall effect because they are colloidal solution. Whereas (a) salt solution and (c) copper sulphate solution are true solution. Their particle size is too small to scatter light. So they do not show Tyndall effect.  
  

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  (a) Growth of a plant, (b) rusting of iron, (d) cooking of food, (e) digestion of food and (g) burning of a candle, are chemical changes.
  

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  (b) A pure substance is one which is made up of only one kind of atoms or molecules, They have the same composition throughout.
  

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Page 11

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  (c) Rusting of an article made up of iron is called corrosion. Corrosion is a chemical change because rust is a chemical compound (hydrated iron oxide,

which is totally different from element iron (Fe). Corrosion is

Physical changes are those changes in which no new substances are formed. The substances do not change their identity and they can be easily returned to their original form by some physical processes, e.g., melting of ice, glowing of an electric bulb, breaking of glass tumbler, etc. On the other hand, chemical changes are those changes in which new substances are formed. The substances involved change their identity and they get converted into entirely new substances, which cannot be returned to their original form. e.g., burning of paper, rusting of iron, burning of magnesium wire, etc.

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    (a) A mixture of sulphur and carbon disulphide is a heterogeneous colloid and shows Tyndall effect. In a colloidal solution, the particles are big enough to scatter light. The scattering of light by colloidal particles is known as Tyndall effect. Colloids are actually heterogeneous in nature though they appear to be homogeneous.
  

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    (d) Tincture of iodine is made by dissolving iodine in alcohol.
  

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    (c) Options (i) Ice and (iv) Air are homogeneous in nature as their particles are not distinctly visible. A homogeneous mixture has a uniform composition throughout its mass. It has no visible boundaries of separation between its various constituents, e.g., air, sugar solution, brass, etc. A heterogeneous mixture does not have a uniform composition throughout its mass. It has visible boundaries of separation between its various constituents, e.g., soil, wood, blood etc.
  

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    (c) Options (i) Melting of iron metal, (iii) Bending of an iron rod and (iv) Drawing a wire of iron metal are physical changes, because in three processes, iron changes its form, not the chemica! composition. In rusting of iron, its chemical composition is changed.
  

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    (a) Options (i) Decaying of wood and (ii) Burning of wood are chemical changes, because in these processes, the chemical composition of wood is changed and new substances are formed, which cannot be converted back into their original form. In (iii) Sawing of wood and (iv) Hammering of a nail into a piece of wood, chemical composition of wood is not changed, therefore, (iii) and (iv) are physical changes
  

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    (c)

  
   is a compound made up of two elements A and B in a fixed ratio. The properties of a compound (e.g., A2B) are entirely different from those of its constituent elements (e.g., A and B). The composition of a compound is fixed.
  

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    (d) In this reaction, X and Y can not be broken down into simpler substances by chemical reactions; therefore, X and v are elements. A compound is a substance made up of two or more elements chemically combined in a fixed proportion by mass. therefore, P is a compound, having a fixed composition.
  

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Page 19

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      (a) Mercury and water are separated by decantation by using separating funnel as the separation of two immiscible liquids by a separating funnel depends on the difference in their densities. Mercury is heavier than water therefore, forms the lower layer and separated from water. (b) Potassium chloride and ammonium chloride are separated by sublimation method because ammonium chloride being a sublimate, sublimes leaving behind the potassium chloride. Sublimation is the process in which a solid changes directly into vapours on heating and vapours change into solid on cooling without going in liquid state.          

  
    (b) Potassium chloride and ammonium chloride are separated by sublimation method because ammonium chloride being a sublimate, sublimes leaving behind the potassium chloride. Sublimation is the process in which a solid changes directly into vapours on heating and vapours change into solid on cooling without going in liquid state.
  
  Separation of potassium chloride and ammonium chloride by sublimation (c) Common salt, water and sand are separated by (i) Decantation (or filtration) process is used to separate sand from common salt solution in water because common salt is soluble in water whereas sand is insoluble in water. Therefore, by filtration, sand will be separated as insoluble substance as residue and filtrate will be common salt solution in water.
  
    (ii) Evaporation process is used to separate common salt from water. Water evaporates and common salt remains as residue.  
  
    (d) (i) Decantation by using separating funnel is used to separate kerosene nil from salt solution in water as they form separate layers (salt is soluble in water). (ii) Evaporation is used to separate salt from water.
  

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    Figure (a) will be more effective condenser in the distillation apparatus because beads present will provide more surface area for cooling of the vapours passing through it.
  

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    Salt can be recovered from its solution by 'crystallisation'. Crystallisation' is a better technique than 'evaporation' because it removes soluble impurities also, which do not get removed in the process of evaporation.
  

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Page 22

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    'Sea-water' is called homogeneous as it contains dissolved salts in it. It may be called heterogeneous as it contains various insoluble components too as sand, microbes, shells made of calcium carbonate and so many other things.
  

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Page 23

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    Acetone is soluble in water, a homogeneous mixture is obtained and hence separation by separating funnel cannot be used. Acetone can be get back by simple distillation because the difference in the boiling points of acetone and water is more than 25oC. Boiling point of acetone ? 56°C Boiling point of water    ? 100°C In distillation flask, acetone will boil at 56°C and change into vapours and can be collected in flask after condensation.
  

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Page 24

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    (a) When a saturated solution of potassium chloride prepared at 60'C is allowed to cool at room temperature, crystals of potassium chloride will be formed. (b) Initially, sugar will obtained as water will get evaporated. But on dry heating sugar gets charred and it turns black.

  
  (c) The black coloured compound, iron sulphide is formed when a mixture of iron filings and sulphur powder is heated strongly.
  
  
  

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Page 25

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    The colloidal particles are smaller and not heavy. They always remain in a state of zig-zag motion, called Brownian movement, which counters the force of gravity acting on colloidal particles and hence, helps in providing stability to colloidal sols by not allowing them to settle down. Apart from this, colloidal particles are charged and repel each other. This fact also do not the particles of colloidal solution to settle down. Whereas particles of suspension are larger, heavy and have less movement, thus settle down due to gravity.
  

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Page 26

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    In smoke and fog. dispersion medium is same, i.e., air but they differ in dispersed phase. In smoke, solid carbon particles are dispersed in air while in fog, liquid water particles are dispersed in air.
  

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