Chemistry: Physical Chemistry: UPSC Previous Year Questions (Science and Technology)

Explanation

Ammonium chloride (NH₄Cl) is a primary component of the electrolyte paste used in dry cells. Zinc chloride (ZnCl₂) serves as the negative electrolyte, and it is often mixed with ammonium chloride in a paste to form the electrolyte for the dry cell. These components, along with a zinc anode and a carbon cathode, are commonly used in the construction of dry cells, which power small devices such as flashlights and radios.

Explanation

Option (d) is correct: Crookes glass is a type of glass that can absorb and block ultraviolet (UV) rays. It is specially designed to cut off UV radiation while allowing visible light to pass through. This makes it useful in applications where protection from UV light is needed, such as in certain laboratory equipment or sun protection glasses. Options (a), (b) and (c) are incorrect: Soda glass is a common type of glass used for general purposes, but it does not have UV-blocking properties. Pyrex glass is known for its heat resistance and is used in laboratory equipment but does not specifically block UV rays. Jena glass is a type of glass with special properties, mainly used in scientific instruments, but it is not specifically designed to block ultraviolet rays.

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

The order of filling up orbitals in an atom is governed by Aufbau’s principle, which states that electrons occupy orbitals in order of increasing energy levels, starting from the lowest energy orbital first. The sequence is determined by the (n + l) rule, where n is the principal quantum number and l is the azimuthal quantum number. If two orbitals have the same (n + l) value, the orbital with the lower n value is filled first.

Explanation

The half-life of a radioactive substance is the time it takes for half of the substance to decay. After each half-life:

At the end of 1 half-life (4 months), 50% of the substance remains. At the end of 2 half-lives (8 months), 25% of the substance remains, meaning 75% (three-fourths) of the substance has decayed. Thus, it takes 8 months for three-fourths of the substance to decay.

Explanation

Astatine, Francium, and Tritium are radioactive elements:

Astatine is a halogen with no stable isotopes.

Francium is an alkali metal, highly unstable and radioactive. Tritium is a radioactive isotope of hydrogen. Zirconium is not radioactive. It is a stable, non-radioactive metal widely used in industries like nuclear reactors and construction.

Explanation

A is matched with 2: Neutrino is a particle with almost zero mass and no electric charge, making it the correct match for zero mass. B is matched with 3: Quarks are fundamental particles with fractional charges, such as +2/3 or -1/3. C is matched with 1: Positrons, the antimatter counterparts of electrons, possess fractional spin, matching this characteristic. D is matched with 4: Photons are elementary particles of light with an integral spin of 1, classifying them as bosons. Direction: The following item consists of two statements. One labeled as the ‘Assertion (A)’ and the other as "Reason (R). You have to answer this item using the code given below: (2000)

Explanation

Explosions in mines are often caused by the presence of methane gas, which mixes with air to form an explosive mixture. This phenomenon is known as a "firedamp explosion." Methane is highly flammable and can ignite when exposed to a spark or heat, leading to devastating consequences in mining environments.