Acids, Bases, and Salts

Litmus is an acid–base indicator. In an acidic medium, blue litmus changes to red, so option A, acid, is correct. A base shows the opposite familiar change by turning red litmus blue. A neutral salt solution or water generally produces no characteristic litmus-colour change under ordinary conditions, so options C and D do not explain the stated observation.

The governing concept is the action of acid–base indicators. A base turns red litmus paper blue because the indicator responds to alkaline conditions; therefore, Base is correct. Acids produce the opposite change by turning blue litmus red. A neutral substance and many salts generally do not change red litmus in this characteristic way.

A natural indicator is a substance obtained from a natural source that shows different colours in acidic and basic media. Litmus, extracted from lichens, becomes red in an acidic solution and blue in a basic solution, so option A is correct. Sodium chloride, sugar, and distilled water are not ordinary natural acid–base indicators.

Soap consists of sodium or potassium salts of higher fatty acids. These are salts formed from a strong base and a weak acid. On hydrolysis in water, they produce
\(\mathrm{OH^-}\) ions; therefore, an aqueous soap solution is generally basic and may turn red litmus blue. In contrast, a neutral solution has a pH of about 7.

The indicator rule is that bases turn red litmus blue, whereas acids turn blue litmus red. Sodium hydroxide dissolves in water to give an alkaline solution, so it changes red litmus to blue and option C is correct. Lemon juice, vinegar, and dilute hydrochloric acid are acidic substances, so they cannot produce this change.

The pH scale expresses the acidic or basic nature of an aqueous solution. At approximately 25°C, pH 7 is neutral; values below 7 indicate greater hydrogen-ion concentration and an acidic solution. Therefore, option B is correct. A basic solution has pH above 7, while “salty” describes taste or composition, not a pH category.

At 25°C, a neutral solution has pH 7. In a solution with pH greater than 7, the concentration of hydrogen ions is relatively lower and that of hydroxide ions is relatively higher; therefore, the solution is basic. Hence, Basic is the correct answer. A neutral solution has pH exactly 7, whereas a solution with pH below 7 is acidic.

Soap usually contains salts of weak acids and strong bases, and its aqueous solution is alkaline. According to the litmus rule, an alkaline solution changes red litmus paper to blue, so option A is correct. In an acidic or neutral solution, red litmus would remain red; it would not become colourless or change twice.

Dilute acids react with active metals such as zinc or magnesium to form a salt and hydrogen gas. For example, \(\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}\). Therefore, hydrogen is the correct answer. Carbon dioxide is produced when an acid reacts with a carbonate or hydrogen carbonate, not in the usual acid–metal reaction.

A soap solution is generally alkaline because soap produces basic ions in water. The governing indicator rule is that bases turn red litmus paper blue; therefore, option A is correct. Green is not the standard result for red litmus, colourless is not expected, and red would remain only in an acidic or neutral medium.

Many acids found in foods have a sour taste, which is a common preliminary property used to identify acidic substances. Curd contains lactic acid, and this contributes to its sour taste; hence option B is correct. Sweetness, bitterness, and saltiness are not the characteristic taste indicated here. Substances must never be tasted in a laboratory merely for identification.

A characteristic classroom property of many aqueous bases is a slippery or soapy feel. Dissolved basic substances interact with skin oils and can produce this sensation, so option B is the expected answer. Rough, dry, or hard are not the usual touch descriptions of a basic solution. In practice, unknown chemicals should never be touched because some bases are corrosive.

Tomato juice is acidic because it contains naturally occurring organic acids, including citric and malic acids. These acids contribute to its characteristic tangy taste, so option A is correct. A basic solution has alkaline properties, a neutral solution has neither acidic nor basic character, and “saline” only means containing dissolved salt; it does not identify the juice’s pH nature.

Turmeric contains curcumin, a natural indicator that remains yellow in acidic or neutral conditions but changes to reddish brown in a basic medium. Therefore, option B is correct. Blue is the familiar change of red litmus in a base, not the normal turmeric response, while green is not the standard result for this indicator.

Methyl orange is an acid–base indicator whose colour depends on the acidity of the solution. In an acidic medium, it changes to red, so option C is correct. In a basic or alkaline medium it appears yellow, while blue and green are not the characteristic colours of methyl orange. Thus, the observation directly identifies the solution as acidic.

Oral bacteria act on sugars to produce acids, which lower the pH and can increase enamel erosion. A basic toothpaste helps neutralise the acid. In contrast, sugary drinks provide more sugar for bacterial acid production, and acidic fruit juices can increase acidity in the mouth.

Sodium carbonate, \(\mathrm{Na_2CO_3}\), is a salt formed from the strong base \(\mathrm{NaOH}\) and the weak acid \(\mathrm{H_2CO_3}\). In water, the carbonate ion \(\mathrm{CO_3^{2-}}\) reacts with water to form \(\mathrm{OH^-}\) ions, making the solution basic. In contrast, ammonium chloride forms an acidic solution.

Direct answer: A, NaCl, is correct. A salt formed from a strong acid and a strong base generally gives a neutral aqueous solution. Sodium chloride can be formed from hydrochloric acid and sodium hydroxide: HCl + NaOH → NaCl + H₂O. In water, NaCl separates into Na⁺ and Cl⁻ ions. These ions do not undergo appreciable hydrolysis under ordinary conditions, so the solution is generally close to neutral, with pH near 7. Option B, NH₄Cl, is generally acidic because NH₄⁺ is the conjugate acid of the weak base ammonia and reacts with water to produce hydronium ions. Option C, Na₂CO₃, is basic because carbonate ions react with water to form hydroxide ions. Option D, CH₃COONa, is basic because acetate ions are the conjugate base of the weak acid acetic acid. Memory cue: strong acid plus strong base usually gives a neutral salt; weak-acid or weak-base ions can hydrolyse and change the pH.

Sodium hydroxide is a soluble base. When it dissolves in water, it produces hydroxide ions, OH⁻, making the solution alkaline; an alkaline solution turns red litmus paper blue. Therefore, option A is correct. Hydrochloric acid turns blue litmus red, whereas sodium chloride solution and distilled water are generally neutral and do not produce this change.

When bases dissolve in water, they produce hydroxide ions, \(OH^-\); therefore, option B is correct. In contrast, hydrogen ions, \(H^+\), are characteristic of acids. Chloride and carbonate ions may occur in some salts, but they are not the general identifying ions of all bases.

An acid ionises in water and forms mobile ions. For example, HCl forms \(\mathrm{H_3O^+}\) and \(\mathrm{Cl^-}\) ions in water. These ions carry electric charge through the solution, so an acidic solution conducts electricity. In contrast, conduction by free electrons occurs mainly in metals, not in aqueous solutions.

Sodium chloride is formed from hydrochloric acid, a strong acid, and sodium hydroxide, a strong base. Their ions do not undergo significant hydrolysis in water, so sodium chloride produces an approximately neutral solution; option A is correct. Hydrochloric acid is acidic, sodium hydroxide is basic, and ammonium chloride generally gives an acidic solution because of the ammonium ion.

Diluting an acid with water is an exothermic process, so heat is released. Acid is therefore added to water slowly in small portions while stirring, allowing the heat to disperse safely. If water is added to concentrated acid, the small amount of water may heat suddenly and splash; therefore, option B is unsafe.

When an acid is added slowly to water, its ions become hydrated and heat is released. Therefore, dilution of an acid is an exothermic process. An endothermic process absorbs heat, whereas heat is released here. This is why acid is added to water, not water to acid, for safety.

Acid rain has a pH below 5.6. Normal rain can have a pH of about 5.6 because dissolved carbon dioxide forms a weak acid. Acids formed from sulphur dioxide and nitrogen oxides lower the pH further. A pH of 7 is neutral, so option B does not represent acid rain.