Science and Technology: UPSC Previous Year Questions (Prelims)

Explanation

The diffusion of light in the atmosphere occurs mainly due to dust particles and aerosols, which scatter sunlight in all directions. This scattering, known as Mie scattering, happens when particles like dust or water droplets are comparable in size to the wavelength of light, scattering all wavelengths almost equally. It causes the sky to appear uniformly bright and ensures visibility even in shadows. Other aerosols like Water Vapors, Smoke, etc can also contribute to the diffusion effect. However, dust particles are the most prominent cause in this context.

Explanation

Option (c) is incorrect: Humidity (water vapor) actually increases the speed of sound in air. This is because water molecules are lighter than nitrogen and oxygen molecules, reducing the overall density of the air. The speed of sound in humid air is higher than in dry air at the same temperature. Options (a), (b) and (d) are correct:

The velocity of sound in air increases with temperature because higher temperatures increase the energy of air molecules, allowing sound to travel faster. The velocity of sound in air is independent of pressure at constant temperature since both pressure and density change proportionally, leaving the speed unchanged. The velocity of sound is unaffected by amplitude and frequency as it depends only on the medium>s properties like temperature, density, and composition.

Explanation

Ionic compounds are chemical compounds composed of positively and negatively charged ions held together by strong electrostatic forces of attraction.

Statement 1 is correct: Ionic compounds generally dissolve in water due to its high polarity, but alcohols are less polar compared to water, which makes most ionic compounds insoluble in alcohol.

Statement 2 is incorrect: In the solid state, the ions in ionic compounds are fixed in a rigid lattice structure and cannot move freely, which prevents the conduction of electricity.

Explanation

Option (a) is correct: The magnetic quantum number of an element specifies the orientation of an orbital in space relative to an external magnetic field. It determines the number of orbitals and their spatial orientation within a given subshell (defined by the azimuthal quantum number, l). Options (b), (c) and (d) are incorrect: Shape is determined by the azimuthal quantum number; Size is determined by the principal quantum number; Spin is determined by the spin quantum number, which specifies the spin orientation of the electron.

Explanation

Option (a) is correct: Liquid sodium is used as a coolant in certain types of nuclear reactors, particularly fast breeder reactors. Sodium is an effective coolant because of its high thermal conductivity and low neutron absorption properties, which are ideal for cooling the reactor core. Options (b), (c) and (d) are incorrect:

Dental amalgams are typically made of silver, mercury, and other metals. Calcium carbonate is not a must-include ingredient in toothpastes. Though it is sometimes used in toothpaste as an abrasive that helps remove plaque, stains, and debris from teeth. Bordeaux mixture is made from copper sulfate and lime (calcium hydroxide). It is used as a fungicide and pesticide in agriculture to prevent fungal diseases like downy mildew in crops. Zinc amalgam is not commonly used in dental fillings, although zinc oxide is sometimes used in dental cements and temporary fillings.

Explanation

Assertion (A) is false: Electron affinity generally decreases as you move down a group due to increasing atomic radii and weaker nuclear attraction for the incoming electron. Additional electron shells increase the shielding effect, further reducing effective nuclear pull. Reason (R) is true: As you go down a group, additional electron shells are added, making the atomic radius increase. This is because the outermost electrons are farther from the nucleus, and the shielding effect from inner electrons increases. Directions: The following Items consist of two statements. One labeled as the ‘Assertion (A)’ and the other as ’Reason (R)’. You have to answer these items using the code given below:(2003)

Explanation

Detergents are cleaning agents used to remove dirt, grease, and stains from surfaces and fabrics. Key additives like sodium sulphate prevent clumping by keeping the powder dry, while sodium silicate maintains alkalinity, enhancing the cleaning efficiency.

Statement 1 is correct: Sodium sulphate is added to keep the washing powder dry by preventing it from absorbing moisture.

Statement 2 is correct: Sodium silicate is used to maintain the alkalinity of the detergent, which enhances the cleaning action by breaking down grease and dirt.

Explanation

Photochemical smog is a type of air pollution that occurs when sunlight reacts with pollutants like nitrogen oxides and volatile organic compounds (VOCs), including hydrocarbons. The reaction leads to the formation of ozone, which is a key component of photochemical smog. Key Components of Photochemical Smog are:

Nitrogen Oxides - Released from vehicles, industrial emissions, and power plants. They react with sunlight to form ozone. Hydrocarbons: Organic compounds that also contribute to ozone formation when exposed to sunlight. Ozone: The primary harmful product of photochemical smog, which can cause respiratory problems and damage to vegetation. Methane is a potent greenhouse gas, but it does not directly form photochemical smog. While it can contribute indirectly to smog formation by reacting with other pollutants, it is not a primary component like ozone, hydrocarbons, or nitrogen oxides.

Explanation

Statement 1 is correct: Spirogyra and Ulothrix are types of blue green algae that can be found in both freshwater and marine environments. They are typically found in ponds, rivers, and even oceans in some cases. Spirogyra is known for its spiral-shaped chloroplasts and forms slimy green filaments/ mats on the water surface, especially when nutrients like nitrogen and phosphorus are abundant while Ulothrix is characterized by its simple, unbranched filaments and typically grows in both freshwater ponds and coastal regions. These algae play an important role in the aquatic food chain.

Statement 2 is correct: Chameleons have a unique ability to move their eyes independently of each other. They can focus one eye on an object in front while the other eye looks in a different direction, even behind them. This independent eye movement enables them to have a 360-degree view of their surroundings, aiding in predator detection and prey surveillance.

Explanation

Saturated fats are typically solid at room temperature and are found in animal products like butter, lard, and fatty cuts of meat, as well as some plant oils like coconut and palm oil. They are composed of fatty acids with no double bonds between carbon atoms, making them less reactive. Unsaturated fats are liquid at room temperature and are mainly found in plant-based oils (such as olive, sunflower, and canola oil) and fish. They contain one or more double bonds between carbon atoms which introduce "kinks" in the structure making them more reactive and beneficial for heart health when consumed in moderation. Assertion (A) is true: Unsaturated fats have at least one double bond between carbon atoms, making them more reactive. These double bonds are sites where chemical reactions can occur more easily compared to the single bonds in saturated fats. Reason (R) is false: Unsaturated fats contain at least one double bond in their carbon chain, not only single bonds. Saturated fats have only single bonds.