Acids, Bases, and Salts

A base is a substance that reacts with an acid to produce salt and water; it need not dissolve in water. An alkali is specifically a base that dissolves in water and gives hydroxide ions in solution. Thus an insoluble substance showing neutralisation with an acid is a base but not an alkali. It is not automatically an acid, a salt, or an indicator.

Initially, red litmus turning blue shows that the solution is basic, because hydroxide ions change the indicator colour. When acid is added in sufficient amount, its hydrogen ions neutralise the hydroxide ions: H⁺ + OH⁻ → H₂O. If the final solution changes neither red nor blue litmus, it is approximately neutral. A strongly acidic or basic solution would still change one indicator.

Acid rain forms when acidic oxides, mainly sulfur dioxide and nitrogen oxides, dissolve in atmospheric moisture and produce acids such as sulfurous, sulfuric, or nitric acid. These acids increase the hydrogen-ion concentration of rainwater, and pH decreases as hydrogen-ion concentration increases. Therefore acid rain has a lower pH than normal rain. Common salt or absence of dissolved gas does not explain the acidity.

The pH scale describes hydrogen-ion concentration: pH = −log[H⁺]. An acidic solution has a hydrogen-ion concentration greater than that of neutral water and therefore has pH below 7 at ordinary conditions. If the acid is very dilute, its hydrogen-ion concentration is only slightly greater than neutral, so its pH may be just below 7, such as 6 or 6.5. It is not necessarily 0.

The pH scale identifies how acidic or basic an aqueous solution is: values near 14 indicate a very strong base. A slippery feel is also commonly associated with alkaline solutions because they react with skin oils, but touching an unknown chemical is unsafe. Therefore option A is correct: it may be strongly basic and caustic. Pure water is near pH 7, a mild acid has pH below 7, and common salt solution is approximately neutral.

Bleaching powder is a chlorine-containing compound, commonly represented as calcium oxychloride. In moist water it releases available chlorine, which forms disinfecting chlorine species that damage or kill many disease-causing microorganisms. Hence option A gives the scientific reason for its use in water treatment. It does not convert water into sugar, produce calcium metal, or merely change the colour; disinfection is the essential function.

Carbonate ions release carbon dioxide when they react with hydrogen ions supplied by an acid: carbonate plus acid produces a salt, water, and CO2. Merely adding sodium carbonate to a solution that is not acidic need not produce visible gas. Thus option A correctly identifies the need for an acidic reaction medium. The gas is carbon dioxide, not hydrogen; sodium carbonate is soluble and is an ionic salt rather than a metal.

Blue litmus changing to red is the standard indicator test for an acidic medium. Passage of current through an aqueous solution requires mobile charged particles, usually ions, to carry charge between the electrodes. Combining both observations gives the strongest conclusion in option A: the solution is acidic and contains ions. A neutral solution can also contain ions, but options B, C, and D do not fit the stated observations.

A pH near 7 describes the acid-base character of the sodium chloride solution, not the absence of charged particles. Sodium chloride dissolves and dissociates into mobile Na+ and Cl− ions, which carry electric current. Therefore option A is correct: neutrality and ionic conductivity can coexist. Options B and C incorrectly restrict conduction, while option D is false because sodium chloride is soluble in water.

A soil pH of 5 indicates appreciable acidity, which can hinder the availability of nutrients and affect crop growth. Slaked lime, calcium hydroxide, is a base; in a carefully measured amount it neutralises part of the excess acidity and raises the soil pH toward a more suitable range. Hence option A is correct. Vinegar, lemon juice, and dilute acid would add acidity and generally worsen the stated problem.

Adding water causes dilution: the concentration of hydrogen ions per unit volume decreases, so the pH usually moves upward toward 7. However, dilution does not remove every hydrogen ion or automatically create a basic solution. If hydrogen-ion concentration is still greater than hydroxide-ion concentration, the solution remains acidic. Therefore option A correctly explains the observation; the other choices contradict dilution and acid-base behaviour.

The governing concept is the distinction between a base and an alkali. A base is a substance that can neutralise an acid, but bases do not all dissolve in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Therefore option A gives the correct correction; the other statements wrongly reverse acid-base properties or confuse solubility with acidity.

The governing reaction pattern is acid plus carbonate or hydrogencarbonate producing a salt, water, and carbon dioxide. For example, an acid reacting with a carbonate releases CO2, while an acid reacting with a hydrogencarbonate also releases CO2 after the intermediate carbonic acid decomposes. Thus option A correctly states the common products. The other options omit the characteristic gas or name products that are not formed.

The governing concept is the pH scale and dilution. Both pH 2 and pH 4 are below 7, so the solution remains acidic after water is added. Because pH increases, the hydrogen-ion concentration decreases; in fact, a two-unit rise represents a large reduction in hydrogen ions. Therefore acidity has decreased but has not disappeared, making option A correct. The other choices confuse dilution with becoming basic or claim the opposite ion change.

The governing concept is the reaction of acids with certain metals. Curd contains lactic acid and is therefore acidic. If it is stored for some time in copper or brass, the acid may react with the metal surface and produce metal compounds or salts that can contaminate the food. Hence option A gives the correct safety reason. Gold formation is impossible here, copper does not always destroy acids, and the claim that curd has no acid is false.

The governing concept is salt hydrolysis and the relative strengths of the parent acid and base. A salt produced from a strong acid and a strong base, such as sodium chloride from hydrochloric acid and sodium hydroxide, generally does not hydrolyse appreciably in water. Its aqueous solution is therefore approximately neutral, with pH near 7. Salts involving a weak acid or weak base are commonly acidic or basic, so options B and C are not the best answer.

The governing concept is that litmus is only one indicator of acidity, not the complete list of acid properties. Acids can react with active metals to release hydrogen, with carbonates to release carbon dioxide, and with bases in neutralisation reactions to form salt and water. Therefore option A correctly rejects the student’s incomplete claim. Options B and C contradict known reactions and ionisation, while D wrongly says acids are always bases.

The governing concept is acid-base neutralisation. Solution A turns blue litmus red, so it is acidic; solution B turns red litmus blue, so it is basic. When suitable amounts are mixed, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while a salt is produced. The slight temperature rise is consistent with heat released during neutralisation. Therefore option B is correct, not precipitation, evaporation, or sublimation.

The governing concept is the relation between salt hydrolysis and the strengths of its parent acid and base. A pH of 8.5 is above 7, and red litmus turning blue confirms that the solution is basic. Salts formed from a weak acid and a strong base, such as sodium carbonate in the usual school-level classification, generally hydrolyse to produce a basic solution. Hence option B is correct; strong-acid/weak-base salts are usually acidic, while strong-acid/strong-base salts are approximately neutral.

Dilution of a concentrated acid with water is usually exothermic, meaning that considerable heat is released. If water is poured into a small amount of concentrated acid, the water may heat suddenly, boil, and eject corrosive acid droplets. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring and using protection. Therefore option B is correct; the other methods increase splash, pressure, or heating hazards.

An acid shows its characteristic effect through hydrogen ions, more accurately hydronium ions, in aqueous moisture. The gas does not change dry blue litmus because there is no sufficient water medium for these ions to form and move. When moisture is present, the gas dissolves and reacts with water to produce an acidic solution, which turns blue litmus red. Hence option A is correct; the other choices do not explain the dependence on moisture.

In the chlor-alkali process, concentrated aqueous sodium chloride is electrolysed. Water is reduced at the cathode, producing hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Hydrogen is combustible and, when a small sample is brought near a burning splint, it burns with a characteristic ‘pop’ sound. Therefore option B identifies the cathode gas. Limewater tests carbon dioxide, while the litmus and turmeric changes are not the hydrogen test.

Indicator colours reveal the approximate nature of a solution. Phenolphthalein is colourless in acidic and neutral media and turns pink in a sufficiently basic medium. Methyl orange is red in an acidic medium, yellow in a basic medium, and orange near its transition range. The simultaneous observations—colourless phenolphthalein and red methyl orange—therefore indicate an acidic solution. Option B is correct; a basic solution would make methyl orange yellow and usually phenolphthalein pink.

Evolution of carbon dioxide when an acid is added is a characteristic test for a carbonate or hydrogencarbonate. Sodium carbonate reacts with an acid, for example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. In water, Na₂CO₃ undergoes hydrolysis and produces a basic solution because carbonate ions react with water to form hydroxide ions. Ammonium chloride is acidic, sodium chloride is nearly neutral, and copper sulphate is not the required carbonate. Thus option C is correct.

Tooth enamel is mainly a mineral containing calcium compounds, and acids can react with and gradually dissolve this mineral. When the pH in the mouth falls below about 5.5, the environment becomes sufficiently acidic for demineralisation of enamel to become significant, especially with repeated or prolonged exposure. Therefore option C is the most suitable range. A pH near 7 is approximately neutral, while values above 7 or near 14 are alkaline rather than acidic causes of typical enamel erosion.