General Science MCQs – Page 32 (Q311-320)

The standard pH scale used to measure acidity and alkalinity generally ranges from 0 to 14, a logarithmic scale devised by Danish chemist Soren Sorensen in 1909, where each whole number change represents a tenfold change in hydrogen ion concentration in a solution. Values below 7 indicate increasing acidity, values above 7 indicate increasing alkalinity or basicity, and 7 itself represents neutrality, exemplified by pure distilled water. Although the scale technically extends slightly beyond 0 and 14 for extremely concentrated acids or bases, the standard 0 to 14 range covers the vast majority of everyday and laboratory solutions.

On the pH scale, a value of 7 is generally considered neutral, meaning the concentration of hydrogen ions and hydroxide ions in the solution are exactly equal, as found in pure distilled water at standard conditions. This neutral point serves as the dividing line on the scale, with values below 7 becoming progressively more acidic as hydrogen ion concentration increases, and values above 7 becoming progressively more basic, or alkaline, as hydroxide ion concentration increases relative to hydrogen ions. Because the scale is logarithmic, a solution with pH 6 is ten times more acidic than one with pH 7.

On the pH scale, values below 7 generally indicate a solution that is acidic, meaning it has a higher concentration of hydrogen ions than pure water, with lower pH values corresponding to stronger acidity. Common acidic substances include lemon juice at around pH 2, vinegar at around pH 3, and black coffee at around pH 5, while battery acid can be as low as pH 0 or even below. Acids typically taste sour, react with certain metals to release hydrogen gas, and turn blue litmus paper red, distinguishing them clearly from basic, or alkaline, solutions found above pH 7 on the scale.

On the pH scale, values above 7 generally indicate a solution that is basic, or alkaline, meaning it has a higher concentration of hydroxide ions relative to hydrogen ions, with higher pH values corresponding to stronger alkalinity. Common basic substances include baking soda solution at around pH 8 to 9, ammonia solution at around pH 11, and strong drain cleaners or bleach that can reach pH 13 or higher. Bases typically feel slippery to the touch, taste bitter, and turn red litmus paper blue, providing a simple laboratory test to distinguish them from acidic solutions found below pH 7.

Litmus paper commonly turns red when dipped into an acidic solution, a classic and simple test used in chemistry to quickly identify the general nature of an unknown solution, since litmus is a natural dye extracted from lichens that changes color in response to hydrogen ion concentration. This color change occurs because acidic conditions protonate the litmus dye molecules, altering their light absorption properties and shifting their visible color toward red. While litmus paper reliably indicates whether a solution is acidic or basic, it does not provide a precise pH value, unlike more sophisticated tools like a digital pH meter or universal indicator paper.

Litmus paper commonly turns blue when dipped into a basic, or alkaline, solution, providing a simple visual test that complements its behavior in acidic conditions, where it turns red instead. This color-changing property comes from litmus dye, a mixture of naturally occurring compounds extracted from lichens, which has been used as a pH indicator since the Middle Ages, making it one of the oldest chemical indicators still in common use. Red litmus paper is typically used to test for basicity, since it will turn blue in a base, while blue litmus paper is used to test for acidity, since it will turn red in an acid.

Acetic acid is the mild organic acid primarily responsible for the sour, tangy taste of vinegar, typically present in concentrations of about 4 to 8 percent in household vinegar, with the remainder mostly water. Acetic acid is produced through the fermentation of ethanol by acetic acid bacteria, a process that has been used for thousands of years to preserve foods through pickling, since the acidic environment inhibits the growth of many spoilage-causing microorganisms. Beyond culinary uses, acetic acid is an important industrial chemical used in the production of plastics, synthetic fibers, and various solvents.

Citric acid is primarily responsible for the sour taste found in lemons and other citrus fruits, present in particularly high concentrations in lemons and limes, which is why these fruits taste noticeably more sour than oranges or grapefruits that contain relatively less of the acid. Beyond its natural presence in citrus, citric acid is widely produced industrially through fermentation processes and used as a common food preservative and flavoring agent in soft drinks, candies, and canned goods, as well as in cleaning products, where its mild acidity helps dissolve mineral deposits and limescale.

Hydrochloric acid is the strong acid naturally present in the human stomach, secreted by specialized parietal cells lining the stomach wall, where it maintains an acidic environment with a pH typically between 1.5 and 3.5, aiding digestion by breaking down food, activating the digestive enzyme pepsin from its inactive precursor pepsinogen, and killing many harmful bacteria ingested with food. The stomach protects itself from this highly corrosive acid through a thick, continuously renewed layer of mucus lining its inner wall, and when this protective barrier is compromised, conditions like gastritis or peptic ulcers can develop, sometimes treated with acid-reducing or antacid medications.

Lactic acid is the mild acid that gives sour milk and yogurt their characteristic tangy taste, produced when lactic acid bacteria ferment lactose, the natural sugar found in milk, converting it into lactic acid and causing the milk proteins to coagulate, or curdle, giving yogurt its characteristic thickened texture. This same fermentation process is used to produce other foods like cheese and sauerkraut. Lactic acid is also produced within human muscle cells during intense exercise when oxygen supply is insufficient for normal aerobic respiration, a process called anaerobic glycolysis, historically, though somewhat inaccurately, associated with the muscle soreness felt after strenuous physical activity.