Matter, Atoms and Molecules RRB PYQ

A precipitation reaction occurs when two solutions combine to form an insoluble solid called a precipitate. These reactions are a type of double displacement reaction, where ions exchange places, resulting in the formation of an insoluble product. This type of reaction is commonly used in analytical chemistry to identify or isolate ions in a solution.

A change of state of matter is a physical change because it only alters the physical properties of the substance without changing its chemical composition. A change of state is generally a reversible change, not an irreversible one. You can undo the change by reversing the conditions, such as cooling steam back into liquid water or freezing water back into ice. Examples of a physical change are melting ice into water, boiling water, and water vapor changing to water. Examples of an irreversible change are wood burning, rusting of iron, souring of milk.

Nuclear force acts between nucleons—protons and neutrons—present inside an atomic nucleus. It is an extremely strong attractive force at nuclear distances and is responsible for holding nucleons together against the electrostatic repulsion between positively charged protons. A neutron is electrically neutral, so electrostatic force does not provide the binding interaction between a neutron and a proton. Nuclear force is a short-range force, effective over distances of the order of a few femtometres (10^-15 m), and is much stronger than gravitational force at nuclear dimensions.

When ammonia solution is added to aluminum chloride (AlCl3), a white precipitate of aluminum hydroxide (Al(OH)3) is formed. This reaction can be represented as: AlCl3 + 3NH3 + 3H2O → Al(OH)3 (white precipitate) + 3NH4Cl. None of the other solutions (Mg(NO3)2, KNO3, Ba(NO3)2) would produce a white precipitate with ammonia solution.

Niels Bohr proposed that electrons do not just fly anywhere around the nucleus. Instead, they revolve in specific, non-radiating circular paths known as discrete orbits or energy levels. While revolving in these discrete orbits, electrons do not radiate energy. An electron only gains or loses energy when it jumps from one discrete orbit to another involving absorption or emission of a photon. For most atoms, the outermost shell is considered stable when it has 8 electrons. This 'movement' of electrons to achieve 8 by sharing or transferring is what causes chemical reactions.

The molar mass of H2SO4 is 98 g/mol. Therefore: Moles = 25g / 98 g/mol ≈ 0.255 moles.

Silver nitrate (AgNO3) and potassium bromide (KBr) react to form silver bromide (AgBr) and potassium nitrate (KNO3). In a double displacement reaction, the cations and anions of two compounds switch places, leading to the formation of two new compounds. This reaction also results in the precipitation of silver bromide (AgBr).

Condensation results in the formation of water droplets on the outer surface of a cold glass during a hot day. Condensation is an exothermic process, meaning it releases heat into the surrounding environment as the gas turns into liquid. Condensation is the exact reverse of evaporation. Condensation begins at a specific temperature called the dew point. At this temperature, the air is fully saturated and cannot hold all its water vapour. As warm air rises, it cools. The water vapour in the upper atmosphere condenses around tiny dust or smoke particles to form clouds.

For aluminium sulphate, Al2(SO4)3: Atomic masses: Al=27, S=32, O=16. Molar mass of Al2(SO4)3 = (2x27)+(3x32)+(12x16) = 54+96+192 = 342 g/mol. Mass of oxygen = 12x16 = 192 g/mol. Percentage of oxygen = (192/342)x100 = 56.14% ≈ 56.1%. Thus, oxygen constitutes approximately 56.1% by mass of Al2(SO4)3.

This equation is balanced because it ensures that the number of atoms for each element on both sides of the equation is equal, adhering to the law of conservation of mass. In this reaction, barium sulphate reacts with aluminium chloride to form barium chloride and aluminium sulphate.