Monday, August 15, 2011

Chemistry Class 9th:Is matter around us pure solved questions

Q.1: Which separation techniques will apply for the separation of the following?
(a) Sodium chloride from its solution in water.
(b) Ammonium Chloride from a mixture containing Sodium Chloride and Ammonium Chloride.
(c) Small pieces of metal in the engine oil of a car.   
(d) Different pigments from an extract of flower petals.
(e) Butter from curd.
(f) Oil from water.
(g) Tea leaves from tea.
(h) Iron pins from sand.
(i) Wheat grains from husk.
(j) Fine mud particles suspended in water.
Ans: (a) Crystallization or Evaporation.
 (b) Sublimation.
(c) Centrifugation or Sedimentation.
(d) Chromatography.
(e) Centrifugation.
(f) Separating funnel.
(g) Hand-picking.
(h) Magnetic separation.
(i) Winnowing.
(j) Centrifugation.
Q.2: Write the steps you would use for making tea. Use the words - solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Ans: Take the solvent, water, in a kettle. Heat it. When the solvent boils, add the solute, milk. Milk and water forms a solution. Then pour some tea leaves over a sieve. Pour slowly hot solution of milk over tea leaves. Colour of tea leaves goes into solution as filtrate. The remaining tea leaves being insoluble remains as residue. Add requisite sugar which dissolves and the tea is ready.
Q.3: Explain the following giving examples:
(a) Saturated solution, (b) Pure substance, (c) Colloid, (e) Suspension.
Ans: (a) Saturated Solution - a solution in which no more of the solid (solute) can be dissolved at a given temperature is called a saturated solution. Suppose 50 gm of a solute is the maximum amount that can be dissolved in 100 gm water at 298 K. Then 150 gm of solution so obtained is the saturated solution at 298 K.
(b) Pure Substance - A pure substance consists of a single of matter or particles and can not be separated into other kind of matter by any physical process. Pure substances always have the same colour, taste and texture at a given temperature and pressure. For example, pure water is always colorless, odorless and tasteless and boils at 373 K at normal atmospheric pressure.
(c) Colloid - Colloids are heterogeneous mixtures the particle size is too small to be seen with a naked eye, but it is big enough to scatter light. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. Colloids are useful in industry and daily life.
A colloid has the following characteristics:
(1) It is a heterogeneous mixture.
(2) The size of particles of a colloid lies between 1 - 100 nm and can not be seen by naked eyes.
(3) The particles of colloid can scatter a beam of light passing through it and make the path visible.
(4) The particles of colloid can not be separated from the mixture by filtration. The process of separation of colloidal particles is known as ‘centrifugation’.
(5) They do not settle down when left undisturbed. In other words colloids are quite stable e.g. smoke, milk, fog, cloud etc.
(d) Suspension 
A ‘suspension’ is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.
A suspension has the following characteristics:
(1) It is a heterogeneous mixture.
(2) The size of particles of a suspension is greater than 100 nm and is visible to naked eyes.
(3) The particles of suspension can scatter a beam of light passing through it.
(4) The particles of a suspension settle down when left undisturbed.
(5) The particles of a suspension can be separated from its mixture by filtration.       
Q.4: Classify each of the following as a homogeneous or heterogeneous mixture: soda water, wood, air, soil, vinegar, filtrated tea.  
Ans: Homogeneous mixture - soda water, air, vinegar, filtered tea.
       Heterogeneous mixture - wood, soil.
Q.5: How would you confirm that a colourless liquid given to you is pure water?
Ans: Every liquid has a characteristic boiling point at 1 atmospheric pressure. If the given colourless liquid boils exactly at 373 K at 1 atmospheric pressure, then it is pure water. If the boiling point is different then the water is contaminated.
Q.6: Which of the following materials fall in the category of a ‘pure substance’?
(a) Ice (b) Milk (c) Iron (d) Hydrochloric acid (e) Calcium oxide (f) Mercury (g) Brick (h) Wood (i) Air.  
Ans: (a), (c), (d), (e) and (f) are pure substances.
Q.7: Which of the following will show “Tyndall effect”?
(a) salt solution (b) milk (c) copper sulphate solution (d) starch solution.  
Ans: (b) and (d) are colloids and will show Tyndall Effect.
Q.8: Classify the following into elements, compounds and mixtures.
(a) sodium                   (b) soil                         (c) sugar solution         (d) silver
(e) calcium carbonate (f) tin               (g) silicon                     (h) coal
(i) air                            (j) soap           (k) methane                 (l) carbon dioxide (m) blood
Ans:  Elements - sodium, silver, tin, silicon.
Compounds - calcium carbonate, methane, carbon dioxide.
Mixtures - soil, sugar solution, coal, air, soap, blood.
Q. 9. In the formation of sodium chloride from its constituent elements, do the properties of its elements change? Explain.
 Ans:  Sodium is a very reactive metal that reacts exothermically with water. If we were to ingest a pinch of sodium, it would burn our intestines. Chlorine is a greenish yellow gas with a characteristic irritating odour and pungent taste. When sodium and chlorine combine to form sodium chloride, the properties of the elements are totally changed. Sodium chloride is a white substance totally safe to be ingested and is used to add flavour to our food.
Q. 10. Briefly describe how to separate, i) Sulphur from a mixture of sulphur and sand. ii) Black CuO from a mixture of CuO and ZnO.
i) Add a solvent to the mixture of sulphur and sand. Sulphur dissolves in carbon disulphide while sand does not. When filtered, the residue is sand. The filtrate is kept open, carbon disulphide evaporates and the sulphur crystals form. 
ii) Add a solvent to the mixture of CuO and ZnO that dissolves only one component e.g. sodium hydroxide. When sodium hydroxide is added to the mixture, ZnO dissolves. Filter to obtain the residue of CuO.
Past related to Ch: Is Matter Around Us Pure?  


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