If it is pure, the substance is either an element or a compound. If a substance can be separated into its elements, it is a compound. If a substance is not chemically pure, it is either a heterogeneous mixture or a homogeneous mixture. If its composition is uniform throughout, it is a homogeneous mixture.
Name the process associated with the following: Settling of sand when a mixture of sand and water is left undisturbed for some time.
Answer
This process is sedimentation as sand does not dissolve completely in water and forms suspension and settles down at the bottom when left undisturbed for some time.
Give an example each for the mixture having the following characteristics. Suggest a suitable method to separate the components of these mixtures: Two immiscible liquids.
Answer
Kerosene and water. The mixture can be separated by using a separating funnel.
Which one amongst the following will show Tyndall effect?
Copper sulphate solution.
Answer
$(b)$ 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.
When blue ink is heated, what do you think has got evaporated from the watch glass?
Answer
This is a experiment of separating dye from ink: Fill half a beaker with water and put a watch glass on the mouth of the beaker. Put few drops of ink on the watch glass. Now start heating the solution or mixture. after that the water evaporate continue heating till there is no further change in the watch glass. When the water is completely evaporated from the watch glass Dye is left in the watch glass.
Name the process associated with the following: Fine beam of light entering through a small hole in a dark room, illuminates the particles in its paths.
Answer
This shows Tyndall effect, i.e., scattering of light by colloidal solution or by particles in a fine suspension. Dust particles are suspended in air which scatter the light coming from small hole.
To obtain a solid from its solution, which technique is better, evaporation or crystallisation?
Answer
Crystallization is a process that separates a pure solid in the form of its crystals from a solution. Crystallization is better than evaporation because during Evaporation:
Some solids decompose or some, like sugar may get charred on heating to dryness.
Some impurities may remain dissolved in the solution even after.
'All mixtures are homogeneous', comment upon this statement.
Answer
No this statement is not correct because all mixtures are not homogeneous. They are heterogeneous too. The appearance and composition of homogeneous mixtures are uniform throughout. Separate constituents are not identifiable in homogeneous mixtures whereas in heterogeneous mixtures separate substances can be easily observed.
Give an example each for the mixture having the following characteristics. Suggest a suitable method to separate the components of these mixtures: Two or more coloured constituents soluble in some solvent.
Answer
Plant pigments. The mixture of coloured constituents can be separated by chromatography.
A mixture of two or more phases of a pure substance is still a pure substance as long as the chemical composition of all phases is the same. A pure substance may exist in different phases.