Question 1

SOAPS & DETERGENTS — Process understanding.  Question: During soap manufacture (saponification), how does adding common salt (NaCl) help in separating soap from the aqueous mixture?

Accepted Answer

Introduction / Context: In the saponification process, triglycerides react with alkali to form soap (sodium or potassium salts of fatty acids) and glycerol. After the reaction, soap must be separated from the mother liquor. Traditional processes add common salt to “salt out” the soap. Understanding the physical chemistry behind this separation is key to mastering industrial soapmaking. Given Data / Assumptions: Saponification yields a mixture of soap, glycerol, unreacted alkali, and water. Common salt (NaCl) is added to this mixture. We want the mechanism by which separation occurs. Concept / Approach: Adding electrolytes like NaCl reduces the solubility of soap in water (a “salting out” effect). The increased ionic strength favors the precipitation (or separation as curd) of the less soluble soap phase while glycerol and salt remain in the aqueous layer. This is not primarily a density manipulation; rather it is a solubility and activity effect driven by electrolyte interactions. Step-by-Step Solution: Recognize that soap is an amphiphilic salt; its solubility depends on ionic strength.Add NaCl → ionic strength increases.Higher ionic strength reduces soap solubility → soap separates as a distinct phase/precipitate.Glycerol remains in the brine, enabling decantation and recovery. Verification / Alternative check: Classical batch soapmaking demonstrates visible “curd” formation after salting, confirming the salting-out phenomenon common to many colloids and proteins as well. Why Other Options Are Wrong: A, C) Density change is not the operative separation mechanism.D) Increasing solubility would hinder, not aid, separation.E) Evaporation is not the purpose of adding salt. Common Pitfalls: Confusing salting out with flotation or with merely thickening the mixture. The key is reduced solubility due to electrolyte effects. Final Answer: By decreasing the solubility of the soap (salting out)

Question 2

HOUSEHOLD CHEMISTRY — Composition of cleaning agents.  Question: Modern detergents used for cleaning clothes and utensils commonly contain which class of active surfactants?

Accepted Answer

Introduction / Context: Detergents clean by lowering surface tension and forming micelles that trap oily dirt. The key component is the surfactant molecule, with a hydrophobic tail and a hydrophilic headgroup. Recognizing the chemical family used in most household detergents helps explain their effectiveness and compatibility with hard water. Given Data / Assumptions: We focus on the principal active surfactant in typical laundry and dishwashing detergents. Common examples include linear alkylbenzene sulphonates (LAS). Other listed ions (bicarbonate, nitrate, silicate) may appear as builders or additives but are not the core surfactant class. Concept / Approach: Anionic sulphonate surfactants (–SO₃⁻ group) such as sodium dodecylbenzene sulphonate dominate due to good detergency and foaming. They perform well in a range of water conditions and are relatively economical. Bicarbonates and silicates may assist as builders or pH buffers, but they do not provide the primary micelle-forming action. Heavy-metal anions like bismuthates are unrelated to household detergency. Step-by-Step Solution: Identify the functional role needed: surface-active cleaning → surfactant.Match the family widely used in consumer detergents → sulphonate-based anionic surfactants.Eliminate ions used for other roles (buffers/builders) or not used at all in such products. Verification / Alternative check: Product ingredient lists and safety data sheets typically list “linear alkylbenzene sulfonate” or “sodium laureth sulfate,” both sulphonate/sulfate anionic surfactants. Why Other Options Are Wrong: A) Bicarbonates can act as mild alkalis/builders but are not the primary detergents.B) Bismuthates are not used in consumer detergents.D) Nitrates are oxidizing salts/fertilizer components, not detergent actives.E) Silicates can be builders/anti-corrosion agents; not the main surfactants. Common Pitfalls: Confusing builders and fillers with the actual surfactant. The cleaning action fundamentally depends on the sulphonate/sulfate headgroup amphiphiles. Final Answer: sulphonates (anionic surfactants)

Question 3

CEMENT TECHNOLOGY — Role of gypsum.  Question: In Portland cement manufacture, gypsum is interground with clinker primarily to:

Accepted Answer

Introduction / Context: Portland cement sets through hydration reactions of clinker phases (notably tricalcium silicate and tricalcium aluminate). Controlling the early setting time is essential for handling, placing, and finishing concrete. Gypsum plays a specific chemical role in moderating the very fast reaction of aluminate phases. Given Data / Assumptions: Gypsum (calcium sulfate dihydrate) is added in a few percent by mass. Objective: regulate initial set, not long-term strength directly. We consider early hydration chemistry, especially of C₃A. Concept / Approach: Without gypsum, the highly reactive C₃A phase hydrates rapidly, causing “flash set.” Sulfate ions from gypsum combine with C₃A to form ettringite initially, which slows down aluminate hydration, providing workable time. Later, silicate hydration dominates strength development; gypsum’s main effect is on setting regulation rather than tensile strength per se. Step-by-Step Solution: Recognize the problem: C₃A reacts too quickly → flash set.Add gypsum → provides sulfate → forms ettringite with C₃A.Ettringite formation moderates the reaction rate → workable setting time.Therefore, gypsum acts primarily as a set retarder. Verification / Alternative check: Standards limit SO₃ content to balance workability and avoid excessive sulfate, confirming gypsum’s regulatory role. Why Other Options Are Wrong: A) Tensile strength results from hydration products over time, not from gypsum addition directly.C) “Colloidal gel” is not the targeted mechanism here.D) Gypsum does not act as a binder of calcium silicate particles.E) Color effects are incidental; the function is chemical control of set. Common Pitfalls: Confusing setting control with strength gain. Gypsum prevents flash set; it is not a plasticizer or strengthener in itself. Final Answer: decrease the rate of setting of cement (act as a retarder)

Question 4

INDUSTRIAL MATERIALS — Source identification.  Question: Wood is the main raw material for the manufacture of which of the following products at industrial scale?

Accepted Answer

Introduction / Context: Many everyday materials trace back to natural resources. Understanding these value chains helps in environmental awareness and industry basics. Wood, specifically from sustainably managed forests, is a critical feedstock for one major product used globally: paper. Given Data / Assumptions: Wood comprises cellulose, hemicellulose, and lignin. Pulping processes free cellulose fibers for papermaking. Other listed products use different primary feedstocks. Concept / Approach: Pulp and paper mills convert logs or wood chips into pulp via mechanical, chemical (kraft/sulfite), or semi-chemical processes. The cellulose fibers are then processed into a vast range of paper and board grades. Paints rely primarily on resins, pigments, and solvents; inks on pigments/dyes and vehicles; traditional gunpowder uses charcoal (which originates from wood) plus sulfur and potassium nitrate, but the principal bulk raw material for papermaking is unambiguously wood. Step-by-Step Solution: Identify the industry that consumes vast quantities of wood → pulp & paper.Recall that paper is formed from cellulose fibers derived from wood pulp.Eliminate alternatives that depend on other primary inputs (resins, minerals, petrochemicals). Verification / Alternative check: Industry statistics show wood pulp as the dominant raw material in paper manufacture, with increasing recycled fiber content augmenting but not replacing virgin wood in many grades. Why Other Options Are Wrong: A) Paint uses binders and pigments; wood is not its main raw material.C) Ink compositions center on vehicles and pigments, not wood.D) Gunpowder includes charcoal (from wood) but also sulfur and nitrate; the question asks for the main industrial product with wood as the core bulk feedstock, which is paper.E) Glass is made from silica, soda ash, and limestone. Common Pitfalls: Equating “contains some wood derivative” with “main raw material.” Only paper manufacturing directly depends on large-scale wood fiber. Final Answer: paper

Question 5

Which one of the following types of waves are used in a night vision apparatus?

Accepted Answer

Infra-red waves

Question 6

The blue colour of the clear sky is due to

Accepted Answer

A clear cloudless day-time sky is blue because molecules in the air dissipate blue light from the sun more than they dissipate red light. When we look towards the sun at sunset, we see red and orange hues on the grounds because the blue light has been scattered out and away from the line of sight.

Question 7

As you go down into a well, your weight

Accepted Answer

decreases slightly

Question 8

The weight of an object will be minimum when it is placed at ?

Accepted Answer

The weight of an object is minimum when it is placed at the center of the earth because when an object is in the center, it experiences gravitational pull from all directions.

Question 9

What does airbag, used for safety of car driver, contain?

Accepted Answer

Sodium azide

Question 10

In fireworks, the green flame is produced because of

Accepted Answer

Barium

Question 11

Potassium Permanganate is used for purifying drinking water, because

Accepted Answer

it is an oxidising agent

Question 12

Which of the following parts of the sun is easily visible only during a total solar eclipse?

Accepted Answer

corona

Question 13

Which of the following is used in pencils?

Accepted Answer

Graphite

Question 14

A married couple adopted a male child. A few years later, twin boys were born to them. The blood group of the couple is AB positive and O negative. The blood group of the three sons is A positive, B positive, and O positive. The blood group of the a…

Accepted Answer

O positive

Question 15

Which of the following has got more heat capacity?

Accepted Answer

Water

Question 16

The radioactive element most commonly detected in humans is

Accepted Answer

potassium-40

Question 17

The gases used in different types of welding would include

Accepted Answer

oxygen and acetylene

Question 18

When a gas is turned into a liquid, the process is called

Accepted Answer

condensation

Question 19

Which is the largest satellite in the solar system?

Accepted Answer

Ganymede , satellite of Jupiter

Question 20

Which of the following plants is not capable of manufacturing own food?

Accepted Answer

Mushroom

General Science Questions