Chemistry MCQ Asked in Previous Year’s WBCS Preliminary Exams [130 MCQs]

Answer – A. Na+ and Cl–
Explanation-Sodium ions (Na+) and chloride ions (Cl–) are the most abundant ions in seawater, including the Bay of Bengal. When saltwater evaporates, the water molecules leave behind the dissolved salts, which primarily consist of sodium and chloride ions. As a result, the concentration of Na+ and Cl– ions increases, making them the dominant ions in the Bay of Bengal water.

Answer – D. Carbonyl dichloride.
Explanation-Phosgene is a common name for carbonyl dichloride (COCl₂). It is a colorless gas that is toxic and highly reactive. Phosgene was historically used as a chemical weapon during World War I. It is also employed in various chemical processes and serves as a precursor in the production of plastics and other organic compounds.

Answer – C. Water + oil
Explanation-A separating funnel is a laboratory apparatus used to separate immiscible liquids or liquids with different densities. Water and oil are immiscible, meaning they do not mix and form separate layers. By using a separating funnel, it is possible to separate the water and oil layers by allowing them to settle into distinct phases. The denser liquid (in this case, water) will settle at the bottom, while the less dense liquid (oil) will accumulate at the top, allowing them to be separated.

Answer –C. Flux
Explanation- In metallurgy, a flux is a substance used to facilitate the removal of impurities, such as rock material or slag, from metals during the smelting or refining process. Fluxes are added to the molten metal to create a chemical reaction that aids in separating impurities and promoting the desired chemical reactions for the purification or extraction of the metal.

Answer –D. for reducing the velocity of the neutron
Explanation-Graphite is used in a nuclear reactor as a moderator to reduce the velocity of the neutron.
A moderator is a material that slows down neutrons. Slow neutrons are more likely to cause fission in uranium-235 atoms, which is the fuel used in nuclear reactors.

Graphite is a good moderator because it is a light element and has a lot of free space. The free space allows the neutrons to travel through the graphite without colliding with other atoms, which would slow them down.

The other options are incorrect.
A. as a lubricant is incorrect because graphite is not a lubricant. Lubricants are substances that reduce friction between moving parts. Graphite is a solid, and it does not reduce friction.

B. as a fuel is incorrect because graphite is not a fuel. Fuels are substances that can undergo combustion to produce heat. Graphite does not undergo combustion.

C. for lining inside the reactor as an insulator is incorrect because graphite is not an insulator. Insulators are materials that do not allow heat to flow through them. Graphite is a conductor, and it allows heat to flow through it.

Answer –D. A supercooled solution
Explanation-Glass is best described as a supercooled solution. It is not a gel, alloy, or eutectic mixture. Glass is made by cooling a molten mixture of various minerals, typically silica, soda ash, and lime, very quickly. This rapid cooling prevents the atoms from arranging themselves into a crystalline structure, resulting in an amorphous solid with a random atomic arrangement.

Answer – C. Nodules of a variety of minerals found on the sea beds.
Explanation-Potato ores are nodules of various minerals found on the sea beds. These nodules resemble potatoes in shape, hence the name. They contain valuable minerals such as manganese, iron, nickel, and cobalt. Potato ores are of interest for their potential as a future source of these minerals.

Answer – C. silica
Explanation- In the open hearth process, a furnace is used to convert cast iron into steel. If the cast iron used has no phosphorus but has manganese as an impurity, the furnace lining should be made of silica (silicon dioxide). Silica has a high melting point and is resistant to the corrosive effects of manganese oxide. The silica lining helps protect the furnace from the detrimental effects of manganese during the steelmaking process.

Answer – C. either protons or neutrons.
Explanation-Nuclei of radioactive elements can emit various types of particles during radioactive decay. The emitted particles can include alpha particles (helium nuclei), beta particles (electrons or positrons), gamma rays (high-energy photons), and sometimes protons or neutrons. The specific type of emitted particles depends on the radioactive isotope undergoing decay and the decay process involved.

Answer –A. neutrons
Explanation- Particles that can be added to the nucleus of an atom without changing its chemical properties are neutrons. Neutrons are electrically neutral particles found in the nucleus of an atom. They do not have a charge, so adding or removing neutrons does not alter the atom’s chemical properties. Protons and electrons, on the other hand, contribute to the atom’s charge and can participate in chemical reactions.

Answer –B. -3.4eV
Explanation- In the Bohr model of the atom, the energy levels of electrons are quantized. The energy of an electron in the nth Bohr orbit is given by the formula E = -13.6/n^2 eV, where n represents the principal quantum number. For the first Bohr orbit (n = 1), the energy is -13.6 eV. For the second Bohr orbit (n = 2), the energy can be calculated as E = -13.6/2^2 = -13.6/4 = -3.4 eV. Therefore, the energy of the electron in the second Bohr orbit is -3.4 eV.

Answer –D. acetic acid
Explanation- Vinegar is a dilute solution of acetic acid (CH3COOH). It is produced by the fermentation of ethanol (found in alcoholic beverages) by acetic acid bacteria. Acetic acid gives vinegar its sour taste and distinctive odor.

Answer – A. sodium bicarbonate and caustic soda.
Explanation- Sodium bicarbonate (NaHCO₃) and caustic soda (sodium hydroxide, NaOH) cannot exist together in a solution because they react with each other to form water and sodium carbonate (Na2CO₃).

This reaction is known as a neutralization reaction:
NaHCO₃ + NaOH → H₂O + Na₂CO₃

The reaction between sodium bicarbonate and caustic soda results in the formation of water (H2O) and sodium carbonate (Na2CO3), indicating that they cannot coexist in a stable solution.

Answer – A. Rubber, and Sulphur are heated together.
Explanation-Vulcanization is a chemical process used to enhance the properties of rubber by heating it with sulfur or other curatives. The rubber is mixed with sulfur and then heated to a high temperature. This process cross-links the polymer chains in the rubber, making it more durable, resistant to abrasion, and less sticky. The heating of rubber and sulfur together promotes the formation of chemical bonds between the rubber molecules, resulting in the transformation of the rubber into a more stable and useful material.

Answer – B. Chemical erosion.
Explanation- Corrosion is a chemical process that occurs when a material, typically a metal, reacts with its environment and undergoes degradation. It involves the oxidation of the metal, resulting in the formation of various corrosion products such as rust. Unlike mechanical erosion caused by physical forces or wear, corrosion is driven by chemical reactions between the metal and substances in its surrounding environment.

Answer –C. Decreases
Explanation-In general, the density of a liquid decreases as it is heated. When a liquid is heated, its molecules gain energy and become more agitated, leading to increased thermal motion. As a result, the average distance between the molecules increases, causing the volume of the liquid to expand. Since density is defined as mass divided by volume, if the volume increases while the mass remains constant, the density of the liquid decreases.

Answer – A. Fusion.
Explanation-Solar energy is produced by the process of nuclear fusion in the Sun’s core. Nuclear fusion occurs when light atomic nuclei, such as hydrogen nuclei, combine to form heavier nuclei, releasing a tremendous amount of energy in the process. In the Sun, hydrogen nuclei fuse to form helium, resulting in the release of large amounts of energy in the form of light and heat.

Answer –C. Argon and Neon
Explanation-Noble gases are a group of elements in the periodic table that have full electron shells, making them stable and unreactive. The noble gases include helium, neon, argon, krypton, xenon, and radon. Among the given options, only argon and neon are noble gases.

Answer –B. Hydrogen peroxide
Explanation- In underground railways, air is often purified by using hydrogen peroxide (H₂O₂). Hydrogen peroxide can help in the removal of pollutants and contaminants from the air, acting as an oxidizing agent. It can react with various organic compounds and airborne particles, breaking them down into simpler, less harmful substances.

Answer –A. Priestley
Explanation-In 1775, the scientist Joseph Priestley conducted experiments that led to the discovery of oxygen gas. He demonstrated that a gas produced during the process of photosynthesis (later named oxygen) supported combustion and sustained the respiration of animals. This experiment played a crucial role in understanding the composition of air and the discovery of oxygen as a distinct gas.

Answer – D. Carbon dioxide.
Explanation-Dry ice is a solid form of carbon dioxide (CO_{2). It is called “dry ice” because it sublimates (turns directly from a solid to a gas) at temperatures below -78.5°C (-109.3°F) without passing through a liquid phase. Dry ice is commonly used as a cooling agent and is often used to keep things frozen during transport or for special effects due to its ability to rapidly change from a solid to a gas.}

Answer – A. Carbon monoxide and Hydrogen.
Explanation-Water gas is a mixture of carbon monoxide (CO) and hydrogen (H₂). It is produced by passing steam over red-hot carbon (such as coal or coke) in a process called the water gas shift reaction. The resulting gas mixture, water gas, is used as a fuel or as a raw material in the synthesis of various chemicals.

Answer – A. Ethylene
Explanation-Ethylene is a gaseous plant hormone that plays a crucial role in the ripening of fruits. It is naturally produced by fruits and triggers various physiological changes during ripening, such as softening, color changes, and aroma development. Ethylene can be used artificially to accelerate the ripening process of fruits or to induce uniform ripening in large-scale fruit storage and transportation.