General Science MCQs – Page 35 (Q341-350)

A chemical bond formed by the sharing of one or more electron pairs between atoms is called a covalent bond, typically occurring between two nonmetal atoms that both need additional electrons to complete their outer shells, allowing them to achieve stability by mutually sharing rather than fully transferring electrons. Covalent bonds can be single, double, or triple, depending on how many electron pairs are shared, and covalently bonded compounds like water and methane often form molecules with distinct, relatively low melting and boiling points compared to ionic compounds, and they generally do not conduct electricity, since no free-moving charged ions are present.

The strong attraction between positive metal ions arranged in a lattice and a surrounding sea of freely moving, delocalized valence electrons is called a metallic bond, a bonding type unique to metals that explains many of their characteristic physical properties. This mobile electron sea allows metals to conduct electricity and heat efficiently, since the electrons can move freely to carry charge and transfer thermal energy, and it also allows metal ion layers to slide past one another without breaking the overall bonding structure, giving metals their characteristic malleability and ductility, properties not found in ionic or covalent compounds.

Common table salt, sodium chloride, is a classic textbook example of an ionic bond, formed when a sodium atom transfers its single valence electron entirely to a chlorine atom, creating a positively charged sodium cation and a negatively charged chloride anion, which then attract each other strongly to form a stable, three-dimensional crystal lattice structure. This ionic bonding explains sodium chloride's high melting point of 801 degrees Celsius, its brittle crystalline structure, and its ability to conduct electricity when dissolved in water or melted, since the ions become mobile and free to carry electric charge under those conditions.

Water and methane are commonly used as classic textbook examples of substances held together by covalent bonds, since both are formed between nonmetal atoms, hydrogen and oxygen in water, and hydrogen and carbon in methane, that share electron pairs rather than fully transferring them. Water's bent molecular shape, resulting from oxygen's two lone electron pairs repelling the shared bonding pairs, makes it a polar molecule capable of hydrogen bonding, while methane's symmetrical tetrahedral shape makes it nonpolar. These structural and bonding differences directly explain why water is liquid at room temperature while methane is a gas, despite both being simple covalent compounds.

A mixture, unlike a chemical compound, is generally made of two or more substances that are physically combined rather than chemically bonded, meaning they can typically be separated again through purely physical methods like filtration, evaporation, or distillation, and they retain their individual chemical properties even while mixed together. Mixtures can be combined in variable, non-fixed proportions, unlike compounds, which always combine their constituent elements in fixed, definite ratios. Common examples of mixtures include air, a combination of gases, and saltwater, a combination of dissolved salt and water, both of which can be separated back into their original components.

A chemical compound differs from a simple mixture mainly because its component elements are combined in fixed, definite proportions through actual chemical bonds, forming an entirely new substance with properties distinct from its constituent elements, and requiring a chemical reaction, rather than simple physical separation, to break it back down into its original elements. For example, water, a compound, always contains hydrogen and oxygen in an exact two-to-one atomic ratio, and its properties, a liquid that extinguishes fire, are completely different from those of its component elements, highly flammable hydrogen gas and oxygen gas that supports combustion.

Salt dissolved uniformly in water is a common example of a homogeneous mixture, meaning its composition and properties are completely uniform throughout the entire mixture, with no visibly distinct particles or separate phases detectable, even under a standard microscope. This uniform, single-phase appearance is characteristic of true solutions, in which the dissolved solute particles are dispersed at the molecular or ionic level throughout the solvent. Homogeneous mixtures contrast with heterogeneous mixtures, like sand and water, where the individual components remain visibly distinct and unevenly distributed throughout the mixture, easily distinguishable to the naked eye.

A mixture of sand and water, where the components remain visibly distinct and unevenly distributed, is an example of a heterogeneous mixture, since the sand particles do not dissolve in the water but instead remain as a separate, visible solid phase that will eventually settle to the bottom due to gravity if left undisturbed. This contrasts with homogeneous mixtures like saltwater, where the dissolved substance is distributed uniformly at the molecular level and cannot be visually distinguished from the solvent. Heterogeneous mixtures can often be separated using simple physical techniques like filtration, decanting, or sedimentation, exploiting the visible, physical distinctness of their components.

Distillation is a separation technique mainly used to separate liquids in a mixture that differ in boiling point, working by heating the mixture until the component with the lower boiling point vaporizes first, then cooling and condensing that vapor separately to collect it as a purified liquid, leaving behind the higher-boiling-point component. This technique is widely used industrially to refine crude oil into its various fractions, such as gasoline, diesel, and kerosene, each with distinct boiling point ranges, and it is also the basis for producing distilled spirits and purifying drinking water by removing dissolved salts and other impurities that do not vaporize at water's boiling point.

Filtration is a common laboratory and industrial technique used mainly to separate insoluble solids from a liquid, by passing the mixture through a porous material, such as filter paper, which allows the liquid and any dissolved substances to pass through while trapping and retaining the larger, undissolved solid particles. This simple physical separation method is widely used in water treatment plants to remove suspended sediment and debris, in laboratories to separate a precipitate from its solution following a chemical reaction, and in everyday applications like coffee filters and vacuum cleaner bags, all relying on the same basic principle of size-based physical separation.