Acids, Bases, and Salts

Hydrochloric acid ionises almost completely in water: HCl(aq) → H+(aq) + Cl−(aq), with H+ more accurately represented in water as H3O+. The hydronium or hydrogen-ion concentration gives the solution its acidic properties and allows it to react with bases. Hydroxide ions indicate basicity, whereas sodium and carbonate ions are not produced by hydrochloric acid. Hence option A is correct.

The governing concept is the acid–base nature of oxides. Many non-metal oxides, such as carbon dioxide and sulfur dioxide, react with water to form acids or acidic solutions. These solutions can turn blue litmus red. Therefore, the general nature of non-metal oxides is acidic, making option A correct. Basic and metallic describe other substances, while “salty” is not the usual classification of an oxide.

An acidic oxide is identified by its reaction with bases and, in many cases, by its ability to form an acid with water. Carbon dioxide is a non-metal oxide; it reacts with sodium hydroxide to produce sodium carbonate and water: CO₂ + 2NaOH → Na₂CO₃ + H₂O. Hence option A is correct. Turning red litmus blue indicates a base, and the other choices misidentify carbon dioxide.

Turmeric contains a natural indicator substance called curcumin. In an alkaline or basic medium, curcumin changes from its usual yellow colour to reddish-brown. Thus, when turmeric paper or turmeric paste contacts a base, the reddish-brown colour appears, so option B is correct. Acidic, neutral, and ordinary sugar solutions do not normally produce this characteristic turmeric change.

Phenolphthalein is an acid–base indicator whose colour depends on the medium. It remains colourless in acidic or neutral conditions but changes to pink, often bright pink in a sufficiently basic solution. Therefore option B is correct. Colourless is associated with its acidic form, whereas red and green are not the standard phenolphthalein colour changes used in this test.

Methyl orange is an acid–base indicator with two familiar colour forms. It appears red in an acidic solution and yellow in a basic or nearly neutral solution. Since the question specifies an acidic medium, the correct observation is red, so option A is correct. Yellow is the likely basic-medium distractor, while blue and green are not the standard colours of methyl orange.

The governing concept is indicator colour change with pH. Methyl orange is red in an acidic medium, but in a basic medium its colour is yellow. Therefore option B is correct. Red is the relevant distractor because it is the colour seen in acid, while black and blue are not the characteristic colours of methyl orange in this test. The observation helps distinguish acidic and basic solutions.

Baking soda is the common name of the salt sodium hydrogen carbonate, whose formula is NaHCO₃. It reacts with acids and releases carbon dioxide, which explains its use in baking and some antacid preparations. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is a different salt found in materials such as limestone.

Common salt is the familiar crystalline ionic compound represented by the formula NaCl. It contains sodium ions, Na+, and chloride ions, Cl−, in a regular lattice, so its chemical name is sodium chloride. Sodium carbonate is a different salt, calcium sulphate occurs in gypsum and plaster-related compounds, and sodium hydroxide is a strong alkali. Hence A is correct.

A base is a substance that neutralises an acid and generally produces hydroxide ions in aqueous solution. The special name for a base that dissolves in water is an alkali. For example, sodium hydroxide and potassium hydroxide are soluble alkalis. An acid is chemically different, a salt is a neutralisation product, and an indicator only shows acidity or basicity. Thus option A is correct.

An alkali is not every kind of base; it is specifically a base that dissolves in water and forms an alkaline solution. Some bases, such as copper(II) oxide, are insoluble or only very slightly soluble in water, so they are bases but not alkalis. Bases can react with acids to form salt and water, and they do contain or produce ions. Therefore option A is correct.

Laboratory acids and bases may be corrosive, poisonous, or irritating even when only a small amount is present. Tasting an unknown chemical can therefore cause burns, poisoning, or other injury, so it is never an acceptable identification method. Safe identification uses approved indicators, pH paper, or other supervised tests. The colour and taste of a substance are not reliable criteria. Thus option A is correct.

Blue litmus turning red is the characteristic test for an acidic solution. The red colour of a universal indicator also corresponds to a low pH, generally indicating strong acidity rather than neutrality or basicity. A neutral solution would leave litmus unchanged and show a colour near pH 7, while a basic solution would turn red litmus blue. Therefore option A is correct.

The pH scale describes the acidity of an aqueous solution: pH 7 is approximately neutral, values below 7 are acidic, and values above 7 are basic. Since pH 4 is below 7, the solution has greater hydrogen-ion concentration than a neutral solution and is acidic. It is not basic or metallic, and neutrality would require a value near 7. Therefore option C is correct.

The pH scale indicates the acidic or basic nature of an aqueous solution. A neutral solution has pH 7 at approximately room temperature; values below 7 are acidic, while values above 7 are basic. Since pH 10 is greater than 7, the solution is basic. Therefore, option B is correct. Option A applies below 7, option C applies to pH 7, and insolubility is unrelated to pH.

A universal indicator is a mixture of indicators that shows different colours over a wide pH range. A neutral aqueous solution has a pH of about 7, and the indicator generally appears green at this central, neutral value. Therefore, option C is correct. Red usually represents an acidic solution, while blue or violet generally indicates a basic solution with a higher pH.

Many active metals displace hydrogen from dilute acids. The general reaction is: metal + acid → salt + hydrogen gas. For example, zinc reacts with hydrochloric acid as Zn + 2HCl → ZnCl₂ + H₂. Thus hydrogen is the gas formed, so option B is correct. Carbon dioxide is associated with acid–carbonate reactions, whereas oxygen and nitrogen are not released in this standard reaction.

Hydrogen is a combustible gas. When a sample of hydrogen is brought carefully near a burning splint, it ignites and produces a characteristic ‘pop’ sound because it reacts rapidly with oxygen. Therefore, option C is the correct identification test. Limewater turning milky indicates carbon dioxide, turmeric changing colour suggests a base, and pink phenolphthalein also indicates alkalinity rather than hydrogen.

An acid reacts with a carbonate to produce a salt, water, and carbon dioxide gas. The general equation is acid + carbonate → salt + water + CO₂. For example, hydrochloric acid reacts with calcium carbonate to form calcium chloride, water, and carbon dioxide. Thus option B is correct. Hydrogen is normally released with active metals, not carbonates, while chlorine and ammonia are not products of this reaction.

The reaction in which an acid reacts with a base and their acidic and basic effects are reduced is called neutralisation. In a typical case, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt. Therefore, option C is correct. Filtration separates solids, evaporation removes solvent, and melting is a physical change involving heating.

Neutralisation is the reaction of an acid with a base. In the ionic view, H⁺ from the acid combines with OH⁻ from the base to form H₂O, while the remaining positive and negative ions combine to form a salt. Thus the main products are salt and water, making option A correct. The other pairs do not represent the standard products of an acid–base neutralisation.

During neutralisation, an acid supplies hydrogen ions, H⁺, and a base supplies hydroxide ions, OH⁻. These ions combine in a 1:1 ratio according to H⁺ + OH⁻ → H₂O, producing water. Therefore, option B is correct. Sodium and chloride ions can form sodium chloride, but they do not form water; the other ion pairs are unrelated to the water-forming step.

A base is a substance that can neutralise an acid, but not every base dissolves in water. A base that is soluble in water is specifically called an alkali. When dissolved, an alkali generally produces hydroxide ions in the solution. Therefore, option C is correct. An acid has opposite acidic properties, a salt is a different reaction product, and an indicator only shows acidity or basicity by colour.

The governing concept is the ionisation of bases in aqueous solution. Sodium hydroxide dissociates in water according to NaOH(aq) → Na⁺(aq) + OH⁻(aq). The hydroxide ion increases the basic character of the solution, so option C is correct. Hydrogen ions are associated with acids, while carbonate and chloride ions are not produced by sodium hydroxide alone.

The governing concept is the ionisation of acids in water. Hydrochloric acid dissociates almost completely: HCl(aq) → H⁺(aq) + Cl⁻(aq), with H⁺ commonly represented as hydronium, H₃O⁺, in water. Thus option A is correct. Hydroxide ions indicate bases, carbonate ions come from carbonates, and sodium ions are not present in pure hydrochloric acid.