10. Which cation most effectively coagulates negatively charged Sb2S3 colloid?
[RPSC Senior Teacher Grade II Science, 04 Jul 2019]
(A) Ba2+
(B) Ca2+
(C) Al3+
(D) Na+
Answer: (C) Al3+
Explanation: For a negatively charged sol, positively charged counter-ions cause coagulation. Under the Schulze–Hardy rule, coagulating power generally increases sharply with counter-ion valency. Trivalent aluminium ions are therefore more effective than the listed divalent or monovalent cations.
9. Which is an example of gas dispersed in a solid?
[RPSC Senior Teacher Grade II Science, 18 Feb 2019]
(A) Cloud
(B) Froth
(C) Fog
(D) Pumice stone
Answer: (D) Pumice stone
Explanation: Pumice develops when bubbles of gas become trapped in cooling volcanic material. The resulting solid contains many gas-filled cavities and is a solid foam. Clouds and fog contain liquid droplets dispersed in air, whereas ordinary froth is gas dispersed in liquid.
8. What happens to the boiling point of pure water when salt is added?
[WBCS Preliminary, 2018]
(A) It increases
(B) It decreases
(C) It remains unchanged
(D) It becomes zero
Answer: (A) It increases
Explanation: Dissolving a nonvolatile solute such as sodium chloride lowers the solvent vapour pressure. The solution consequently needs a higher temperature to reach atmospheric pressure and boil. This boiling-point elevation is a colligative property governed by the concentration of dissolved particles.
7. Fog is a colloid of which dispersed phase and medium?
[RPSC Senior Teacher Grade II Science, 01 Jul 2017]
(A) Liquid particles dispersed in gas
(B) Gas particles dispersed in liquid
(C) Solid particles dispersed in liquid
(D) Solid particles dispersed in gas
Answer: (A) Liquid particles dispersed in gas
Explanation: Fog is made of very fine liquid water droplets suspended in air. Thus the dispersed phase is liquid and the dispersion medium is gaseous. Such colloids are called liquid aerosols; smoke, by comparison, is largely solid particles dispersed in gas.
6. Pasteurisation is related to which of the following?
[MPPSC State Service Preliminary, General Studies, 2016]
(A) Sterilisation of milk
(B) Dehydration of milk
(C) Fermentation of milk
(D) Distillation of milk
Answer: (A) Sterilisation of milk
Explanation: Pasteurisation uses controlled heating followed by cooling to reduce harmful microorganisms in milk and improve safety. Among the given exam options, sterilisation is the intended response. Strictly, however, pasteurisation is not complete sterilisation because it does not eliminate every resistant microbial spore.
5. Water gas is a mixture of which gases?
[CGPSC Preliminary General Studies, 2016]
(A) Carbon monoxide and hydrogen
(B) Carbon monoxide and water vapour
(C) Carbon dioxide and hydrogen
(D) Carbon dioxide and water vapour
Answer: (A) Carbon monoxide and hydrogen
Explanation: Water gas is chiefly a mixture of carbon monoxide and hydrogen. It can be generated by passing steam over hot carbon: C + H2O → CO + H2. The product mixture has served historically as a combustible industrial gas.
4. Which of these protective colloids has the greatest protective power based on its gold number?
[RPSC Senior Teacher Grade II Science, 2015]
(A) Gelatin (0.005–0.01)
(B) Haemoglobin (0.03–0.07)
(C) Potato starch (25)
(D) Gum arabic (0.15–0.25)
Answer: (A) Gelatin (0.005–0.01)
Explanation: The gold number measures the amount of protective colloid required to prevent coagulation of a standard gold sol. A lower gold number indicates stronger protective ability. Gelatin has the smallest listed range and therefore offers the highest protection among these choices.
3. Which of the following are chemical changes?
1. Crystallisation of sodium chloride
2. Melting of ice
3. Souring of milk
[UPSC Civil Services Preliminary, General Studies Paper I, 24 Aug 2014]
(A) 1 and 2 only
(B) 3 only
(C) 1, 2 and 3
(D) None of the above
Answer: (B) 3 only
Explanation: Crystallisation of sodium chloride and melting of ice change physical state without producing a new substance. Souring of milk is different because bacteria convert lactose into lactic acid, altering composition. Therefore only the third process represents chemical change.
2. Which of the following is an example of a chemical change?
[RAS/RTS Preliminary, 2003]
(A) White light separating into seven colours through a prism
(B) Vegetables becoming soft when cooked
(C) A wet lump of soil becoming fragile after drying
(D) Salt dissolving in water
Answer: (B) Vegetables becoming soft when cooked
Explanation: Cooking vegetables involves numerous heat-driven chemical reactions that alter cell-wall components, pigments and other molecules. Dispersion of light is optical; drying soil and dissolving salt are predominantly physical processes. The cooking option is therefore classified as a chemical change.
1. Which statement about aqueous solutions is NOT true?
[UPSC Civil Services Preliminary, 2002]
(A) Boiling point of any water solution is higher than pure water
(B) Addition of solute to a solution increases its water potential
(C) Water vapour pressure of any solution is lower than vapour pressure of pure water
(D) When a solution is separated with a semipermeable membrane, water flow can be stopped by giving pressure to the solution
Answer: (B) Addition of solute to a solution increases its water potential
Explanation: Adding solute decreases the chemical potential and water potential of a solvent rather than increasing them. For typical nonvolatile solutes, boiling-point elevation and vapour-pressure lowering occur. Applying the appropriate osmotic pressure can prevent net solvent flow across a semipermeable membrane.
