Chemistry

281. White Phosphorus glows in the dark due to:

White Phosphorus glows in the dark due to:

[amp_mcq option1=”amorphous character” option2=”slow oxidation” option3=”high ignition temperature” option4=”good conducting property of electricity” correct=”option2″]

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The correct answer is B) slow oxidation.
White phosphorus exhibits phosphorescence, which is a form of chemiluminescence resulting from its slow oxidation when exposed to air. This process releases energy in the form of light, causing it to glow faintly in the dark. This is often referred to as ‘cold light’.
Amorphous character, high ignition temperature (white phosphorus has a very low ignition temperature, around 30°C), and good electrical conductivity are not the reasons for its glow. Its low ignition temperature makes it highly reactive and dangerous.

282. The main constituent of Vinegar is:

The main constituent of Vinegar is:

[amp_mcq option1=”Acetic acid” option2=”Ascorbic acid” option3=”Citric acid” option4=”Tartaric acid” correct=”option1″]

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The correct answer is A) Acetic acid.
Vinegar is primarily an aqueous solution of acetic acid. Typically, it contains 5-20% acetic acid by volume. Acetic acid is responsible for vinegar’s characteristic sour taste and pungent smell.
Other options are incorrect. Ascorbic acid is Vitamin C. Citric acid is found in citrus fruits. Tartaric acid is found in grapes and tamarinds. While small amounts of other acids may be present depending on the production method, acetic acid is the main constituent.

283. Boric acid is an acid because its molecule:

Boric acid is an acid because its molecule:

[amp_mcq option1=”accepts OH– from water” option2=”combines with proton from water molecule” option3=”contains replaceable H+ ion” option4=”gives up a proton” correct=”option1″]

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Boric acid (H₃BO₃) is a weak monoprotic Lewis acid. It does not directly donate a proton (H⁺) from its own molecule in aqueous solution. Instead, it accepts a hydroxide ion (OH⁻) from a water molecule, releasing a proton from the water molecule in the process. The reaction is: H₃BO₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺. Therefore, it acts as an acid by accepting an OH⁻ ion from water.
Boric acid’s acidity is explained by its behavior as a Lewis acid, accepting an electron pair from the hydroxide ion of water.
Although boric acid contains three hydrogen atoms, they are not easily ionizable as protons in the way that typical Brønsted-Lowry acids like HCl or H₂SO₄ are. Its acidic nature arises from its ability to form a coordinate covalent bond with the hydroxide ion.

284. Ammonia (NH 3 ) obtained from different sources always has same propor

Ammonia (NH3) obtained from different sources always has same proportion of Nitrogen and Hydrogen. It proves the validity of law of:

[amp_mcq option1=”Reciprocal proportion” option2=”Constant proportion” option3=”Multiple proportions” option4=”None of the above” correct=”option2″]

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The statement “Ammonia (NH3) obtained from different sources always has same proportion of Nitrogen and Hydrogen” is a direct application of the Law of Constant Proportions (also known as the Law of Definite Proportions). This law states that a given chemical compound always contains its component elements in fixed proportion by mass, regardless of the source or method of preparation.
The Law of Constant Proportions explains why the elemental composition of a specific compound is always the same.
The Law of Reciprocal Proportion relates the masses of two elements that combine separately with a fixed mass of a third element. The Law of Multiple Proportions states that if two elements can combine to form more than one compound, then the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers. The given statement specifically addresses the fixed composition of a single compound, which is the domain of the Law of Constant Proportions.

285. Addition of gypsum to cement:

Addition of gypsum to cement:

[amp_mcq option1=”reduces setting time of cement” option2=”produces very light colour cement” option3=”increases setting time of cement” option4=”shining surface is formed” correct=”option3″]

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The correct answer is C. Addition of gypsum to cement increases the setting time of cement.
– Cement clinker, when mixed with water, undergoes rapid hydration reactions, especially involving tricalcium aluminate (C₃A), which can lead to flash setting (very rapid hardening).
– Gypsum (calcium sulfate dihydrate) is added to the cement clinker during grinding to act as a retarder.
– Gypsum reacts with C₃A to form ettringite, which precipitates on the surface of the C₃A particles, forming a barrier that slows down the rapid hydration of C₃A.
– This allows sufficient time for the cement paste or concrete to be mixed, transported, placed, and finished before it begins to harden.
The amount of gypsum added is carefully controlled (typically 2-5% by weight of clinker) as too little or too much can adversely affect the setting properties and strength development of the cement.

286. Consider the following reaction: CH₄(g) + H₂O(g) 1270 K CO(g) + 3

Consider the following reaction:
CH₄(g) + H₂O(g)

1270 K

CO(g) + 3H₂(g)
In the reaction given above, the mixture of CO and H₂, is:

[amp_mcq option1=”natural gas” option2=”water gas” option3=”producer gas” option4=”industrial gas” correct=”option2″]

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The correct answer is B. The mixture of CO and H₂ produced by the steam reforming of methane is known as water gas.
– The reaction CH₄(g) + H₂O(g) → CO(g) + 3H₂(g) is called steam reforming of methane.
– The resulting mixture of carbon monoxide (CO) and hydrogen (H₂) is commonly referred to as synthesis gas or, specifically when produced via steam reactions with carbonaceous materials or hydrocarbons, water gas.
– Producer gas is formed by reacting carbonaceous fuel with air and steam, containing CO, H₂, N₂, and CO₂.
– Natural gas is primarily methane.
Water gas and synthesis gas are important industrial feedstocks used in the production of various chemicals, including methanol, ammonia, and synthetic fuels (via the Fischer-Tropsch process).

287. Which one among the following oxides has the highest melting point ?

Which one among the following oxides has the highest melting point ?

[amp_mcq option1=”Na₂O” option2=”MgO” option3=”Fe₂O₃” option4=”CuO” correct=”option2″]

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Among the given options, Magnesium oxide (MgO) has the highest melting point.
Oxides of highly electropositive metals (like those in Group 1 and 2) and transition metals are generally ionic compounds with high melting points due to strong electrostatic forces in the crystal lattice. MgO is a Group 2 metal oxide with high lattice energy primarily due to the +2 and -2 charges of Mg²⁺ and O²⁻ ions and their relatively small ionic radii, resulting in a very high melting point (~2852 °C).
Comparing the options: Na₂O (~1132 °C), MgO (~2852 °C), Fe₂O₃ (~1565 °C, decomposes), CuO (~1326 °C, decomposes). MgO’s melting point is significantly higher than the others listed, making it a refractory material.

288. Which among the following is the correct composition of Borax ?

Which among the following is the correct composition of Borax ?

[amp_mcq option1=”Sodium, Boron, Magnesium and Hydrogen” option2=”Sodium, Boron, Oxygen and Hydrogen” option3=”Potassium, Boron, Oxygen and Hydrogen” option4=”Sodium, Boron, Nitrogen and Hydrogen” correct=”option2″]

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The correct composition of Borax includes Sodium, Boron, Oxygen, and Hydrogen.
Borax is commonly found as the mineral sodium tetraborate decahydrate, with the chemical formula Na₂B₄O₇·10H₂O. This formula clearly shows the presence of Sodium (Na), Boron (B), Oxygen (O), and Hydrogen (H).
The B₄O₇²⁻ anion in borax is actually a complex polyborate ion, often more accurately represented as [B₄O₅(OH)₄]²⁻ in its hydrated forms. The decahydrate contains 10 water molecules of crystallization.

289. Soap with water forms :

Soap with water forms :

[amp_mcq option1=”Metallotropic liquid crystal” option2=”Thermotropic liquid crystal” option3=”Homogeneous solution” option4=”Lyotropic liquid crystal” correct=”option4″]

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Soap with water can form lyotropic liquid crystals, especially at higher concentrations.
Soap molecules are amphiphilic, having a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. In water, they aggregate into structures like micelles, bilayers, or vesicles depending on concentration and temperature. These ordered structures often exhibit liquid crystalline phases, which are classified as lyotropic because their formation depends on the concentration of the solute (soap) in the solvent (water).
A homogeneous solution is formed at very low soap concentrations where molecules are dispersed individually. However, the characteristic behavior of soap forming micelles and other structured phases at higher concentrations leads to lyotropic liquid crystal phases. Thermotropic liquid crystals are formed by pure substances or mixtures that show liquid crystalline phases at different temperatures without a solvent. Metallotropic liquid crystals are formed from molten salts.

290. Which one among the following is known as Milk of Magnesia ?

Which one among the following is known as Milk of Magnesia ?

[amp_mcq option1=”Magnesium bicarbonate” option2=”Magnesium carbonate” option3=”Magnesium sulphate” option4=”Magnesium hydroxide” correct=”option4″]

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Milk of Magnesia is the common name for an aqueous suspension of magnesium hydroxide.
Magnesium hydroxide (Mg(OH)₂) is a slightly soluble base. When suspended in water, it forms a milky white liquid known as Milk of Magnesia. It is commonly used as an antacid to neutralize stomach acid and as a laxative.
Magnesium carbonate is also used as an antacid but is not called Milk of Magnesia. Magnesium sulphate (Epsom salt) is used as a laxative and for baths. Magnesium bicarbonate exists only in aqueous solution.
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