Acids, Bases, and Salts

Tooth enamel is mainly mineral-based and can be attacked when the mouth becomes sufficiently acidic. The commonly taught critical pH for enamel is about 5.5; below this value, demineralisation and tooth decay can begin more readily. Thus option A is correct. pH 7 is neutral, while pH 10 and pH 14 are alkaline and do not represent the acidic threshold asked here.

Food particles and bacterial activity can produce acids in the mouth. A mildly basic toothpaste helps neutralise or reduce these acids, limiting their harmful action on tooth enamel and helping prevent decay. Therefore option A is correct. Toothpaste is not intended to make teeth sour, convert saliva into gas, or transform teeth into metal; those choices do not describe acid-base protection.

Antacids are generally mild bases, such as magnesium hydroxide or aluminium hydroxide. They react with excess hydrochloric acid in the stomach and reduce its acidic effect, often producing salt and water. Hence option A correctly states their main function. Producing more hydrochloric acid or converting food into acid would worsen acidity, and changing stomach temperature is unrelated to neutralisation.

An ant sting commonly contains methanoic acid, which causes local irritation. Baking soda, or sodium hydrogen carbonate, is mildly basic and can neutralise part of this acid when applied appropriately as a dilute paste. The reduced acidity helps lessen the burning sensation, so option A is correct. Increasing acid, causing rusting, or stopping gas is not the relevant chemical action.

Toothpaste is generally mildly basic, whereas acids produced by bacterial action on food particles can attack tooth enamel. The basic substances in toothpaste react with and neutralise these acids, reducing their harmful effect. This is an acid–base neutralisation process that produces less acidic products; simple dilution is not the main reason. Therefore, option B is correct.

Quicklime is calcium oxide, CaO, a basic oxide. When added carefully to acidic soil, it reacts with the excess acidic substances and helps neutralise them, bringing the soil closer to a suitable pH for crops. Hydrochloric acid and vinegar are acids and would increase acidity, while sodium chloride is not the appropriate neutralising material. Hence, option A is correct.

Carbonate ions react with dilute HCl to release carbon dioxide: \(\mathrm{CO_3^{2-}+2H^+\rightarrow CO_2+H_2O}\). The \(\mathrm{CO_2}\) reacts with lime water, \(\mathrm{Ca(OH)_2}\), and forms an insoluble white precipitate of \(\mathrm{CaCO_3}\). This precipitate makes the lime water appear milky. Chloride, sulfate, and nitrate ions do not release \(\mathrm{CO_2}\) with dilute HCl. Exam tip: gas evolution followed by milky lime water is the standard test for a carbonate ion.

Bleaching powder, commonly represented in school chemistry as calcium oxychloride, releases chlorine or chlorine-containing species that destroy many disease-causing microorganisms. Therefore, it is widely used to disinfect drinking water and in sanitation. Baking uses substances such as baking soda, it does not increase stomach acid, and it has no role in melting iron. Thus, option A is correct.

Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses part of its water of crystallisation and forms calcium sulfate hemihydrate, CaSO₄·½H₂O, known as Plaster of Paris. Common salt and sodium chloride are the same substance and do not produce it; baking soda also gives a different product on heating. Therefore, option A is correct.

Antacids contain mild bases, such as suitable hydroxide, carbonate, or bicarbonate compounds. These bases react with excess hydrochloric acid in the stomach, reducing the concentration of hydrogen ions and relieving acidity. The process is neutralisation, not acid production or conversion into a stronger acid. Cooling food is unrelated to the chemical action. Therefore, option A correctly revises the statement.

In the absence of water, dry HCl gas does not ionise to form H⁺ or H₃O⁺ ions. Therefore, dry blue litmus paper shows no colour change. In contrast, moist blue litmus turns red because HCl dissolves and ionises in water; thus, option B applies only to moist litmus. Exam tip: HCl shows acidic behaviour only after ionisation in water.

Blue copper sulfate crystals are hydrated copper sulfate, CuSO₄·5H₂O, containing a fixed amount of water of crystallisation. Heating removes this water and produces anhydrous copper sulfate, which is white. The copper does not become metal, oxygen is not added, and common salt is not formed. If water is added again, the blue hydrated colour can return. Hence, option A is correct.

Sodium hydroxide is a strong base and dissociates to provide OH⁻ ions. These ions react with hydrogen ions from the acid: H⁺ + OH⁻ → H₂O. Consequently, the hydrogen-ion concentration decreases and the pH rises, initially moving the solution towards neutrality. Sodium hydroxide is not an acid, and a pH change does not require adding pure water separately. Thus, option A is correct.

A hydrated salt contains a definite number of water molecules incorporated into its crystal structure. In CuSO₄·5H₂O, the dot and 5H₂O show that each formula unit of copper sulfate is associated with five water molecules; it is blue hydrated copper sulfate. The other listed formulas do not show such fixed crystallisation water in this context. Therefore, option B is correct.

Baking powder contains baking soda (sodium hydrogen carbonate) together with a mild edible acid such as tartaric acid. On receiving moisture and heat, they react to release carbon dioxide, which makes cake or bread batter rise. Sodium chloride is common salt and is not the acidic component of baking powder. Exam tip: baking soda is sodium hydrogen carbonate alone, whereas baking powder also contains an edible acid.

Baking powder contains sodium hydrogen carbonate and an edible acid. In the presence of moisture and heat, they react to release carbon dioxide gas. Its bubbles spread through the cake batter and make it rise. Water vapour may form on heating, but carbon dioxide is the main gas responsible for raising the cake. Exam tip: Link baking powder and the rising of cakes with carbon dioxide.

Zinc oxide, ZnO, is an amphoteric oxide, meaning it can react with both acids and bases. With hydrochloric acid it forms zinc chloride and water; with sodium hydroxide it forms a soluble zincate compound. Sodium chloride and copper sulfate do not show this amphoteric behaviour, while calcium carbonate mainly reacts with acids. Hence, option A is the best and unambiguous answer.

Sodium chloride is formed from the strong acid hydrochloric acid and the strong base sodium hydroxide. Its ions do not hydrolyse appreciably in water, so its aqueous solution is approximately neutral. Sodium carbonate and sodium acetate, salts of weak acids, give basic solutions, whereas ammonium chloride, from a weak base and strong acid, gives an acidic solution. Therefore, option A is correct.

Acid rain is rainfall with a pH generally below 5.6. Clean rainwater can have a pH of about 5.6 because dissolved carbon dioxide forms a weak acid; acids formed from sulfur dioxide and nitrogen oxides lower it further. A pH of 7.0 is neutral, so it is not the acid-rain limit. Exam tip: Remember the standard cutoff for acid rain as pH below 5.6.

The governing concept is the Arrhenius definition of an acid. When an acid dissolves in water, it increases the concentration of hydrogen ions, often represented as H⁺; for example, HCl separates into H⁺ and Cl⁻ in aqueous solution. A base generally produces OH⁻ ions, while a salt dissociates into its constituent ions and an indicator mainly changes colour. Therefore, option A is correct.