Acids, Bases, and Salts

The pH scale classifies aqueous solutions: pH 7 is neutral, a pH below 7 indicates acidity, and a pH above 7 indicates basicity. Because the given pH is 11, it is clearly greater than 7, so the solution is basic and has a comparatively greater hydroxide-ion concentration. It cannot be acidic or neutral, while metallic describes a material rather than its pH nature.

An active metal can displace hydrogen from a dilute acid. The general reaction is: metal + acid → salt + hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Therefore hydrogen is released, making option A correct. Oxygen and nitrogen are not produced in this ordinary acid-metal reaction, while carbon dioxide is characteristic of an acid reacting with a carbonate or bicarbonate.

Hydrogen is a combustible gas. When a sample of hydrogen is brought near a burning splint, it combines rapidly with oxygen and burns, producing the characteristic ‘pop’ sound. Thus option B is the identification test. Limewater turning milky indicates carbon dioxide, turmeric changing colour suggests a basic medium, and blue litmus remaining blue is not a specific hydrogen test.

Acids react with carbonates in a characteristic reaction that produces a salt, water, and carbon dioxide. In general, acid + carbonate → salt + water + CO₂; for example, 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. Therefore option B is correct. Hydrogen is associated with many acid-metal reactions, whereas oxygen and chlorine are not products of this reaction.

Carbon dioxide is identified by passing it through limewater, an aqueous solution of calcium hydroxide. The gas reacts to form insoluble calcium carbonate, making the limewater appear milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Hence option B is correct. A pop sound identifies hydrogen, while litmus or unchanged turmeric does not specifically confirm carbon dioxide.

The reaction between an acid and a base is called neutralisation because the acidic and basic properties counteract each other. 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 A is correct. Evaporation, melting, and filtration are physical processes, not acid-base reactions.

In a typical neutralisation reaction, an acid reacts with a base. Hydrogen ions from the acid combine with hydroxide ions from the base to form water: H⁺ + OH⁻ → H₂O. The other positive and negative ions remain together as a salt, so the main products are salt and water. Therefore option A is correct; the other pairs are unrelated to ordinary neutralisation.

Sodium hydroxide, NaOH, is an ionic compound that dissociates almost completely in water: NaOH → Na⁺ + OH⁻. The high concentration of hydroxide ions gives it strong basic or alkaline properties, so option A is correct. It is not an acid or a neutral salt. Although sodium hydroxide contains a non-metal element, the question asks for the nature of the compound, not the classification of one constituent element.

Hydrochloric acid, HCl, is classified as a strong acid because it ionises almost completely in water: HCl → H⁺ + Cl⁻. This produces a high concentration of hydrogen ions and gives the solution strongly acidic properties. Therefore option A is correct. A weak base, neutral salt, and alkali describe different chemical categories and cannot correctly classify hydrochloric acid.

Ethanoic acid, CH₃COOH, is classified as a weak acid because it ionises only partially in water and establishes an equilibrium between molecules and ions. It still produces hydrogen ions and shows acidic properties, but less extensively than a strong acid such as hydrochloric acid. Therefore, option B is correct; the other choices describe a strong acid, a base, or a salt.

Most metal oxides are basic oxides. They commonly react with acids in a neutralisation reaction to form salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. This acid-neutralising behaviour supports option B. “Acidic” is generally associated with many non-metal oxides, while neutral and salty do not describe the usual chemical nature of metal oxides.

Non-metal oxides are generally acidic. Many dissolve or react with water to form acids, and these acidic solutions can turn blue litmus red. For example, carbon dioxide reacts with water to form carbonic acid. Thus option B is correct. Basic describes the usual behaviour of metal oxides, while metallic and salt-free are not acid–base classifications for these oxides.

Turmeric contains a natural indicator substance, mainly curcumin. In its usual yellow form it changes to reddish-brown when it contacts a basic or alkaline substance, such as a soap solution. This colour change indicates basicity, so option B is correct. Acidic and neutral substances do not normally produce this reddish-brown turmeric test, and sugar is not the relevant chemical category.

Phenolphthalein is a synthetic acid–base indicator. It is colourless in an acidic or sufficiently neutral medium, but its indicator form becomes pink to reddish-pink in a basic solution. Since the question asks specifically about a basic solution, option B, pink, is correct. Colourless belongs to the acidic range, while blue and green are not the standard phenolphthalein result.

Methyl orange is an acid–base indicator whose colour depends on the solution’s pH. In an acidic solution it appears red; as the medium becomes less acidic and then basic, it changes through orange toward yellow. Therefore option B is correct. Yellow is associated with the basic side of its transition, whereas green and blue are not the expected methyl-orange colours in this test.

A universal indicator is a mixture of indicators that displays different colours across the pH scale. A neutral aqueous solution has a pH close to 7, and the corresponding universal-indicator colour is generally green. Therefore option B is correct. Red and orange indicate acidic conditions, while violet represents a strongly basic region, not neutrality.

Bacteria in the mouth can convert food sugars into acids. When the pH around the teeth falls below about 5.5, the mineral hydroxyapatite in enamel can begin to dissolve more readily, increasing the risk of dental caries. Hence option B is correct. A pH of 7 is neutral, while 10 and 14 are alkaline and do not represent the critical acidic threshold.

After eating, oral bacteria may produce acids from sugars, lowering the pH at the tooth surface and promoting enamel demineralisation. A mildly basic toothpaste helps neutralise some of these acids, reducing acidity and supporting tooth protection. Therefore option A is correct. Dissolving enamel, making pH zero, or increasing acid would worsen damage rather than prevent it.

Acidic soil has a pH below the desirable range for many crops. Slaked lime, calcium hydroxide, is a basic substance that reacts with excess soil acidity and raises the pH toward a more suitable level. Thus option A is correct. Vinegar, lemon juice, and dilute acid are acidic materials, so adding them would generally increase rather than correct soil acidity.

The governing concept is neutralisation and soil pH management. Highly basic soil has a pH above 7, so adding more alkaline substances such as lime or sodium hydroxide would make the problem worse. Organic matter, such as compost or manure, decomposes gradually and can produce mildly acidic substances that help reduce excessive alkalinity and improve soil structure. Therefore, option B is correct; soap is not a suitable soil amendment.