Acids, Bases, and Salts

The governing concept is the indicator test for acids and bases. Litmus is a natural acid–base indicator: in an acidic solution, blue litmus changes to red because the acidic medium affects the indicator dye. Blue is the unchanged colour in a neutral or non-acidic condition, while yellow and green are not the standard litmus result. Hence, option A, red, is correct.

The governing concept is the litmus test for a base. A basic or alkaline solution changes red litmus to blue because the indicator responds to the presence of basic ions, commonly associated with OH⁻ in water. Red is the original colour of the test paper, whereas white and orange are not the standard outcomes. Therefore, the correct observation is blue, given in option B.

The governing concept is the common property of acids. Many familiar acidic substances have a sour taste; for example, lemon juice contains citric acid and vinegar contains acetic acid. Thus, sourness is generally associated with an acidic nature. Basic, neutral, and metallic are different classifications and do not explain the usual sour taste. Chemicals must never be tasted in a laboratory, so this property is learned from safe examples. Option A is correct.

The governing concept is the characteristic physical behaviour of bases. Many bases feel slippery or soapy because they react with oils and affect the skin’s surface; soap solutions are commonly alkaline. Acids, ordinary salts, and gases are not identified by this typical soapy sensation. However, unknown substances should never be touched because some bases are corrosive. Therefore, option B, base, is the correct answer.

The governing concept is the identification of naturally occurring acids in food. Lemon juice contains citric acid, which contributes to its characteristic sour taste. Lactic acid is associated with curd and other fermented milk products, acetic acid is the principal acid in vinegar, and formic acid is associated with ant stings. Therefore, the acid identified in lemon juice is citric acid, so option A is correct.

Vinegar is mainly a dilute aqueous solution of acetic acid, so option B is correct. The acid gives vinegar its characteristic sour taste and pungent smell. Citric acid is commonly associated with citrus fruits, lactic acid with curd and fermentation, and oxalic acid with some plants such as spinach. These comparisons help distinguish the distractors while reinforcing the key association: vinegar contains acetic acid.

The governing concept is the formation of acids during fermentation. Bacteria convert some lactose in milk into lactic acid as curd forms, and this lactic acid produces the characteristic sour taste. Citric acid is linked with citrus fruits, formic acid with sources such as ant stings, and sulphuric acid is a strong mineral acid rather than the normal acid in curd. Therefore, option A is correct.

An ant sting contains formic acid, which is also called methanoic acid. This acid can cause irritation or a burning sensation at the sting site. Citric acid occurs in sour fruits such as lemons, while acetic acid is found in vinegar. Exam tip: Remember ant sting with formic (methanoic) acid.

The governing concept is the association of natural food substances with their characteristic acids. Tomato contains organic acids, including oxalic acid, and this contributes to its mildly sour taste. Hydrochloric, nitric, and sulphuric acids are strong mineral acids commonly studied as laboratory or industrial acids, not the expected natural acid in tomato for this school-level question. Therefore, option A, oxalic acid, is the intended correct answer.

The governing concept is the acid–metal displacement reaction. An active metal displaces hydrogen from a dilute acid, producing a salt and hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Hydrogen can be tested cautiously by the squeaky-pop test under supervision. Oxygen, nitrogen, and chlorine are not the general products of this reaction. Therefore, option A is correct.

The governing concept is the indicator property of acids and bases. Hydrochloric acid dissolves in water and produces hydrogen ions, giving the solution acidic character; acids change blue litmus to red. Therefore option B is correct. Sodium hydroxide is basic and changes red litmus to blue, while sodium chloride solution and distilled water are generally neutral and do not produce this red colour change.

According to the aqueous theory of acids and bases, an acid ionises or reacts with water to increase the concentration of hydrogen ions, written as H⁺ in school-level chemistry. Hence option A is correct. A base generally increases hydroxide ions, OH⁻, while a salt dissociates into its constituent ions and an indicator mainly changes colour to show acidity or basicity.

Litmus is an acid-base indicator: a basic solution changes red litmus to blue. Sodium hydroxide dissolves in water and provides hydroxide ions, so its solution is basic; therefore option C is correct. Hydrochloric acid and vinegar are acidic and turn blue litmus red. Distilled water is approximately neutral, so it normally causes no litmus colour change.

An acid reacts with a metal hydrogen carbonate to form a salt, water and carbon dioxide. For example, \(\mathrm{NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2}\). Therefore, carbon dioxide is the correct answer. Hydrogen is generally produced when an acid reacts with a metal, not with a hydrogen carbonate. Exam tip: Remember that acids produce \(\mathrm{CO_2}\) with carbonates and hydrogen carbonates.

The governing idea is that hydrogen chloride shows acidic behaviour in the presence of water. Dry HCl gas cannot release hydrogen ions on completely dry litmus, so dry blue litmus does not change. In moisture, HCl dissolves and forms an acidic solution, turning blue litmus red. Thus option B is correct; it also rejects the opposite colour change and the incorrect claim of neutrality.

A basic oxide is generally a metal oxide that reacts with water to form a base or reacts with an acid to form salt and water. Magnesium oxide, MgO, is a metal oxide and can form magnesium hydroxide: MgO + H₂O → Mg(OH)₂. Therefore option B is correct. CO₂ is an acidic non-metal oxide, HCl is an acid, and NaOH is a base rather than an oxide.

The error is ignoring the need for water in showing acidity. Dry hydrogen chloride molecules do not produce hydrogen ions on dry paper, so dry blue litmus remains unchanged. When the gas dissolves in moisture, it forms an acidic solution containing H⁺ ions and turns moist blue litmus red. Therefore option A is correct; B reverses the colour change, C wrongly calls HCl neutral, and D reverses the role of bases.

The pH scale indicates the acidic or basic nature of an aqueous solution. At approximately 25 °C, pH 7 is neutral; values below 7 indicate an increased hydrogen-ion concentration and therefore acidic character. Thus option A is correct. A basic solution has pH above 7, a neutral solution is around 7, and “metallic” is not a pH classification.

On the pH scale, a value of 7 is approximately neutral at room temperature, while a value greater than 7 indicates a basic or alkaline solution. Therefore option B is correct. Such a solution has relatively fewer hydrogen ions and greater hydroxide-ion influence, and it can turn red litmus blue. An acidic solution has pH below 7; neutral is around 7, and “salty” does not by itself define pH.

The pH scale compares hydrogen-ion concentration with the neutral reference. At approximately 25 °C, pH 7 represents a neutral solution, in which acidic and basic effects are balanced. Thus option A is correct. A pH below 7 indicates acidity and a pH above 7 indicates basicity. “Corrosive” describes a possible chemical effect, not a specific pH classification, so it is not the answer.