Chemistry 2012 Jamb Examination

Group VII elements, known as halogens, are very good oxidising agents. They readily gain electrons to form halide ions because they have high electron affinities and high electronegativities. These properties enable them to oxidise other substances by accepting electrons. For example, chlorine can oxidise bromide ions to bromine in solution. Their ability to accept electrons makes them strong oxidising agents rather than electron donors.

X-rays are used to study the arrangement of atoms in crystal lattices through a process known as X-ray crystallography. When X-rays are directed at a crystal, they are diffracted by the regular array of atoms, producing a diffraction pattern that can be analysed to determine the crystal’s structure. Other types of radiation like alpha and beta particles lack the short wavelength required to interact with atomic-scale structures effectively.

Air is a mixture because its composition varies depending on location and environmental conditions, satisfying statement I. It can also be separated into its components, such as oxygen, nitrogen, and carbon dioxide, by physical processes like fractional distillation, satisfying statement II. The mere presence of noble gases does not indicate that air is a mixture, hence statement III is not relevant. Therefore, I and II correctly show that air is a mixture.

Hard water contains calcium and magnesium ions that prevent soap from lathering easily. Softening is achieved by adding soluble sodium compounds such as sodium carbonate or sodium phosphate. These react with calcium and magnesium ions to form insoluble precipitates like calcium carbonate and magnesium carbonate, which are removed by filtration. This process leaves behind soft water that no longer contains the hardness-causing ions.

Chlorination is done to disinfect water by killing pathogenic microorganisms such as bacteria and viruses. Chlorine acts as an oxidising agent, destroying the cell walls and enzymatic systems of these microbes, making the water safe for drinking. While it may slightly alter the taste or odour, its main purpose is sterilisation, not improving flavour or appearance.

From the balanced equation:\[ 2H_2 + O_2 \rightarrow 2H_2O \] 2 volumes of hydrogen react with 1 volume of oxygen. Hence, 300 cm³ of O₂ requires \( 2 \times 300 = 600 \; \text{cm}^3 \) of H₂. Initially, 1000 cm³ of hydrogen is available, so the leftover hydrogen after the reaction is:\[ 1000 - 600 = 400 \; \text{cm}^3 \] Therefore, 400 cm³ of hydrogen remains unreacted after the explosion.

All four phenomena—evaporation, sublimation, diffusion, and Brownian motion—demonstrate that matter is made up of tiny, moving particles. Evaporation and sublimation show that particles of matter have energy to escape into the gaseous phase. Diffusion shows that particles move from regions of high concentration to low concentration, while Brownian motion provides visible evidence of random molecular motion. Together, they confirm the particulate nature of matter.

Element X (atomic number 11) is sodium, a metal, and element Y (atomic number 17) is chlorine, a non-metal. Sodium loses one electron to form Na⁺, while chlorine gains one electron to form Cl⁻. The electrostatic attraction between these oppositely charged ions forms an ionic bond, characteristic of metal–non-metal combinations.

A neutral magnesium atom has an atomic number of 12, with the configuration 1s²2s²2p⁶3s². When it forms Mg²⁺, it loses two electrons from the outermost 3s orbital, leaving 1s²2s²2p⁶. This configuration corresponds to a noble gas (neon), indicating a stable ion.

The mixture contains sand (insoluble) and methanoic acid (soluble in water). Filtration removes the sand, while distillation separates pure water from methanoic acid based on their different boiling points. Therefore, filtration followed by distillation effectively produces pure water.