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.
Answer: Mixture.
Q.5: Define with example: (a) Mixture (b) Metalloid (c) Compound
Answer:
(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.
Further Study
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