Answer: Option C. -> Sublimation
:
C

Ammonium chloride is a substance which directly converts to vapour upon heating, without going through the liquid state. This process is called sublimation. The vapours can be collected and condensed into a separate flask. On the other hand, NaCl does not get vaporised at that temperature and can be separated easily.

Answer: Option A. -> 15%
:
A
Mass of the solute (sodium chloride) = 15 g
Mass of the solvent (water) = 85 g
Mass of the solution = Mass of the solute + Mass of the solvent
= 15 + 85 = 100 g
Concentration (mass by mass percentage) = $\frac{\text{mass of solute}}{\text{mass of solution}}\times 100$
= $\frac{\text{15}}{\text{100}}\times 100$ = 15%

Answer: Option A. -> Suspension > Colloidal > True solution
:
A
A true solution is a homogeneous mixture. So, the particles will be completely dissolved.
Suspension and colloidal solutions are heterogeneous mixtures. The particles in colloidal solutions are relatively smaller than suspensions. However, the particles of a suspension can be seen with the naked eye, as opposed to a colloid.
Hence, the size of particles in suspension, true solution and colloidal solution varies in the order: Suspension > Colloid > True solution.

Answer: Option B. -> 23.07 %
:
B

Volume of solute (ethyl alcohol) = 30 mL

Volume of solvent (water) = 100 mL

Total volume of solution = 30 + 100 = 130 mL

Volume by volume % = $\frac{\text{Volume of solute}}{\text{Volume of solution}}\times 100$
$=\frac{30}{130}\times 100=23.07\%$

Answer: Option D. -> Sugar crystals
:
D

A pure substance consists of only a single type of particle - atoms or molecules. Pure substances are classified as elements and compounds.
Sugar is a pure substance as it is made up of only one kind of molecule which is the sugar molecule $\left({\text{C}}_{12}{\text{H}}_{22}{\text{O}}_{11}\right)$.
The other options salt water, muddy water, and soft drink are not pure substances because these are mixtures and contains more than one type of particles.

Answer: Option C. -> Centrifugation
:
C

Centrifugation is used to separate cream from milk. Centrifugation works on the principle of density difference. When a mixture is spun rapidly, the denser particles are forced to the bottom and the lighter particles stay on top.
Similarly, when milk is spun rapidly, the less dense fat globules which make cream rise to the top. These can be removed or 'skimmed' off the milk to make low fat milk called skimmed milk. 

Answer: Option C. -> Heterogeneous- aqueous solution of alcohol
:
C and D

A mixture is a combination of two or more pure substances in which each pure substance retains its individual chemical properties.

A homogeneous mixture is a solid, liquid, or gaseous mixture that has the same proportions of its components throughout any given sample. Conversely, a heterogeneous mixture has components whose proportions vary throughout the sample.

Options (a) and (b) are correctly labelled. But, options (c) and (d) are wrong or incorrectly labeled. Let's see the reason for that. Oxygen and nitrogen will form a homogeneous mixture in a scuba tank because gases mix uniformly in any proportion to form a homogeneous mixture. Also, aqueous solution of alcohol is a homogeneous mixture, because the two liquids are miscible, soluble in all proportions.

Answer: Option A. -> Threshing
:
A and D

Threshing is the process of loosening the edible part of grain from the husks and straw to which it is attached. It is the step in grain preparation after reaping and before winnowing, which separates the grain from the chaff(dust). 

When rice is washed with water, the dust particles get mixed with water and get separated. Since dust along with water is removed, so the process is decantation.

Sedimentation is just letting it settle in a liquid medium and the impurities get sedimented. It is not suitable here because it will destroy the quality of rice if they are kept for a long time. 

Also, Sieving is defined as a method in which two or more components of different sizes are separated from a mixture on the basis of the difference in their sizes. So, the removal of dust particles from rice is difficult with this method as they both almost have the same size.

Answer: Option A. -> It involves conversion from liquid to gaseous state.
:
A and B
Evaporation is the conversion of a substance from the liquid to its gaseous state from the surface of the liquid. Its rate is increased by a number of factors such as a high surface area and wind. It happens below the boiling point of the liquid. At the boiling point, the liquid is said to boil or vaporize. Above the boiling point, there is no liquid left to vaporize.

Answer: Option B. -> False
:
B
A compound is a substance composed of two or more elements, chemically combined together in a fixed proportion. The properties of a compound are different from those of its constituent elements.
For example, let us consider water (${\text{H}}_2\text{O}$). Water is a compound with hydrogen and oxygen as constituent elements. Oxygen is required for combustion, hydrogen is combustible while water extinguishes fire. Oxygen and hydrogen are gases at room temperature while water is a liquid. Clearly, the properties of water are completely different from those of oxygen and hydrogen.