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The correct answer is 1 and 2 only.

Key Points

• A precipitation reaction occurs when two aqueous solutions containing soluble salts are mixed together, resulting in a double displacement reaction that yields an insoluble ionic solid. Hence, statement 1 is correct.
• The insoluble solid that emerges from the liquid mixture is scientifically referred to as a precipitate, and its formation indicates that the product has exceeded its solubility limit in the given solvent. Hence, statement 2 is correct.
• While some acid-base reactions might produce a precipitate, the vast majority of precipitation reactions involve salts; conversely, acid-base reactions are fundamentally neutralization reactions that primarily produce water and a soluble salt. Hence, statement 3 is incorrect.

 Additional Information

• Common Precipitation Examples:
• Silver Chloride (AgCl): When Silver Nitrate (AgNO₃) reacts with Sodium Chloride (NaCl), a white precipitate of AgCl is formed.
• Barium Sulphate (BaSO₄): The reaction between Barium Chloride (BaCl₂) and Sodium Sulphate (Na₂SO₄) produces a white insoluble solid.
• Lead Iodide (PbI₂): Mixing Lead Nitrate and Potassium Iodide results in a striking yellow precipitate.
• Industrial and Laboratory Uses:
• Qualitative Analysis: Used to detect the presence of specific cations or anions in an unknown solution based on the color and solubility of the precipitate.
• Water Treatment: Used to remove contaminants like magnesium, calcium, and heavy metals from hard water or industrial wastewater.
• Pigment Production: Many industrial dyes and pigments are manufactured through controlled precipitation processes.

The correct answer is Sodium chloride (NaCl).

Key Points

• Sodium chloride (NaCl) is classified as a neutral salt because it is the product of a neutralization reaction between a strong acid (Hydrochloric acid, HCl) and a strong base (Sodium hydroxide, NaOH).
• When dissolved in water, NaCl dissociates completely into its constituent ions, Na⁺ and Cl⁻. These ions are the conjugate species of a strong base and a strong acid, respectively, and therefore have negligible tendency to react with water.
• Since neither the cation nor the anion undergoes salt hydrolysis, the concentration of hydrogen ions (H⁺) and hydroxyl ions (OH⁻) remains equal to that of pure water.
• Consequently, the pH value of the resulting aqueous solution is approximately 7.0 at 25°C, characterizing the solution as neutral.

 Additional Information

• Potassium hydroxide (KOH):
• It is not a salt but a strong base. It dissociates to release a high concentration of OH⁻ ions, resulting in a strongly alkaline solution with a pH typically greater than 13.
• Ammonium chloride (NH₄Cl):
• This is an acidic salt formed from a strong acid (HCl) and a weak base (Ammonium hydroxide). The NH₄⁺ ion undergoes cationic hydrolysis, releasing H⁺ ions into the solution and lowering the pH below 7.
• Sodium acetate (CH₃COONa):
• This is a basic salt derived from a weak acid (Acetic acid) and a strong base (NaOH). The acetate ion (CH₃COO⁻) undergoes anionic hydrolysis, producing OH⁻ ions and making the solution alkaline (pH > 7).

The correct answer is Three.

Key Points

• Among the five substances buried by the gardener, Banana peels, Newspaper, and Cotton cloth are the three that are classified as biodegradable.
• Banana peels are organic waste products from plants that consist of biological tissue, which microorganisms like bacteria and fungi can decompose into humus within approximately 2 to 5 weeks.
• Newspaper is primarily made from wood pulp, which contains cellulose; this organic compound is easily broken down by soil enzymes and moisture, typically taking 10 to 30 days to disappear.
• Cotton cloth is a natural textile derived from the Gossypium plant; because it is composed of natural fibers, it is susceptible to biological decay and generally decomposes within 2 to 5 months in a garden environment.

 Additional Information

• Biodegradable substances are materials that can be broken down into simpler, non-toxic substances by the action of living organisms like decomposers.
• Natural materials like paper, wood, animal waste, and plant remains are essential for maintaining the nutrient cycle in the soil.
• The process of biodegradation requires specific environmental conditions including moisture, oxygen, and the presence of aerobic or anaerobic bacteria.
• Non-biodegradable substances are materials that cannot be decomposed by biological processes and persist in the environment for long periods.
• Plastic bottles are made from synthetic polymers like Polyethylene Terephthalate (PET), which lack the chemical bonds that bacteria can recognize and break, lasting for up to 450 years.
• Metal spoons, often made of stainless steel or aluminum, are inorganic and do not undergo biological decay, though they may eventually undergo corrosion or oxidation over centuries.

The correct answer is Electrons are embedded in a positively charged sphere.

Key Points

• J.J. Thomson proposed his model of the atom in 1898, shortly after he discovered the electron, the first subatomic particle.
• According to this model, an atom is envisioned as a positively charged sphere where negatively charged electrons are embedded into it, similar to seeds in a fruit.
• This model is widely known as the Plum Pudding Model or the Watermelon Model due to the specific arrangement of its components.
• In the Watermelon analogy, the red edible part represents the positive charge distributed throughout the atom, while the black seeds represent the electrons.
• Thomson hypothesized that the magnitude of the positive and negative charges is equal, which ensures that the atom remains electrically neutral.

 Additional Information

• Rutherford Atomic Model:
• Unlike Thomson's static model, Ernest Rutherford proposed in 1911 that electrons revolve around a nucleus in circular paths.
• This model was based on the Alpha-particle scattering experiment, which proved the existence of a dense, central nucleus.
• Bohr Atomic Model:
• Proposed by Niels Bohr in 1913, it refined the arrangement by stating that electrons move in discrete orbits with fixed energy levels.
• Shortcomings of Thomson Model:
• It could not explain the stability of the atom or the results of later experiments showing that most of an atom is empty space.
• It failed to account for the scattering of alpha particles through large angles, which necessitated the concept of a central positive core.

The correct answer is Regulate movement of substances across the cell.

Key Points

• The plasma membrane, also referred to as the cell membrane, is a biological membrane that separates the interior of all cells from the outside environment.
• It is selectively permeable, which is its most vital characteristic, allowing it to control the influx and efflux of nutrients, ions, and metabolic waste products.
• The structure is primarily composed of a phospholipid bilayer with embedded integral proteins and peripheral proteins that act as channels or pumps.
• It maintains homeostasis within the cell by facilitating transport mechanisms such as simple diffusion, facilitated diffusion, and active transport using ATP.
• The Fluid Mosaic Model describes the membrane as a dynamic structure where proteins float in or on the fluid lipid bilayer, enabling cell signaling and recognition.

 Additional Information

• ATP Synthesis:
• In eukaryotic cells, the primary site for ATP (Adenosine Triphosphate) production is the mitochondria via the process of oxidative phosphorylation.
• Cell Wall:
• Provides rigid support and structural integrity; it is found in plants (cellulose), fungi (chitin), and prokaryotes (peptidoglycan) but is absent in animal cells.
• Nucleus:
• Acts as the genetic control center of the cell, housing the chromosomes and DNA (Deoxyribonucleic Acid), protecting them from the metabolic activities of the cytoplasm.

The correct answer is Transporting food from leaves.

Key Points

• Xylem is a complex conducting tissue in vascular plants that primarily facilitates the upward movement of water and dissolved minerals from the roots.
• It consists of four specialized components: tracheids, vessel elements, xylem parenchyma, and xylem fibers.
• Tracheids and vessels are the primary water-conducting elements that form long, hollow tubes to ensure vertical transport of fluids throughout the plant.
• Xylem fibers are highly lignified, dead cells with thick walls that function to provide mechanical support and structural rigidity to the plant body.
• Xylem parenchyma is the only living component of the xylem tissue, specifically designed for storing food materials such as starch and assisting in lateral water conduction.
• The process of transporting organic solutes and food from the leaves to other parts of the plant is called translocation, which is the exclusive function of the phloem tissue.

 Additional Information

• Phloem Tissue:
• Sieve Tube Elements: These are long, tube-like structures that lack a nucleus at maturity and are responsible for the bidirectional transport of sucrose.
• Companion Cells: These specialized cells are closely associated with sieve tubes and maintain the pressure gradient required for nutrient movement.
• Phloem Parenchyma: Responsible for storing food and secretion of substances like resins and mucilage.
• Direction of Flow: While xylem flow is unidirectional (root to leaf), phloem flow is multidirectional, moving nutrients from source to sink.
• Vascular Bundle:
• The combination of xylem and phloem constitutes the vascular bundle, which serves as the circulatory system for higher plants.
• The cambium layer, found in dicotyledonous plants between xylem and phloem, is responsible for secondary growth and thickening of the stem.

The correct answer is It is the sum of protons and neutrons in an atom.

Key Points

• The mass number (represented by the symbol A) of an atom is defined as the total number of protons and neutrons present in its nucleus. Since these two particles reside in the nucleus, they are collectively known as nucleons. Hence, statement 1 is correct.
• In many light elements, the mass number is approximately twice the atomic number (e.g., Helium has 2 protons and 2 neutrons, A=4), but this is not a universal law. As elements become heavier, the proportion of neutrons increases to provide nuclear stability, making the mass number significantly more than twice the atomic number. Hence, statement 2 is incorrect.
• The number of electrons in a neutral atom is equal to the atomic number (Z), which is the number of protons. The mass number focuses on the dense particles within the nucleus, and electrons have negligible mass compared to nucleons. Hence, statement 3 is incorrect.
• While the sum of protons and electrons equals the total charge components of a neutral atom, it does not define the mass number. The mass number is strictly the count of heavy subatomic particles in the nucleus. Hence, statement 4 is incorrect.

 Additional Information

• Atomic Notation:
• In standard chemical notation, an element is written as ^A_ZX, where A is the mass number, Z is the atomic number, and X is the chemical symbol.
• The number of neutrons (n) can be calculated by the formula: n = A − Z.
• Isotopes:
• Isotopes are atoms of the same element that have the same atomic number (protons) but different mass numbers due to a different number of neutrons.
• For example, Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons) are isotopes used in radiocarbon dating.
• Atomic Mass vs. Mass Number:
• The mass number is always a whole number representing a count of particles.
• The atomic mass is the weighted average of the masses of all naturally occurring isotopes of that element, which is why it often appears as a decimal on the Periodic Table.

The correct answer is Methyl chloride (CH₃Cl).

When methane reacts with chlorine in the presence of ultraviolet (UV) light, a free-radical substitution reaction occurs, and the first product formed is methyl chloride (chloromethane), CH₃Cl.

Key Points

• The first substitution product of methane and chlorine is methyl chloride (CH₃Cl).
• The reaction takes place in the presence of UV light or sunlight, which initiates free-radical formation.
• One hydrogen atom of methane is replaced by one chlorine atom during the first substitution step.
• Further chlorination can produce dichloromethane (CH₂Cl₂), chloroform (CHCl₃), and carbon tetrachloride (CCl₄).

Additional Information

• Substitution Reaction
• A reaction in which one atom or group in a molecule is replaced by another atom or group.
• Alkanes typically undergo free-radical substitution reactions with halogens.
• Methyl Chloride (CH₃Cl)
• Also known as chloromethane.
• It is the first product formed during the chlorination of methane.
• Used in the production of silicones and other organic compounds.
• Dichloromethane (CH₂Cl₂)
• Formed during further chlorination of methyl chloride.
• Commonly used as an industrial solvent.
• Carbon Tetrachloride (CCl₄)
• The final product when all four hydrogen atoms of methane are replaced by chlorine atoms.
• Historically used in fire extinguishers and cleaning agents.
• Free Radical Mechanism
• Initiation: Cl₂ → 2Cl· (under UV light)
• Propagation: CH₄ + Cl· → CH₃· + HCl
• Propagation: CH₃· + Cl₂ → CH₃Cl + Cl·

The correct answer is Sand settles at the bottom, while salt dissolves in water.

Key Points

• Salt (Sodium Chloride, NaCl) is an ionic compound that is highly soluble in water; its ions dissociate and are surrounded by water molecules through a process called hydration, forming a homogeneous solution.
• Sand, primarily composed of Silicon Dioxide (SiO₂), is insoluble in water because the strong covalent bonds within its crystal lattice cannot be overcome by the dipole-dipole interactions of water.
• The combination of sand and water forms a suspension, a type of heterogeneous mixture where large particles are visible and do not chemically bond with the solvent.
• Because sand has a higher density (approximately 2.65 g/cm³) than water (1.0 g/cm³), the particles undergo sedimentation and sink to the bottom of the beaker due to gravity once stirring stops.

 Additional Information

• Mixture Classification:
• Homogeneous Mixture: A mixture with a uniform composition, such as the brine solution (saltwater) where the solute is evenly distributed.
• Heterogeneous Mixture: A mixture where components remain physically separate, like the sand-water suspension.
• Separation Techniques:
• Filtration: This method uses a porous barrier to separate the insoluble sand from the liquid phase based on particle size.
• Decantation: This involves carefully pouring out the supernatant liquid (the salt solution) after the sand has completely settled.
• Evaporation: This process recovers the dissolved salt by heating the solution until the water vaporizes, leaving the salt crystals behind.
• Solubility Limit: At room temperature (25°C), the solubility of salt in water is approximately 36 grams per 100 ml.

The correct answer is Both (iii) and (iv).

Key Points

• When white light is incident on a glass prism, it undergoes dispersion, splitting into its constituent colors (VIBGYOR) because each color travels at a different speed in the glass medium.
• Statement (i) explains that red light has the longest wavelength in the visible spectrum; since the angle of deviation is inversely proportional to wavelength, red bends the least. Hence, statement (i) is correct.
• Statement (ii) pertains to the speed of light, which is calculated as \( v = \frac{c}{n} \). Because violet light experiences the highest refractive index, it travels at the slowest speed within the prism. Hence, statement (ii) is correct.
• Statement (iii) suggests that green has a higher refractive index than violet. In reality, according to Cauchy's formula, the refractive index decreases as wavelength increases; since violet has a shorter wavelength than green, violet has a higher refractive index. Hence, statement (iii) is incorrect.
• Statement (iv) claims the refractive index of red is more than blue. However, red light has a significantly longer wavelength than blue light, resulting in a lower refractive index for red. Hence, statement (iv) is incorrect.

 Additional Information

• Refractive Index and Wavelength Relationship:
• The refractive index (n) of a material is not constant but varies with the frequency of light, a phenomenon known as chromatic dispersion.
• Violet Light: It has the shortest wavelength (approximately 400 nm) and highest frequency, leading to the maximum deviation from the original path.
• Red Light: It has the longest wavelength (approximately 700 nm) and lowest frequency, leading to the minimum deviation.
• Visible Spectrum Order: The sequence from longest to shortest wavelength is Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
• Speed in Vacuum: All colors of light travel at the same speed of approximately \( 3 \times 10^{8} \) m/s in a vacuum, but their speeds differ when they enter a dense medium like glass.