UPSC NDA-1

31. Which one of the following is NOT true for bleaching powder ?

Which one of the following is NOT true for bleaching powder ?

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It is used as a reducing agent in chemical industries
It is used for bleaching wood pulp in paper factories
It is used for disinfecting drinking water
It is used for bleaching linen in textile industry
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The correct answer is A) It is used as a reducing agent in chemical industries.
Bleaching powder, chemically Calcium Oxychloride (CaOCl₂), is a strong oxidizing agent due to the presence of available chlorine. It acts by releasing nascent oxygen or chlorine, which oxidizes coloured substances or kills microbes.
A) Reducing agents cause reduction and are themselves oxidized. Bleaching powder’s action is primarily oxidation, not reduction. Therefore, it is used as an *oxidizing* agent, not a reducing agent. This statement is NOT true.
B), C), and D) are correct uses of bleaching powder. It is widely used as a bleaching agent for materials like wood pulp and textiles (linen) and as a disinfectant for water supplies and surfaces.
The bleaching and disinfecting properties of bleaching powder are due to its decomposition in the presence of carbon dioxide or acids, releasing hypochlorous acid (HOCl) or chlorine gas, which are powerful oxidizing agents. CaOCl₂ + CO₂ → CaCO₃ + Cl₂ or CaOCl₂ + 2HCl → CaCl₂ + H₂O + Cl₂ or CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂

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32. Which one of the following is the chemical formula of Washing Soda ?

Which one of the following is the chemical formula of Washing Soda ?

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NaHCO₃
Na₂CO₃·10H₂O
Na₂CO₃·5H₂O
NaOH
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The correct answer is B) Na₂CO₃·10H₂O.
Washing soda is the common name for hydrated sodium carbonate. While anhydrous sodium carbonate is Na₂CO₃ (soda ash), the most common hydrated form used as washing soda is sodium carbonate decahydrate, with the chemical formula Na₂CO₃·10H₂O. This compound contains ten molecules of water of crystallization per molecule of sodium carbonate.
Sodium carbonate is used in various applications, including as a water softener (hence “washing soda”), in glass manufacturing, in the production of detergents and other chemicals. Sodium bicarbonate (NaHCO₃) is baking soda, which is different and used as a leavening agent and antacid. Sodium hydroxide (NaOH) is caustic soda, a strong base used in soap making and other industrial processes.

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33. Brine is an aqueous solution of

Brine is an aqueous solution of

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NaCl
NaOH
NaHCO₃
Na₂CO₃
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The correct answer is A) NaCl.
Brine is an aqueous solution of sodium chloride (NaCl), commonly known as table salt. It is a solution of salt dissolved in water, typically with a high concentration of NaCl. Seawater is a naturally occurring brine, although brine can also be produced by dissolving rock salt deposits in water.
Brine is used for various purposes, including food preservation (pickling), industrial processes (like chlor-alkali production), road de-icing, and as a heat transfer fluid. The concentration of NaCl in brine can vary widely, but it is generally significantly higher than that of normal drinking water.

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34. Which one of the following gives the highest amount of hydrogen ions (

Which one of the following gives the highest amount of hydrogen ions (H⁺) ?

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Sodium hydroxide solution
Milk of magnesia
Lemon juice
Gastric juice
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The correct answer is D) Gastric juice.
The amount of hydrogen ions (H⁺) in a solution determines its acidity, measured by pH. A lower pH indicates a higher concentration of H⁺ ions and thus higher acidity.
A) Sodium hydroxide solution (NaOH) is a strong base, having a very high pH and thus a very low concentration of H⁺ ions.
B) Milk of magnesia (Mg(OH)₂) is a weak base, having a high pH but lower than NaOH, still a very low concentration of H⁺ ions compared to acids.
C) Lemon juice contains citric acid and has a pH typically between 2.0 and 2.5, indicating a significant concentration of H⁺ ions.
D) Gastric juice in the stomach contains hydrochloric acid (HCl) and has a very low pH, typically ranging from 1.5 to 3.5. This extremely low pH indicates a very high concentration of H⁺ ions, necessary for digesting proteins and killing ingested microorganisms. Comparing the typical pH values, gastric juice is the most acidic among the options, meaning it has the highest concentration of hydrogen ions.
pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log₁₀[H⁺]. Therefore, a decrease of 1 unit in pH corresponds to a tenfold increase in H⁺ concentration. Gastric juice’s low pH (e.g., 2) means a much higher H⁺ concentration than lemon juice (e.g., 2.5).

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35. Which of the following properties is true for a tooth paste ?

Which of the following properties is true for a tooth paste ?

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It is acidic
It is neutral
It is basic
It is made up of Calcium phosphate, the material of tooth enamel
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The correct answer is C) It is basic.
Toothpaste is typically alkaline (basic) with a pH generally ranging from 7.5 to 10. The basic nature of toothpaste helps to neutralize the acids produced by bacteria in the mouth after eating. These bacterial acids are responsible for eroding tooth enamel and causing cavities. By creating a more alkaline environment, toothpaste helps protect teeth from decay.
Tooth enamel is primarily composed of calcium phosphate in the form of hydroxyapatite. While toothpaste often contains ingredients like calcium carbonate, dicalcium phosphate, or fluoride compounds to help clean, strengthen enamel, or aid remineralization, toothpaste itself is not *made up* of enamel. Its primary function related to pH is to counteract acidity.

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36. The accidental touch of Nettle leaves creates a burning sensation, whi

The accidental touch of Nettle leaves creates a burning sensation, which is due to inject of

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Hydrochloric acid
Methanoic acid
Citric acid
Sulphuric acid
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The correct answer is B) Methanoic acid.
When you touch a nettle plant, the stinging hairs (trichomes) inject irritating chemicals into the skin. One of the primary compounds responsible for the burning sensation and rash is methanoic acid, also known as formic acid (HCOOH). Other irritants present include histamine, acetylcholine, and serotonin, which contribute to the inflammatory response.
Formic acid is a simple carboxylic acid found naturally in the venom of ants and bee stings, as well as in nettle stings. Its chemical formula is HCOOH. While other substances are involved in the nettle sting reaction, methanoic acid is a major contributor to the immediate burning pain.

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37. Which one of the following frequency ranges is sensitive to human ears

Which one of the following frequency ranges is sensitive to human ears ?

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0 – 200 Hz
20 – 20,000 Hz
200 – 20,000 Hz only
2,000 – 20,000 Hz only
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The correct answer is B) 20 – 20,000 Hz.
The typical range of frequencies that the human ear can perceive is from approximately 20 Hertz (Hz) to 20,000 Hertz (or 20 kilohertz, kHz). Sounds with frequencies below 20 Hz are called infrasound, and those above 20,000 Hz are called ultrasound; neither is audible to humans under normal conditions. While the sensitivity varies significantly with age (higher frequencies are often less audible as people get older), the range 20-20,000 Hz represents the general limits of human hearing.
The human ear is most sensitive to frequencies between 1,000 Hz and 4,000 Hz. The ability to hear high frequencies tends to decrease with age, a phenomenon known as presbycusis. Other animals have different hearing ranges; for example, dogs can hear much higher frequencies than humans (up to 45,000-65,000 Hz).

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38. Which one of the following is the natural phenomenon based on which a

Which one of the following is the natural phenomenon based on which a simple periscope works ?

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Reflection of light
Refraction of light
Dispersion of light
Total internal reflection of light
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A simple periscope uses two plane mirrors to change the direction of light rays. The mirrors are typically placed parallel to each other at an angle of 45 degrees to the main tube. Light rays from the object travel to the top mirror, are reflected downwards, travel through the tube, and are then reflected by the bottom mirror towards the observer’s eye. This bouncing of light rays off the mirrors is due to the phenomenon of reflection of light.
– A simple periscope allows observation of objects located above or below the observer’s direct line of sight.
– It utilizes plane mirrors placed at angles to redirect light.
– The change in direction of light upon hitting a mirror surface is called reflection.
More advanced periscopes, like those used in submarines, may also incorporate lenses and prisms (which use reflection or total internal reflection) to provide magnification, a wider field of view, and optical stability, but the fundamental principle in a simple periscope is reflection from plane mirrors.

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39. Which one of the following devices is non-ohmic ?

Which one of the following devices is non-ohmic ?

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Conducting copper coil
Electric heating coil
Semi conductor diode
Rheostat
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An ohmic device obeys Ohm’s Law (V = IR) where resistance (R) is constant regardless of voltage (V) or current (I). A non-ohmic device does not obey Ohm’s Law; its resistance varies with the applied voltage or current.
– Conducting copper coil (A) and Electric heating coil (B) (like Nichrome wire) are generally considered ohmic conductors, although their resistance can change with temperature.
– A Rheostat (D) is a variable resistor, but for any fixed resistance setting, it acts as an ohmic resistor.
– A Semiconductor diode (C) is a device that exhibits a highly non-linear current-voltage characteristic. It conducts current effectively in only one direction (forward bias) and only above a certain threshold voltage. Its resistance changes dramatically depending on the applied voltage and direction. Therefore, it is a classic example of a non-ohmic device.
– Ohmic devices have constant resistance independent of voltage or current.
– Non-ohmic devices have resistance that varies with voltage or current.
– Metal conductors are typically ohmic (assuming constant temperature).
– Semiconductor devices like diodes and transistors are typically non-ohmic.
The V-I graph for an ohmic device is a straight line passing through the origin with a slope equal to the resistance. The V-I graph for a non-ohmic device is curved. Examples of non-ohmic devices include vacuum tubes, gas discharge lamps, and semiconductor components like diodes and transistors.

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40. Whether an object will float or sink in a liquid, depends on

Whether an object will float or sink in a liquid, depends on

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mass of the object only
mass of the object and density of liquid only
difference in the densities of the object and liquid
mass and shape of the object only
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Whether an object floats or sinks in a liquid is determined by comparing the density of the object to the density of the liquid. According to Archimedes’ principle and the concept of buoyancy, an object will sink if its density is greater than the density of the liquid, float if its density is less than the density of the liquid, and be suspended if its density is equal to the density of the liquid. The difference in densities dictates the outcome. Options A, B, and D are insufficient as they do not solely or correctly identify the determining factor. Mass alone doesn’t account for volume, and while shape can influence *how* an object floats (by affecting the volume of liquid displaced before full submersion), the fundamental condition for sinking is based on the object’s average density relative to the liquid’s density.
– Buoyancy force is the upward force exerted by a fluid that opposes the weight of an immersed object.
– An object floats if the buoyant force is equal to or greater than its weight.
– The buoyant force is equal to the weight of the fluid displaced by the object.
– Floating/sinking is determined by comparing the density of the object (or its average density for irregularly shaped objects) to the density of the liquid.
Density is defined as mass per unit volume (ρ = m/V). The weight of an object is W_object = m_object * g = ρ_object * V_object * g. The maximum buoyant force when fully submerged is F_buoyant_max = W_liquid_displaced = ρ_liquid * V_object * g. The object sinks if W_object > F_buoyant_max, which simplifies to ρ_object > ρ_liquid. The object floats if W_object <= F_buoyant_max (partially or fully submerged), which simplifies to ρ_object <= ρ_liquid.

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