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The correct answer is Convex lens.

Key Points Convex lens is used in magnifying glasses to produce a magnified image of an object. The lens is usually mounted in a frame with a handle.

Convex lenses:

• Thicker in the centre than the edges
• The lens that converges (brings together) light rays.
• Forms real images and virtual images depending on the position of the object.

Uses of convex lenses:  

• It is used in the magnifying glass
• Can be used in the spyhole of doors
• Used in microscopes to generate extremely magnified images of very small objects.
• It is also widely used in camera lenses which not only focus on the image but also magnify the image.

Additional Information

• Concave lens:  it is thinner and flatter in the middle, causing refracted light rays to diverge.
• Plano Concave Lens:  Plano concave lens is an optical lens with one concave surface and one flat surface. It has a negative focal length and can be used for light projection

The correct answer is Platinum.

Key Points

• Paramagnetic substances are those which develop feeble magnetization in the direction of the magnetizing field.
• Such substances are feebly attracted by magnets and tend to move from weaker to stronger parts of a magnetic field.
• Magnetic susceptibility is small and positive i.e., χ > 0.
• Example: Manganese, aluminium, chromium, platinum, etc.

Additional Information

• Ferromagnetic substances are those which develop strong magnetization in the direction of the magnetizing field.
• They are strongly attracted by a magnet and tend to move from the weaker to the stronger part of a magnetic field.
• Magnetic susceptibility is very large and positive i.e. χ > 1000
• Example: Iron, cobalt, nickel, gadolinium, and alloys like alnico.
• Diamagnetic substances are those which develop feeble magnetization in the opposite direction of the magnetizing field.
• Such substances are feebly repelled by magnets and tend to move from stronger to weaker parts of a magnetic field.
• Magnetic susceptibility is small and negative i.e. -1 ≤ χ ≤ 0.
• Examples: Bismuth, mercury, copper, lead, zinc, etc.

The correct answer is protons and neutrons.

Key Points

• The nucleus (plural, nuclei) is a positively charged region at the centre of the atom.
• The atomic nucleus is the small, dense region consisting of protons and neutrons at the centre of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.
• Electrons are present outside the nucleus.
• The nucleus of the atom is extremely small. Its radius is only about 1/100,000 of the total radius of the atom.

The correct answer is Smelting.

Key Points

• The metallurgical process where a metal is obtained in a fused state is called smelting.
• Smelting is a form of extractive metallurgy to produce a metal from its ore. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gasses or slag and leaving just the metal behind.
• The reducing agent is commonly a source of carbon such as **coke, charcoal, and coal.**​

Additional InformationRoasting:

• Roasting is a cooking method that uses dry heat where hot air covers the food, cooking it evenly on all sides with temperatures of at least 150 °C (300 °F) from an open flame, oven, or another heat source.

Calcination:

• Calcination is the heating of solids to a high temperature for the purpose of removing volatile substances, oxidizing a portion of the mass, or rendering them friable.

Froth flotation

• Froth flotation, is a process for selectively separating hydrophobic materials from hydrophilic.

The correct answer is Zinc and Copper.

Key Points

• Brass is an alloy of copper and zinc, in proportions which can be varied to achieve varying mechanical, electrical, and chemical properties.
• It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.

Additional Information Brass Properties

• Brass often has a bright gold appearance, however, it can also be reddish-gold or silvery-white. A higher percentage of copper yields a rosy tone, while more zinc makes the alloy appear silver.
• Brass has higher malleability than either bronze or zinc.
• Brass has desirable acoustic properties appropriate for use in musical instruments.
• The metal exhibits low friction.
• Brass is a soft metal that may be used in cases when a low chance of sparking is necessary.
• The alloy has a relatively low melting point.
• It's a good conductor of heat.
• Brass resists corrosion, including galvanic corrosion from saltwater.
• Brass is easy to cast.

The correct answer is  CaO.

Key Points

• Calcium oxide, commonly known as quicklime or burnt lime, is a widely used chemical compound.
• It is a white, caustic, alkaline, crystalline solid at room temperature.
• Quicklime is known to be fairly caustic and alkaline under standard conditions.
• Quicklime is known to be an irritant to the eyes and the skin.
• It is advisable to use a full personal protective equipment kit while handling this compound (including goggles, head hood, polyurethane and rubber gloves, cotton work suit, and leather boots).

The correct answer is Increases then decreases**.**

Concept:

• Corrosion refers to the eating up of metals by the action of air, moisture, or a chemical on their surface.
• The reaction of iron and oxygen forms an iron oxide which is a reddish-brown oxide.
• This is called Rust or corrosion of iron metal.
• This reaction happens only in presence of oxygen and moisture.
• The presence of impurities like Moisture, Acid, salt, Temperature, Presence of acid, etc. is some factor that affects corrosion.

Explanation:

• When the iron nail gets rusted, the weight of the nail increases.
• It is because a layer of iron oxide is formed on the surface of the iron nail.
• The iron oxide layer is adding oxygen and hydrogen to the nail.
• Here is the chemical reaction of Rust:-
• 2Fe₂+ 3O₂ → 2Fe₂O₃
• Fe₂O₃ + xH₂O → Fe₂O₃. xH₂O (rust)

Thus, the correct answer is Increases then decreases

The correct answer is Platinum.

Key Points

White metal

• ​White metals are an important category of metals used in various industrial applications, just as alloys are.
• In its most basic form, white metal is an alloy of Lead and Lithium, or other metals like cadmium, bismuth, and zinc.
• The term “white metals” are generally used to describe a series of metal alloys with decorative brightness and is mostly used as a base for plated silverware, ornaments, or novelties.
• White metal is an alloy of Lead and Lithium metal.
• White metal bearings are used in motor engines.

Note:  As per the official answer key the correct answer given is option E

The correct answer is 1 only.

Noble metals

• These metals have high heat resistance, high corrosion resistance, high melting points, and unique catalytic properties.
• They are usually found in nature in their pure form. (Hence, Statement 1 is correct)
• Gold (Ag), Silver (Ag), Platinum (Pt) and Platinum metals are Noble metals. (Hence, statement 2 is incorrect)
• Here, Pt, osmium (Os), iridium (Ir), ruthenium (Ru), rhodium (Rh), and palladium (Pd), are collectively called platinum metals.
• Because they are so nonreactive, they don't corrode easily. This makes them ideal for jewelry and coins.

The correct answer is option (3) both statements are true.

Key Points

• Salt changes the freezing point of water, and causes the water to reach a lower temperature in order to freeze.Because salt particles make it harder for water particles to freeze back onto the ice, the ice that is in contact with dissolved salt melts faster. When the saltwater flows over the surface it melts the ice on its way, creating channels, like rivers, over the surface of the ice ball.
• Adding salt to boiling water actually increases the boiling point temperature of the water. This process is called boiling point elevation. When you add a solute like salt to a solvent like water, it makes water an impure solvent and raises its boiling point above that of pure solvents. Hence, statement 1 is correct.
• When methyl alcohol is added to water, boiling point of water decreases due to hydrogen bonding and when a volatile solute is added to a volatile solvent elevation in boiling point is observed.  Hence, statement 2 is correct.