Purity and Composition

Metalloids are unique substances that exhibit characteristics of both metals and non-metals. Which of the following best describes their classification?

1. Atom
2. Mixture
3. Compound
4. Element

Answer: (d) Element

A scientist separates benzene (boiling point: 80°C), toluene (boiling point: 110°C), and xylene (boiling point: 140°C) using fractional distillation. Why must the fractionating column have multiple plates?

1. To increase the surface area for repeated condensation and vaporisation cycles.
2. To reduce the risk of overheating the liquids.
3. To allow liquids with the highest boiling points to evaporate first.
4. To prevent any liquid from condensing prematurely.

Answer: (a) To increase the surface area for repeated condensation and vaporisation cycles.

Sodium hydrogen carbonate, NaHCO₃, is baking soda. What is the percentage composition by mass of hydrogen (H) in sodium hydrogen carbonate? (Na = 22.99 g/mol, H = 1.008 g/mol, C = 12.01 g/mol, O = 16.00 g/mol)

1. 1.20%
2. 2.34%
3. 4.12%
4. 6.00%

Answer: (b) 2.34%

Electrons can either be shared or transferred between atoms depending on the type of chemical bond in a compound. Based on the type of bonding, identify the compounds in which electrons are shared between atoms: I. Ammonia II. Sodium chloride III. Nitrogen dioxide IV. Potassium bromide

1. I and II
2. III and IV
3. I and III
4. II and IV

Answer: (c) I and III

When light passes through different mixtures, it may become visible due to the scattering of light, a phenomenon known as the Tyndall effect. Which of the following is NOT a type of mixture in which the Tyndall effect can be observed?

1. Solution
2. Colloid
3. Aerosol
4. Emulsion

Answer: (a) Solution

In the following question, you will find an assertion and a reason. Select the appropriate option that applies. Assertion (A): During a chemical reaction, the total mass of reactants equals the total mass of products. Reason (R): Atoms are destroyed during chemical reactions but their mass remains constant.

1. Both A and R are true, and R is the correct explanation of A.
2. Both A and R are true, but R is not the correct explanation of A.
3. A is true, but R is false.
4. A is false, and R is true.

Answer: (c) A is true, but R is false.

Ice containing only the isotope ²H (deuterium) sinks and does not melt when dropped into ordinary distilled water maintained at 3°C. This behaviour differs from normal ice, which floats and melts under similar conditions. Which of the following statements correctly explains the behaviour of ²H₂O (heavy water) ice in comparison to normal H₂O ice?

1. ²H₂O ice has a higher melting point than normal ice
2. ²H has a higher natural abundance compared to ¹H
3. ²H₂O has entirely different chemical behaviour compared to H₂O
4. ²H₂O ice has a lower density than normal ice-cold water

Answer: (a) ²H₂O ice has a higher melting point than normal ice

A student heats 20 g of a mixture of potassium chlorate and manganese dioxide in an open crucible. The reaction produces oxygen gas and leaves 15 g of residue. The student concludes that the law of conservation of mass is invalid. Which error in reasoning explains the student’s conclusion?

1. The residue was not pure manganese dioxide.
2. The oxygen gas was not accounted for because the system was open.
3. The initial mass of the mixture was incorrect.
4. The law of conservation of mass does not apply to gas-phase reactions.

Answer: (b) The oxygen gas was not accounted for because the system was open.

Seawater is a mixture containing dissolved salts, gases, and organic matter. Suppose you are a scientist tasked with separating seawater into its pure components for research purposes. Which of the following separation techniques would you choose?

1. Distillation, Sedimentation, centrifugation
2. Distillation, evaporation, filtration, chromatography
3. Evaporation, decantation, electrolysis, centrifugation
4. Filtration, chromatography, decantation

Answer: (b) Distillation, evaporation, filtration, chromatography

Carefully read through the passage given below and answer the following questions. Formation and Properties of Compounds The structure and properties of compounds are directly influenced by the type of chemical bonds between their atoms. Covalent bonds, formed by electron sharing, create discrete molecules with predictable geometries. For example, water (H₂O) has a bent molecular structure due to the electron repulsion around oxygen, influencing its high boiling point and polarity. This polarity makes water an excellent solvent for ionic and polar substances. In contrast, ionic bonds, resulting from electron transfer between metals and non-metals, produce crystalline lattices with high melting points. Sodium chloride (NaCl) forms a cubic structure, where strong electrostatic forces between ions account for its brittleness and ability to conduct electricity in molten or dissolved states. Some compounds, like silicon dioxide (SiO₂), exhibit network covalent bonding, creating rigid, three-dimensional lattices with remarkable hardness and thermal stability. By understanding these bonding mechanisms, chemists can design materials with tailored properties for specific applications, such as semiconductors or drug delivery systems. If the melting point of a compound is found to be 800°C, what does this suggest about the type of bonding present?

1. The compound likely has strong covalent bonds.
2. It contains weak intermolecular forces between molecules.
3. It must be a metallic compound.
4. It is an ionic compound with a crystalline lattice.

Answer: (d) It is an ionic compound with a crystalline lattice.