Chapter-2, Is Matter Around Us Pure
CBSE Board Ncert Science
Hot Questions (following CCE questions pattern)
Q.1: How can matter be classified on the basis of its chemical constitution?
Answer: On the basis of its chemical constitution matter can be classified as element, compound and mixture.
Q.2: How will you obtain pure copper sulphate from an impure sample?
Answer: By crystallization.
Q.3: What should be the minimum difference in temperature between the boiling points of two miscible liquids that these can be separated by simple distillation method?
Answer: 20 - 25OK
Q.4: Name the material which shows the average properties of the constituents contained in it.
Q.5: Define with example: (a) Mixture (b) Metalloid (c) Compound
(a) Mixture - When more than one substance (element, compound etc.) are mixed in any proportion, it forms a mixture. In a mixture the constituents are not chemically combined and can be separated into pure substances using appropriate separation techniques. The properties of a mixture vary according to the proportion and properties of its constituents. Depending upon the composition and nature of components we can have different types of mixtures.
Mixtures are also classified as homogeneous and heterogeneous on the basis of their physical properties. For example - air, and alloys like brass, steel etc. are homogeneous mixtures while, a mixture of sand in water, water and kerosene oil are heterogeneous mixtures.
(b) Metalloid - Metalloids are elements which have properties intermediate between those of metals and non-metals. Examples are silicon, germanium, boron etc.
(c) Compound - A compound is a substance which is formed by the combination of two or more elements chemically combined in a fixed proportion by weight. The properties of a compound are entirely different from its constituents. a compound can be decomposed into two or more simpler substances by the chemical or electrochemical reactions. For example, water is a compound formed by combination of 89% oxygen and 11% hydrogen by weight and can be easily decomposed into its constituents. Its properties viz., density, physical state, reactivity, odor etc. are quite different from the properties of hydrogen and oxygen. Similarly salt, sugars are examples of other compounds.
Q.6: What are ‘fractionating column’ and ‘fractional distillation’?
Answer: When the boiling points of two liquids are very close i.e. difference is less than 20-25OC, the separation is not successfully affected by using simple distillation process but is affected by fitting the flask with a fractionating column. A fractionating column is a long tube provided with obstructions or packed with glass beads. The obstructions provided slow down the passage of vapours upwards and that of the liquid downwards. Thus, they provide surface for the vapours to cool and condense repeatedly. (Check fig. from NCERT Text Book).
Q.7: Give two important industrial applications of fractional distillation.
Answer: Two important industrial applications of fractional distillation are -
(1) Separation of petroleum products such as petrol, diesel and other organic compounds from crude petroleum.
(2) Separation of various gases like oxygen, nitrogen, argon etc. from air.
Q.8: Name the separating processes for the following:
(a) A solid substance dissolved in a liquid.
(b) Fine insoluble particles suspended in a liquid.
(c) Salt from sea water.
(d) Three different coloured solid substances soluble in a mixture of solvents.
(e) Two immiscible liquids.
(f) Two immiscible liquids having 10OC difference in their boiling points.
(g) Mixture of Nitrogen and Helium gases.
Answer: (a) Either by evaporation or distillation or crystallization. (b) Centrifugation and filtration or decantation. (c) Evaporation or crystallization. (d) Chromatography. (e) Separating funnel. (f) Fractional distillation. (g) Cooling and fractional distillation.
Q.9: How is air separated from its constituents?
Answer: Air is homogeneous mixture of several gases which have boiling points very close to each other and also much below room temperature. Hence, the constituents of air can be separated by fractional distillation. Air is first converted into liquid form under higher pressure and reduced temperature. Then at constant high pressure, it is warmed up in fractionating column. Different fractions of air are then obtained in gaseous form at different temperatures. The gas with the lowest boiling point is obtained first and so on.