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

Answer – B. 32°F
Explanation-Pure water freezes at a temperature of 32°F or 0°C. This is the freezing point of water under standard atmospheric pressure.

Answer – C. Propane
Explanation-LPG is a flammable hydrocarbon gas mixture used as fuel in heating appliances, cooking equipment, and vehicles. While it can contain other hydrocarbons, propane is one of the primary components of LPG.

Answer –D. Chlorofluorocarbon
Explanation-Among the options listed, Chlorofluorocarbon (CFC) is the substance that is commonly found in refrigerators. CFCs are a type of synthetic chemical compound that was widely used in refrigeration and air conditioning systems in the past. They were commonly used as refrigerants due to their low boiling points and stability. Refrigerators typically contain a closed system of refrigerant, which is responsible for absorbing heat from the inside of the fridge and releasing it outside. In older refrigerators, CFCs such as Freon were commonly used as the refrigerant. However, it’s important to note that due to the detrimental effects of CFCs on the ozone layer, their use has been largely phased out in many countries. Carbon dioxide (CO₂), methane (CH₄), and helium (He) are not typically present in refrigerators as refrigerants. Carbon dioxide is a greenhouse gas that is naturally present in the Earth’s atmosphere and is not used as a refrigerant in household appliances. Methane is also a potent greenhouse gas and is not used in refrigeration systems. Helium is an inert gas and does not possess the necessary properties for efficient refrigeration.

Answer – A. F-
Explanation-O₂ has 16 electrons, and F- also has 16 electrons. Oxygen (O) has the electron configuration 1s² 2s² 2p⁴. This means it has 2 electrons in the 1s orbital, 2 electrons in the 2s orbital, and 4 electrons in the 2p orbital. Fluorine (F) has the electron configuration 1s² 2s² 2p⁵. This means it has 2 electrons in the 1s orbital, 2 electrons in the 2s orbital, and 5 electrons in the 2p orbital. To become isoelectronic with fluorine, oxygen needs to gain two electrons. By gaining two electrons, the oxygen atom will have the same electron configuration as fluorine. Therefore, O₂ is isoelectronic with F-.

Answer –A. Radioactivity
Explanation-The correct answer is A. Radioactivity. The unit “Curie” (Ci) is a measure of radioactivity. It is named after Marie and Pierre Curie, who were renowned physicists and pioneers in the field of radioactivity. The Curie is defined as the amount of radioactive material that undergoes 3.7 × 10^10 disintegrations per second, which is equivalent to one gram of radium-226. Radioactivity refers to the spontaneous emission of radiation from unstable atomic nuclei. It is commonly associated with the decay of radioactive isotopes, such as uranium, radium, and plutonium. The Curie unit is used to quantify the intensity of radioactivity and is widely used in scientific and medical contexts to measure the amount of radioactive substances or the activity of radioactive sources.

Answer – C. Copper and Tin
Explanation-Bronze is an alloy of copper and tin. It is a hard, strong, and corrosion-resistant metal that has been used for centuries for making tools, weapons, and other objects. The exact proportions of copper and tin in bronze can vary, but most bronze alloys contain about 88% copper and 12% tin.

Answer –B. ¹⁴₆C, also known as carbon-14.
Explanation-Carbon-14 dating, also called radiocarbon dating, is a method used to determine the age of organic materials, such as archaeological artifacts or ancient remains. Carbon-14 (¹⁴C) is an isotope of carbon that is present in the atmosphere in small quantities. It is formed through cosmic ray interactions with nitrogen-14 (¹⁴N) in the upper atmosphere. Living organisms, including humans, take in carbon from the environment through processes such as breathing and consuming plants or animals. While an organism is alive, the ratio of carbon-14 to stable carbon isotopes (carbon-12 and carbon-13) remains constant. However, when an organism dies, it no longer takes in carbon from the environment, and the carbon-14 in its body begins to decay at a known rate. By measuring the remaining amount of carbon-14 in a sample and comparing it to the known decay rate, scientists can estimate the age of the sample. Carbon-14 dating is particularly useful for dating organic materials up to about 50,000 years old. It has been widely used in archaeology and anthropology to determine the age of ancient artifacts, fossils, and human remains, providing valuable insights into the timeline of human history and the development of civilizations.

Answer –D. Solid CO₂
Explanation-Dry ice refers to solid carbon dioxide. It is called “dry ice” because it does not melt into a liquid when heated but instead undergoes sublimation, directly converting from a solid to a gas. Dry ice is extremely cold, with a temperature of approximately -78.5°C (-109.3°F), far colder than the freezing point of water.

Answer –D. Sn and Pb
Explanation-Solder is a metal alloy used for joining or bonding metal parts together. The most common type of solder used in electronics and plumbing is a mixture of tin (Sn) and lead (Pb), known as tin-lead solder. However, due to environmental concerns, lead-free solder is now widely used, which typically consists of tin and other metals such as silver, copper, or bismuth.

Answer –A. Sulphur
Explanation-When you chop an onion, it releases a compound called syn-propanethial-S-oxide. This compound is formed by enzymes in the onion breaking down a sulfur-containing compound called alliin. When syn-propanethial-S-oxide comes into contact with the moisture in your eyes, it forms sulfuric acid, which irritates the eyes and triggers tear production. This is why chopping onions can make you cry.

Answer –D. allotropes
Explanation-Graphite, carbon, and diamonds are all allotropes of carbon. Allotropes are different forms of an element that exist in the same physical state but have different structures and properties. In the case of carbon, graphite has a layered structure and is soft and opaque, carbon atoms in diamond are arranged in a rigid three-dimensional lattice, making it the hardest known natural substance, and carbon atoms in carbon nanotubes have a cylindrical structure with unique electrical and mechanical properties.

Answer –B. oxidizing
Explanation-Potassium permanganate (KMnO₄) is used to purify water because it is a strong oxidizing agent. It can react with various impurities and contaminants in water, such as organic compounds, iron, and hydrogen sulfide, by accepting electrons and undergoing reduction itself. This oxidation process helps to remove or neutralize harmful substances, making the water safer to drink or use.

Answer – C. Henri Becquerel
Explanation-Henri Becquerel discovered natural radioactivity. In 1896, while studying the effects of X-rays, Becquerel accidentally discovered that uranium salts emitted radiation that could penetrate opaque materials. This discovery laid the foundation for further research in the field of radioactivity and led to significant contributions by other scientists like Marie Curie and Ernest Rutherford.

Answer – D. NaNO_{3
Explanation-It provides plants with a source of nitrogen, which is an essential nutrient for their growth and development. Nitrogen is a key component of proteins and other vital cellular components. Fertilizers like sodium nitrate help replenish nitrogen in the soil and support healthy plant growth.}

Answer – B. Nuclear Fusion
Explanation- The hydrogen bomb, also known as a thermonuclear bomb, is based on nuclear fusion reactions. It involves the fusion of hydrogen isotopes, typically deuterium and tritium, to form helium and release a tremendous amount of energy. This process is similar to the energy-producing reactions that occur in the core of the Sun. In contrast, nuclear fission is the process used in atomic bombs and nuclear power plants, where heavy atomic nuclei are split apart.

Answer – A. nitrogen
Explanation-The gas that fills an electric bulb is usually nitrogen. Nitrogen gas is used to fill incandescent light bulbs because it is an inert gas that prevents the filament from oxidizing and burning out when exposed to air. Nitrogen is non-reactive and helps to maintain a stable environment inside the bulb, prolonging its lifespan.

Answer – B. propane and butane.
Explanation-L.P.G. is commonly used as a fuel for heating, cooking, and various industrial applications. Propane and butane are hydrocarbon gases that are easily liquefied under moderate pressure, making them suitable for storage and transportation. The specific ratio of propane to butane may vary, depending on the region and the desired properties of the L.P.G. mixture.

Answer – B. Sulphur
Explanation-Hair setting typically involves the use of various chemicals to achieve the desired results. These chemicals include: 1. Aerosol Propellant: The propellant is used to provide a fine mist or spray for even distribution of the hair-setting product. It helps to moisturize the hair and facilitate its application. 2. Alcohol: Alcohol is utilized to enhance the hold of the hair by increasing its surface tension. It aids in keeping the hair in place and achieving the desired style. 3. Polyvinylpyrrolidone: This chemical is commonly used as a binder or film-forming agent in hair-setting products. It helps to hold the hair together and maintain the desired shape. 4. Formaldehyde: Formaldehyde is sometimes used in hair-setting products for its preserving properties. It helps to extend the longevity of the hair setting by preventing the growth of microorganisms. 5. Aromatic substances: Aromatic substances are added to hair-setting products to provide a pleasant fragrance. They contribute to the overall sensory experience and leave the hair smelling pleasant after styling. 6. Sulphur: Sulphur is utilized in hair-setting products to enhance their longevity. It helps to hold the hair in place for a longer duration. It is important to note that the specific formulation and ingredients used in hair-setting products may vary depending on the brand and product type.

Answer – A. Urea
Explanation-Urea is a commonly used nitrogenous fertilizer that contains approximately 46% nitrogen by weight. It is highly soluble in water and provides a readily available source of nitrogen for plants.

Answer – A. Lemon juice
Explanation-Lemon juice is acidic in nature due to the presence of citric acid. It has a pH of around 2 to 3, making it acidic. Lime water B. has a pH greater than 7, while human blood C. has a slightly alkaline pH of around 7.4. Antacid D. is used to neutralize excess stomach acid and typically has a pH greater than 7.

Answer – C. basic
Explanation-Regularly brushing our teeth is a recommended practice for preventing tooth decay. The toothpaste commonly used for this purpose is alkaline or basic in nature. When saliva builds up in the mouth, it lowers the pH levels, resulting in an acidic environment in the oral cavity. This acidity promotes the growth of bacteria, which can lead to tooth decay. By maintaining good oral hygiene and using toothpaste with alkaline properties, we can help counteract the acidic conditions and protect our teeth from decay.

Answer –B. Limestone
Explanation-Cement is produced by heating limestone (calcium carbonate) along with other materials, such as clay, in a kiln at high temperatures. The resulting product, known as clinker, is then ground into a fine powder and mixed with gypsum to produce cement.

Answer – A. Copper & Zinc
Explanation- It is an alloy that combines the properties of both metals. The proportion of copper and zinc can vary, but typically brass contains around 60-70% copper and 30-40% zinc. Brass is widely used in various applications, including musical instruments, plumbing fixtures, decorative items, and electrical connectors.

Answer –A. increase
Explanation- Adding salt to water on the stove can lead to a slightly higher boiling temperature, but the effect is not significant. When salt is added to water, it dissociates into Na+ and Cl- ions. These ions interact with the water molecules, causing an increase in the intermolecular attraction. As a result, more energy is required to break these bonds, resulting in a slightly higher boiling point for the water. For example, if you add 20 grams of salt to five liters of water, the boiling point may increase from 100°C to around 100.04°C. However, it’s important to note that this effect is minimal and may not have a noticeable impact on everyday cooking.

Answer –D. oxidation
Explanation- The bleaching action of chlorine is due to its strong oxidizing properties. Chlorine can react with certain compounds, such as color pigments, and oxidize them, resulting in the removal of color.

Answer –B. Dilute acetic acid
Explanation-The chemical name of vinegar is acetic acid, which is a weak acid. Vinegar is primarily composed of water and acetic acid, along with small amounts of other substances.

Answer –A. Polyvinyl Chloride
Explanation-PVC stands for Polyvinyl Chloride, which is a commonly used thermoplastic polymer in the plastic industry. It is derived from vinyl chloride monomers through a polymerization process. PVC is known for its versatility, durability, and cost-effectiveness, making it widely used in various applications such as pipes, fittings, cables, flooring, and more.

Answer –B. Gypsum
Explanation- Plaster of Paris is produced by heating gypsum, which is a soft sulfate mineral composed of calcium sulfate dihydrate. When heated, gypsum undergoes a chemical reaction and loses its water content, resulting in the formation of calcium sulfate hemihydrate, commonly known as plaster of Paris.