Chemistry

201. What is the colour of the precipitate obtained by passing CO₂ gas thro

What is the colour of the precipitate obtained by passing CO₂ gas through lime water ?

[amp_mcq option1=”Green” option2=”Blue” option3=”White” option4=”Brown” correct=”option3″]

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Lime water is an aqueous solution of calcium hydroxide (Ca(OH)₂). When carbon dioxide (CO₂) gas is passed through lime water, it reacts with calcium hydroxide to form calcium carbonate (CaCO₃) and water. Calcium carbonate is insoluble in water and precipitates out as a fine suspension, which appears white.
– Reaction: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
– Calcium carbonate (CaCO₃) is the precipitate formed.
– Calcium carbonate is a white solid.
If CO₂ is passed through lime water for a prolonged period, the white precipitate of calcium carbonate dissolves as it reacts further with CO₂ and water to form soluble calcium bicarbonate (Ca(HCO₃)₂): CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq).

202. What is the number of water molecules present in a Ferrous Sulphate cr

What is the number of water molecules present in a Ferrous Sulphate crystal ?

[amp_mcq option1=”1″ option2=”3″ option3=”5″ option4=”7″ correct=”option4″]

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Ferrous Sulphate commonly exists as a hydrated salt known as green vitriol. The chemical formula for this hydrated form is FeSO₄·7H₂O. This indicates that each formula unit of ferrous sulphate is associated with 7 molecules of water of crystallization.
– Ferrous Sulphate forms hydrated crystals.
– The most common hydrate is the heptahydrate (FeSO₄·7H₂O).
Other less common hydrates of ferrous sulphate also exist, such as the monohydrate (FeSO₄·H₂O) and tetrahydrate (FeSO₄·4H₂O). However, unless otherwise specified, ferrous sulphate crystals usually refer to the heptahydrate.

203. Which one of the following compounds is used in ‘black and white’ phot

Which one of the following compounds is used in ‘black and white’ photography ?

[amp_mcq option1=”AgF” option2=”AgBr” option3=”AgCl” option4=”Ag₂SO₄” correct=”option2″]

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Silver halides are widely used in ‘black and white’ photography due to their sensitivity to light. When light strikes a silver halide crystal, it initiates a chemical change that can later be developed into a visible image. Silver bromide (AgBr) is the most common silver halide used in photographic films, although silver chloride (AgCl) and silver iodide (AgI) are also used, particularly in photographic papers. Among the options provided, AgBr is a standard compound used in black and white photography.
– Silver halides are photosensitive compounds.
– AgBr is widely used in photographic films.
The photographic process involves exposing the silver halide crystals on film or paper to light, creating a latent image. This image is then developed using chemical solutions which convert the exposed silver halide into metallic silver (appearing black). Unexposed silver halide is removed during fixing.

204. The pH value of Milk of Magnesia is approximately

The pH value of Milk of Magnesia is approximately

[amp_mcq option1=”Zero” option2=”7″ option3=”10″ option4=”14″ correct=”option3″]

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Milk of Magnesia is a suspension of magnesium hydroxide (Mg(OH)₂). Magnesium hydroxide is a base. Bases have pH values greater than 7. Milk of Magnesia is a mild base commonly used as an antacid and laxative. Its pH value is approximately 10 to 11. Among the given options, 10 is the closest value.
– Milk of Magnesia is magnesium hydroxide, a basic substance.
– Its pH is alkaline, typically in the range of 10-11.
Antacids work by neutralizing excess stomach acid (hydrochloric acid). The reaction between Mg(OH)₂ and HCl is: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l).

205. Which one of the following was the first mineral acid discovered ?

Which one of the following was the first mineral acid discovered ?

[amp_mcq option1=”Sulphuric acid” option2=”Hydrochloric acid” option3=”Nitric acid” option4=”Phosphoric acid” correct=”option3″]

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Nitric acid was one of the first mineral acids discovered.
Mineral acids are acids derived from inorganic compounds. Historically, strong mineral acids like nitric acid, sulfuric acid, and hydrochloric acid were developed by alchemists and early chemists during the Middle Ages. Nitric acid (aqua fortis) is often credited to the Arab alchemist Jabir ibn Hayyan (Geber) in the 8th century, produced by distilling saltpetre (potassium nitrate) with vitriol (sulfates).
While the precise historical order and attribution can be complex due to the nature of early chemical writings, nitric acid is widely recognized as one of the earliest strong mineral acids whose preparation from mineral sources was documented. Sulphuric acid and hydrochloric acid were also known and developed around similar periods by various alchemists like Al-Razi. Phosphoric acid was discovered later, notably by Hennig Brand in the 17th century.

206. Which one of the following is the best example of desiccant ?

Which one of the following is the best example of desiccant ?

[amp_mcq option1=”Silica gel” option2=”Polystyrene” option3=”Sodium chloride” option4=”Sodium carbonate” correct=”option1″]

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Silica gel is the best example of a desiccant among the given options.
A desiccant is a substance that absorbs or adsorbs water, thus creating or maintaining a state of dryness. Silica gel is widely used for this purpose due to its porous structure, which allows it to adsorb a significant amount of water vapour.
Silica gel is often found in small packets in product packaging (like electronics, shoes, leather goods, etc.) to protect them from moisture damage. While other substances like calcium chloride or activated alumina are also used as desiccants, silica gel is a common and effective example. Polystyrene is a type of plastic. Sodium chloride (common salt) can absorb moisture (hygroscopic) but is not primarily used as a general industrial desiccant in the same way as silica gel. Sodium carbonate can absorb moisture to form hydrates but is also not the primary example of a desiccant.

207. Which one of the following statements regarding Bleaching powder and D

Which one of the following statements regarding Bleaching powder and D.D.T. is correct ?

[amp_mcq option1=”Both are inorganic compounds” option2=”Both are organic compounds” option3=”Both contain chlorine” option4=”Both contain calcium” correct=”option3″]

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The correct answer is (C) Both contain chlorine.
Let’s look at the chemical composition of both substances:
– Bleaching powder is Calcium oxychloride (CaOCl2). It is an inorganic compound containing calcium and chlorine.
– D.D.T. is Dichloro-diphenyl-trichloroethane ((ClC6H4)2CH(CCl3)). It is an organic compound (containing carbon and hydrogen, with chlorine substituents) containing chlorine but no calcium.
Comparing the options:
A) Both are inorganic compounds: False (DDT is organic).
B) Both are organic compounds: False (Bleaching powder is inorganic).
C) Both contain chlorine: True (Bleaching powder has chlorine, and DDT has multiple chlorine atoms).
D) Both contain calcium: False (DDT does not contain calcium).
Bleaching powder is used as a bleaching agent and disinfectant. DDT was a widely used insecticide but is now banned or restricted in many countries due to its environmental persistence and harmful effects.

208. Which one of the following is the correct sequence of change in colour

Which one of the following is the correct sequence of change in colours when a turmeric stain on white clothes is scrubbed by soap and then washed with water ?

[amp_mcq option1=”Yellow – pink – blue” option2=”Yellow – reddish brown – yellow” option3=”Yellow – reddish brown – blue” option4=”Yellow – blue – pink” correct=”option2″]

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The correct answer is (B) Yellow – reddish brown – yellow.
Turmeric is a natural acid-base indicator. Its original colour is yellow. In acidic or neutral solutions, it remains yellow. In basic (alkaline) solutions, it turns reddish-brown. Soap solution is basic. Therefore, when a turmeric stain (yellow) on white clothes is scrubbed with soap, it turns reddish-brown. When the cloth is then washed with plain water, which is usually neutral or slightly acidic due to dissolved CO2, the basic soap is rinsed away, and the turmeric indicator returns to its yellow colour.
Other indicators change colours differently in acidic and basic media. For example, litmus paper is red in acid and blue in base; phenolphthalein is colourless in acid and pink in base. Turmeric’s characteristic change from yellow to reddish-brown is a common demonstration of its indicator property.

209. Number of molecules of water of crystallization in copper sulphate, so

Number of molecules of water of crystallization in copper sulphate, sodium carbonate and Gypsum are

[amp_mcq option1=”5, 10 and 2 respectively” option2=”10, 2 and 5 respectively” option3=”5, 2 and 10 respectively” option4=”2, 5 and 10 respectively” correct=”option1″]

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The correct answer is (A) 5, 10 and 2 respectively.
The number of molecules of water of crystallization are specific to the hydrated forms of these compounds:
– Copper sulphate exists commonly as copper(II) sulphate pentahydrate, CuSO4·5H2O, which has 5 molecules of water.
– Sodium carbonate exists commonly as sodium carbonate decahydrate (washing soda), Na2CO3·10H2O, which has 10 molecules of water.
– Gypsum is calcium sulphate dihydrate, CaSO4·2H2O, which has 2 molecules of water.
Water of crystallization refers to water molecules that are chemically bonded to ions in the crystal structure of a salt. These water molecules are part of the crystalline structure but can be removed by heating the salt.

210. At nearly 70°C, sodium bicarbonate shows the property of gradually dec

At nearly 70°C, sodium bicarbonate shows the property of gradually decomposing, which makes it usable in bakery products. The product of decomposition responsible for this use of sodium bicarbonate is

[amp_mcq option1=”Carbon dioxide” option2=”Hydrogen” option3=”Water vapour” option4=”Oxygen” correct=”option1″]

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The correct answer is (A) Carbon dioxide.
Sodium bicarbonate (baking soda, NaHCO3) is used in bakery products as a leavening agent. When heated, it decomposes according to the reaction: 2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g). The gaseous products, water vapour and especially carbon dioxide, are trapped within the dough, causing it to expand and become light and porous. Carbon dioxide is the primary gas responsible for the leavening action.
The resulting sodium carbonate (Na2CO3) is a residual solid. While water vapour also contributes to the rise, carbon dioxide is the main leavening gas produced from this decomposition at baking temperatures. Hydrogen and oxygen are not significant products of this specific decomposition reaction.
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