Physical and Chemical Changes, solution

Total Questions: 33

11. Phosphatase test is used for the analysis of: [M.P. P.C.S. (Pre) 2021]

Correct Answer: (a) Milk
Solution:Milk Pasteurization: The alkaline phosphatase enzyme is naturally present in raw milk. This enzyme is destroyed when milk is adequately pasteurized. Therefore, a phosphatase test is used to verify if milk has been properly pasteurized or if it has been contaminated with raw milk after pasteurization. Its presence indicates insufficient heat treatment, which means potential pathogenic bacteria might still be present.

While "phosphatase" is a general term for a class of enzymes found in various biological systems (including the human body, where an alkaline phosphatase test can indicate liver or bone conditions), in the context of food safety and specifically as a routine quality control test, the phosphatase test almost exclusively refers to its application in milk to confirm proper pasteurization.

12. Because of impurities the boiling point of a liquid: [M.P.P.C.S. (Pre) 2004]

Correct Answer: (a) Increases
Solution:

This phenomenon is known as boiling point elevation, and it's one of the colligative properties of solutions.

Vapor Pressure: A liquid boils when its vapor pressure equals the surrounding atmospheric pressure.

Effect of Impurities (Non-volatile Solute): When a non-volatile impurity (a solute that doesn't easily evaporate) is added to a liquid (solvent), the solute particles occupy some of the surface area of the liquid. This reduces the number of solvent molecules at the surface that can escape into the gas phase, thereby lowering the vapor pressure of the solution compared to the pure solvent at the same temperature.

Boiling Point Elevation: To make the solution's vapor pressure equal to the atmospheric pressure (and thus boil), a higher temperature is required. This increase in the boiling point is directly proportional to the concentration of the impurity

13. Give a thought to these statements: [U.P. Lower Sub. (Pre) 1998, I.A.S. (Pre) 1995]

Statement (A): Temperature below 0°C is achieved by the mixture of salt and ice.
Reason (R): Salt increases the freezing point of ice.

In the context of the above statements, which one of the following is correct:

Correct Answer: (c) (A) is correct but (R) is wrong
Solution:When some impurities are mixed with any liquid, its boiling point increases. On the other hand, the freezing point generally gets lowered below 0°C when ice is mixed with salts (NaCl or KNO₃, etc.). Its mixture is called freezing mixture. That is why statement (A) is correct, while reason (R) is wrong.

14. The amount of solute present per unit volume or per unit mass of the solution/solvent is known as ______ in Chemistry. [69th B.P.S.C. (Pre) 2023]

Correct Answer: (d) Concentration of solution
Solution:The amount of solute present per unit volume or unit mass of the solution/solvent is called 'Concentration of Solution'. The concentration of a solution is the measure of how the solute is present in a fixed amount (mass or volume) of the solution or the amount of solute dissolved in a given mass or volume of solvent.

15. Consider the following statements: [67th B.P.S.C. (Pre) (Re. Exam) 2022]

  1. Addition of salt to pure water increases the boiling point of water and decreases the freezing point of water.
  2. When methyl alcohol is added to water, the boiling point of water decreases.

Which of the above statements is/are correct?

Correct Answer: (d) Both 1 and 2
Solution:When salt is dissolved in pure water, it increases the boiling point and decreases the freezing point of water. While when methyl alcohol is added to water, it decreases the boiling point of water. This happens due to hydrogen bonding.

16. Milk is an example of: [42nd B.P.S.C. (Pre) 1997, U.P.P.C.S. (Pre) 1996]

Correct Answer: (c) An emulsion
Solution:Emulsion: An emulsion is a type of colloid where tiny droplets of one liquid are dispersed in another liquid, and the two liquids are normally immiscible (don't mix). In the case of milk, tiny globules of fat (a liquid) are dispersed in water (another liquid). Proteins like casein act as emulsifiers, preventing the fat droplets from coalescing and separating.

17. Which of these is not a colloid: [U.P.P.C.S. (Pre) 2002]

Correct Answer: (d) Honey
Solution:Among the given options, honey is not a colloid. At room temperature, honey is a supercooled liquid, in which glucose precipitates into solid granules. This forms a semisolid solution of precipitated glucose crystals in a solution of fructose and other ingredients. Milk, blood and icecream are colloids.

18. Which type of colloidal system is expressed in the fog: [I.A.S. (Pre) 1993]

Correct Answer: (c) Liquid in a gas
Solution:

Fog is essentially a cloud that is very close to the ground. It forms when water vapor in the air condenses into tiny liquid water droplets.

The tiny liquid water droplets are the individual particles that are spread out. So, the liquid is the dispersed phase.

These liquid droplets are suspended and spread throughout the air. The air (which is a mixture of gases like nitrogen, oxygen, etc.) is the continuous medium. So, the gas is the dispersion medium.

Therefore, when liquid droplets dispersed in a gas, the classification of the colloidal system is "liquid in a gas".

19. Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R): [U.P. P.C.S. (Pre) 2022]

Assertion (A): A solution of table salt in a glass of water is homogeneous.
Reason (R): A solution having different compositions throughout is homogeneous.

Correct Answer: (a) (A) is true but (R) is false
Solution:Homogeneous solutions have the same composition and properties throughout the solution. None of the components involved in the homogeneous solution can be detected or observed separately. A solution of table salt in a glass of water is an example of a homogeneous solution.

A solution having different compositions and properties throughout is a heterogeneous mixture. Individual components of a heterogeneous mixture can be discreetly observed. Hence, option (a) is the correct answer.

20. If an unsaturated sugar solution is prepared by mixing 100 grams of sugar with half litre of water, which of the following physical quantities will not change? [U.P.P.C.S. (Pre) 2017]

Correct Answer: (d) Volume
Solution:If an unsaturated sugar solution is prepared by mixing 100 grams of sugar with half liter of water then volume of the solution remains constant. this is because the sugar particles occupy the empty space between the molecules of liquids, and don't occupy any new space . That's why volume of the given solution remain constant.