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

Answer – C. Sublimation
Explanation-Progression, regression, and repression are all psychological terms related to human behavior or mental processes. Sublimation, on the other hand, refers to a physical process where a substance transitions directly from a solid to a gaseous state without passing through the liquid state.

Answer – D. Fumeric acid.
Explanation-Succinate dehydrogenase is an enzyme that catalyzes the oxidation of succinate to fumarate. This reaction is part of the citric acid cycle, which is a series of reactions that produce energy in cells. In the reaction, succinate is oxidized by the loss of two electrons and two hydrogen atoms. The electrons are transferred to flavin adenine dinucleotide (FAD), which is reduced to FADH2. The hydrogen atoms are transferred to ubiquinone (UQ), which is reduced to ubiquinol (UQH2).

The reaction catalyzed by succinate dehydrogenase is as follows:
succinate + FAD + H₂O → fumarate + FADH₂ + H+

Answer –C. Lead
Explanation-Uranium is a radioactive element with an atomic number of 92. It decays through a series of steps, each of which produces a different element. The final step in the decay chain of uranium-238 is the production of lead-206.

The radioactive decay of uranium produces a variety of other elements, including thorium, radium, and radon. However, the final product of the decay chain is always lead.

Here is a table of the elements produced by the radioactive decay of uranium:

Answer – A. J. Chadwick
Explanation-Sir James Chadwick discovered the neutron in 1932. He was a British physicist who worked at the Cavendish Laboratory in Cambridge, England. Chadwick bombarded beryllium with alpha particles from a radioactive source. He found that the beryllium emitted radiation that was not deflected by an electric field. This suggested that the radiation was made up of neutral particles, which Chadwick called neutrons.

Chadwick’s discovery of the neutron was a major breakthrough in physics. It led to a better understanding of the structure of the atom and the nature of nuclear reactions. Chadwick was awarded the Nobel Prize in Physics in 1935 for his discovery.

Here is a brief explanation of the other scientists you mentioned:
.J. Thomson was an English physicist who discovered the electron in 1897. He also conducted research on cathode rays and X-rays.

Ernest Rutherford was a New Zealand-born physicist who is known for his work on radioactivity and the structure of the atom. He was awarded the Nobel Prize in Physics in 1908.

Neil Bohr was a Danish physicist who developed a model of the atom that explained the structure of spectral lines. He was awarded the Nobel Prize in Physics in 1922.

Answer – B. ZnSO₄,7H₂O
Explanation-White vitriol refers to zinc sulfate heptahydrate, which is represented by the chemical formula ZnSO₄,7H₂O. It is a white crystalline compound commonly used in various industrial and laboratory applications.

Answer –C. Fuming HNO3
Explanation-Iron is not dissolved in fuming HNO₃ (nitric acid). Fuming HNO3 is a highly concentrated and strong oxidizing agent that can react with various substances, but it does not readily dissolve iron.

Answer – B. -1/2
Explanation-The oxidation number of oxygen in superoxide is –1/2. This is because the superoxide ion, O2–, has a total charge of –1. The two oxygen atoms share two electrons, so each oxygen atom has a charge of –1/2.

Here is a breakdown of the oxidation numbers in the superoxide ion:
Oxygen atom 1: –1/2
Oxygen atom 2: –1/2
Total: –1

Answer – D. High coefficient of expansion
Explanation-The main property of mercury that makes it suitable for use in a clinical thermometer is its high coefficient of expansion. Mercury expands and contracts significantly with temperature changes, allowing for accurate and sensitive temperature measurements in the thermometer.

Answer –A. Emulsion
Explanation-Milk is an emulsion of fat globules in water. Emulsions are mixtures of two or more liquids that are normally immiscible (unable to mix together). In milk, the fat globules are stabilized by proteins in the milk. The proteins form a layer around the fat globules, preventing them from clumping together.

Here is a table of the different types of colloids and their examples:

Answer –A. H₂O₂ (hydrogen peroxide)
Explanation-Hydrogen peroxide can act as only a reducing agent. It readily donates its oxygen atom to other substances, undergoing reduction itself in the process.

Answer –A. Lyman series
Explanation-The spectral lines of the hydrogen atom corresponding to emissions in the ultraviolet region are part of the Lyman series. The Lyman series consists of transitions where an electron in a higher energy level jumps to the first energy level (n=1) and emits ultraviolet radiation.

Answer –B. NO (nitric oxide)
Explanation-Nitric oxide (NO) is paramagnetic. It has an unpaired electron, which makes it attracted to an external magnetic field and gives it paramagnetic properties.

Answer – C. Fractional distillation
Explanation-Ethyl alcohol (ethanol) is completely miscible with water, meaning they mix together in all proportions. To separate ethyl alcohol from a mixture with water, fractional distillation can be used. Ethanol has a lower boiling point than water, allowing it to evaporate and be collected separately during the distillation process.

Answer –B. CH_{3OH (methanol)
Explanation-Hydrogen peroxide (H2O2) has the same number of electrons as methanol (CH3OH). Both molecules have a total of 10 electrons. HCHO (formaldehyde) has 12 electrons, CH2 = CH2 (ethylene) has 10 electrons, and CH2CHO (acetaldehyde) has 14 electrons, which are different from the number of electrons in hydrogen peroxide.}

Answer – D. Silicon
Explanation-Silicon is not a metal; it is a metalloid or semimetal. Platinum, mercury, and nickel are examples of metals.

Answer –A. Priestley
Explanation-Oxygen was discovered by Joseph Priestley in the 18th century. He is credited with isolating oxygen gas and demonstrating its properties.

Answer –A. Cobalt chloride and water
Explanation-Cobalt chloride is a common ingredient in sympathetic ink. When cobalt chloride is exposed to heat, it turns from colorless to blue. This makes it a good choice for invisible ink, as the message can be revealed by heating the paper.

Here is a list of other ingredients that can be used to make sympathetic ink:
Copper chloride: Turns from colorless to blue when exposed to heat or light.
Iron chloride: Turns from colorless to brown when exposed to heat or light.
Potassium iodide: Turns from colorless to yellow when exposed to heat or light.
Sodium thiosulfate: Turns from colorless to white when exposed to heat or light.

The type of sympathetic ink you use will depend on the method you want to use to reveal the message. For example, if you want to reveal the message by heating the paper, you would use a cobalt chloride ink. If you want to reveal the message by exposing the paper to light, you would use a copper chloride or potassium iodide ink.

Answer –B. Effusion
Explanation- Effusion is the process by which a gas passes through a small opening into an area of lower pressure. The rate of effusion is inversely proportional to the square root of the molar mass of the gas. This means that a gas with a lower molar mass will effuse more quickly than a gas with a higher molar mass.

L.P.G. is a mixture of propane and butane, which have molar masses of 44.1 and 58.1 grams per mole, respectively. This means that propane will effuse more quickly than butane.

The pressure inside an L.P.G. cylinder is much higher than the pressure outside the cylinder. This pressure difference drives the effusion of the gas out of the cylinder.

The process of effusion is used in a variety of applications, including the separation of gases, the purification of gases, and the measurement of gas flow rates.

Answer –B. Butane and Propane
Explanation- L.P.G. (liquefied petroleum gas) cylinders contain a mixture of butane and propane gases. These gases are commonly used as fuel for cooking, heating, and other purposes. Isobutane may also be present in small amounts in the mixture.

Answer – D. Helium ion
Explanation- When a helium atom loses an electron, it becomes a helium ion. An ion is an atom or molecule that has gained or lost electrons, resulting in a net positive or negative charge. In the case of helium, which normally has two electrons, when it loses one electron, it becomes a helium ion with a net positive charge. The helium ion is represented as He+.

Answer – D. Chemical energy
Explanation-A storage cell, also known as a battery, stores chemical energy. Batteries are electrochemical devices that convert stored chemical energy into electrical energy. They consist of one or more cells, each containing chemical compounds that can undergo reversible chemical reactions to produce and store electrical energy. When the battery is connected to an external circuit, these chemical reactions occur, and the stored chemical energy is converted into electrical energy that can be used to power various devices.

Answer –D. there is sodium in the solar atmosphere.
Explanation- Fraunhofer lines are dark lines or gaps observed in the spectrum of the Sun’s electromagnetic radiation. These lines indicate the absorption of specific wavelengths of light by various elements present in the Sun’s atmosphere. The sodium D line, specifically, corresponds to the absorption of light by sodium atoms.

The presence of Fraunhofer lines in the sodium D wavelength in the solar spectrum proves that there is sodium in the solar atmosphere. The dark lines indicate that sodium atoms in the Sun’s outer layers absorb certain wavelengths of light, leading to the observed gaps in the spectrum. This phenomenon provides evidence of the presence of sodium in the Sun’s atmosphere.

Answer –B. Nuclear fusion
Explanation- A hydrogen bomb, also known as a thermonuclear bomb or H-bomb, operates on the principle of nuclear fusion. Nuclear fusion involves the merging or combining of atomic nuclei to release a tremendous amount of energy. In a hydrogen bomb, the fusion reaction occurs when isotopes of hydrogen, specifically deuterium and tritium, combine under extreme temperature and pressure conditions to form helium, releasing a vast amount of energy in the process.

Answer – B. 6 lit.
Explanation-Density is defined as mass per unit volume. In this case, the density of air is given as 1.30 g/lit, which means that for every liter of air, the mass is 1.30 grams. To find the volume occupied by 7.8 grams of air, we can set up a proportion:

Density = Mass / Volume
1.30 g/lit = 7.8 g / Volume
Volume = 7.8 g / 1.30 g/lit
Volume = 6 lit

Answer –A. Wrought iron
Explanation-Wrought iron is considered the purest form of iron. It is a form of iron that contains very low amounts of carbon and other impurities. Wrought iron is relatively soft, malleable, and ductile, making it suitable for forging and shaping into various structures. It has a fibrous structure due to the presence of slag inclusions, which gives it its characteristic grainy appearance.

Answer –A. Nuclear fusion reaction
Explanation- Solar energy is primarily derived from nuclear fusion reactions occurring in the Sun’s core. The Sun’s core is a hot and dense region where hydrogen nuclei (protons) fuse together to form helium nuclei. This process, known as nuclear fusion, releases an enormous amount of energy in the form of light and heat. The energy radiated by the Sun as sunlight is what we harness as solar energy here on Earth.

Answer –C. Curie
Explanation- The unit of radioactivity is the Curie (Ci), named after Marie Curie, a renowned physicist and chemist. The Curie is a unit of measurement used to quantify the activity or rate of radioactive decay of a substance. It is defined as the amount of radioactive material in which there are 3.7 x 10¹⁰ radioactive disintegrations (decays) per second.

Answer – C. Graphite
Explanation-An element is a pure substance that cannot be broken down into a simpler substance by chemical means. Table salt, water, and glass are all compounds, which are substances that are made up of two or more elements. Table salt is made up of sodium and chlorine. Water is made up of hydrogen and oxygen. Glass is made up of silicon, oxygen, and other elements. Graphite is an allotrope of carbon, which means that it is a different form of carbon. Other allotropes of carbon include diamond and charcoal.

Answer – B. Will decrease
Explanation-Relative humidity is a measure of the amount of moisture present in the air compared to the maximum amount it can hold at a particular temperature. As the temperature increases, the air’s capacity to hold moisture also increases. If the temperature inside a room increases from 30°C to 40°C, and no additional moisture is added or removed from the air, the relative humidity will decrease. This is because the same amount of moisture will now represent a smaller percentage of the increased moisture-holding capacity of the air.

Answer –C. They carry an electrical charge
Explanation-Colloidal particles are small particles of one substance dispersed throughout another substance. The particles are so small that they do not settle out of the dispersion medium, even under the influence of gravity.

The particles in a colloidal solution carry an electrical charge. This charge prevents the particles from clumping together and settling out. The charge on the particles is balanced by the charge on the dispersion medium. This balance of charge prevents the particles from attracting each other and forming a precipitate.

Answer –B. Some gas molecules to ionize
Explanation- Lightning discharges induce rain formation by ionizing some gas molecules in the air. These ionized molecules then act as nuclei for water droplets to form around.

When lightning strikes, it creates a very strong electrical field. This field ionizes some of the gas molecules in the air. Ionized molecules are molecules that have lost or gained electrons. When a molecule becomes ionized, it becomes more reactive.

Water vapor molecules are very stable, but they can become reactive when they interact with ionized molecules. When a water vapor molecule interacts with an ionized molecule, it can lose an electron. This causes the water vapor molecule to become positively charged.

Positively charged water vapor molecules are attracted to negatively charged water vapor molecules. This attraction causes the water vapor molecules to clump together. As more and more water vapor molecules clump together, they form water droplets.

The water droplets are too small to fall as rain, but they can grow larger by colliding with other water droplets. As the water droplets grow larger, they eventually become heavy enough to fall as rain.

Answer –C. a hypertonic solution.
Explanation- A hypertonic solution is a solution that has a higher concentration of solutes than the cell sap. When a plant cell is placed in a hypertonic solution, water will move out of the cell by osmosis, causing the cell to shrink.

Water moves from an area of high water potential to an area of low water potential. The water potential of a solution is determined by the concentration of solutes in the solution. A solution with a higher concentration of solutes has a lower water potential than a solution with a lower concentration of solutes.

The cell sap is the fluid inside a plant cell. The cell sap contains dissolved solutes, which lower the water potential of the cell sap. When a plant cell is placed in a hypertonic solution, the water potential of the hypertonic solution is lower than the water potential of the cell sap. This causes water to move out of the cell by osmosis, causing the cell to shrink.

Answer – D. may be reversible or irreversible and exothermic or endothermic.
Explanation-Reactions involving chemical decomposition, where a compound breaks down into simpler substances, can exhibit various characteristics. They may be reversible or irreversible, depending on the conditions and the nature of the substances involved. Additionally, these reactions can be exothermic (release heat) or endothermic (absorb heat), depending on the energy changes involved during the decomposition process. Therefore, the statement “may be reversible or irreversible and exothermic or endothermic” encompasses the possible variations in reactions involving chemical decomposition.

Answer –C. Silver
Explanation- Among the given options, silver is the best conductor of heat and electricity. Silver has the highest electrical conductivity and thermal conductivity among all metals. Its high conductivity is due to the presence of a large number of free electrons that can easily move and carry electric current or transfer heat energy. Copper is also an excellent conductor but has slightly lower conductivity than silver. Sodium and potassium, while conducting electricity, have lower conductivities compared to silver and copper.