General Science MCQs – Page 36 (Q351-360)

Chromatography is a technique commonly used in laboratories to separate mixtures of pigments or dyes, working on the principle that different substances travel at different rates across an absorbent material, such as filter paper, based on their differing solubility and attraction to that material, causing the mixture's individual components to spread out and separate into distinct visible bands or spots. Simple paper chromatography is often demonstrated in schools by separating the different colored pigments that make up black ink or plant chlorophyll extracts. More advanced forms, like gas and liquid chromatography, are essential analytical tools in forensic science, pharmaceuticals, and environmental testing.

Evaporation is a common technique used to recover a dissolved solid from a solution by removing the solvent, usually water, typically through gentle heating, which causes the more volatile solvent to vaporize and escape while the less volatile dissolved solid remains behind as solid crystals once all the liquid has evaporated away. This technique is commonly used to recover salt from saltwater, and on a much larger industrial scale, solar evaporation ponds are used to harvest sea salt by allowing sunlight and wind to naturally evaporate seawater over time. Evaporation differs from distillation, which is used specifically to separate and collect two or more liquids based on differing boiling points.

A catalyst is a substance that speeds up the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy, while itself remaining chemically unchanged at the end of the reaction, meaning it is not consumed and can be reused repeatedly. Catalysts work by temporarily interacting with reactant molecules, often binding them in a specific orientation that makes bond breaking and forming easier, without becoming part of the final product themselves. Industrial catalysts, such as platinum in catalytic converters, and biological catalysts, called enzymes, are essential in countless chemical and biological processes, dramatically increasing reaction rates that would otherwise proceed far too slowly.

Enzymes found in living organisms function biologically as catalysts, specifically protein-based, or in some cases RNA-based, catalysts that dramatically accelerate specific biochemical reactions within cells by lowering the activation energy required, while remaining chemically unchanged and reusable after each reaction cycle. Each enzyme typically has a highly specific three-dimensional shape, called its active site, that binds only to particular substrate molecules, following a lock-and-key or induced-fit model of interaction. Enzymes are essential for virtually all metabolic processes, including digestion, DNA replication, and cellular respiration, and their activity can be affected by factors like temperature and pH.

A chemical reaction that releases heat energy to its surroundings is generally classified as an exothermic reaction, meaning the total energy of the products is lower than that of the reactants, with the difference released as heat, and often light as well, causing the surrounding temperature to rise. Combustion, such as burning wood or fuel, and many oxidation reactions, including cellular respiration, are classic examples of exothermic processes. Exothermic reactions are generally, though not always, more thermodynamically favorable and spontaneous than endothermic ones, since releasing energy typically moves a system toward a more stable, lower-energy state.

A chemical reaction that absorbs heat energy from its surroundings is generally classified as an endothermic reaction, meaning the total energy of the products is higher than that of the reactants, with the additional energy drawn in from the surrounding environment, often causing a noticeable drop in surrounding temperature. Photosynthesis, which absorbs light energy to build glucose from carbon dioxide and water, and the melting of ice, which absorbs heat to break intermolecular bonds, are classic examples of endothermic processes. Endothermic reactions require a continuous energy input to proceed and are generally less spontaneous than exothermic reactions under normal conditions.

Combustion of fuel and cellular respiration are both common examples of exothermic reactions, since both processes release stored chemical energy as heat, and often light in the case of combustion, as high-energy reactant bonds break and lower-energy product bonds form. In fact, cellular respiration is essentially a slow, controlled, biologically regulated form of combustion, breaking down glucose using oxygen to release energy gradually as usable ATP rather than as an uncontrolled burst of heat and flame, while both processes ultimately produce carbon dioxide and water as byproducts, following remarkably similar overall chemical equations despite their very different rates and mechanisms.

Photosynthesis in green plants is a classic textbook example of an endothermic reaction, since it requires a continuous input of light energy, absorbed by chlorophyll pigments, to drive the conversion of low-energy carbon dioxide and water into higher-energy glucose and oxygen, storing that absorbed solar energy within the chemical bonds of the glucose molecule produced. This absorbed energy is later released again through the exothermic process of cellular respiration, when the glucose is broken down to release usable energy, making photosynthesis and respiration complementary opposite processes, one endothermic and energy-storing, the other exothermic and energy-releasing.

The chemical formula for sucrose, commonly known as table sugar, is C12H22O11, a disaccharide formed by the chemical combination of two simpler monosaccharide sugars, glucose and fructose, joined together through a condensation reaction that releases a water molecule. Sucrose is extracted commercially primarily from sugar cane and sugar beet, and when digested in the human body, it is broken back down by the enzyme sucrase into its constituent glucose and fructose molecules for absorption. Unlike glucose, sucrose does not directly participate in cellular respiration and must first be broken down through digestion before its component sugars can be metabolized for energy.

When a reactive metal reacts with a dilute acid, hydrogen gas is commonly released as a product, alongside a corresponding metal salt, following a single displacement reaction in which the more reactive metal displaces the hydrogen from the acid. For example, zinc reacting with dilute hydrochloric acid produces zinc chloride and hydrogen gas, a classic reaction demonstrated in school chemistry laboratories. The vigor of this reaction generally depends on the metal's position in the reactivity series, with highly reactive metals like sodium and potassium reacting violently or explosively, while less reactive metals like copper do not react with dilute acids at all.