Acids, Bases, and Salts

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.

Tooth enamel is mainly a mineral that can be attacked when the mouth becomes sufficiently acidic. Bacteria metabolise sugars and produce acids, lowering the pH around the teeth. When the pH falls below about 5.5, enamel demineralisation becomes more likely and the risk of tooth decay rises. A pH of 7 is neutral, while 10 and 14 are alkaline. Hence option A is correct.

After food is eaten, oral bacteria can produce acids from fermentable sugars. These acids lower the pH and may attack tooth enamel, increasing the risk of decay. A basic toothpaste supplies alkaline substances that neutralise some of the excess acid and help move the mouth toward a safer pH. It does not dissolve enamel or make pH zero. 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.

Red litmus turning blue indicates that the solution is basic. Turmeric also changes from yellow to reddish-brown in the presence of a base, confirming the same conclusion. In aqueous school-level acid-base theory, basicity is associated with an excess concentration of hydroxide ions, OH⁻, rather than hydrogen ions. Chloride and sulphate ions alone do not explain these indicator changes. Hence option B 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.

Limewater is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. Calcium carbonate is an insoluble white precipitate, so it makes the solution appear milky. Therefore, option A is correct; the other choices name substances that are not formed in this test.

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 solubility. A base is a substance that can neutralise an acid, whereas an alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Some bases, such as copper(II) oxide, are insoluble or only sparingly soluble in water. Therefore, option B is correct; options A, C and D contradict the definitions and properties of bases.

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.