Acids, Bases, and Salts

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.

The governing concept is the identification and use of common salts. Baking soda is the common name for sodium hydrogen carbonate, whose chemical formula is NaHCO₃. When heated, it produces carbon dioxide, and this gas helps dough or batter rise. Sodium chloride is common salt, calcium carbonate is found in substances such as limestone, and calcium sulphate hemihydrate is plaster of Paris. Thus, option B is correct.

The governing concept is water of crystallisation in salts. Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O, so each formula unit contains ten molecules of crystallisation water. It is used for cleaning and for removing the permanent hardness of water. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and sodium chloride is common salt. Therefore, option A is correct.

The governing concept is the identification of an important everyday salt. Common salt is sodium chloride, with the chemical formula NaCl. It is obtained mainly from seawater or rock salt and is used in food as well as in the chlor-alkali process to produce sodium hydroxide, chlorine, and hydrogen. Sodium carbonate, calcium sulphate, and sodium hydroxide are different compounds with different uses. Thus, option A is correct.

The governing concept is the distinction between bases and alkalis. A base is a substance that can neutralise an acid, but some bases do not dissolve in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in aqueous solution. Therefore, the water-soluble base in the question is an alkali, so option B is correct. Acid, salt and indicator are different classes of substances.

The governing concept is the definition of an alkali. Every alkali is a base, but an alkali must also be soluble in water. Some bases, such as copper(II) oxide, do not dissolve appreciably in water; they can still neutralise acids but are not called alkalis. Thus option A correctly explains the one-way relationship. The other statements either confuse acids with bases or give chemically false claims.

The governing concept is acid–base neutralisation. An ant sting commonly introduces formic acid, which can cause irritation. A mild solution of baking soda, a weak base, can partially neutralise the acid and may reduce discomfort. Therefore option A is the suitable school-level answer. Lemon juice, vinegar and another dilute acid are acidic substances, so applying them would not provide the intended neutralising effect. Any serious reaction requires proper medical care.

The governing concept is the pH scale, on which 7 is neutral, values below 7 are acidic and values above 7 are basic at ordinary school-level interpretation. Because 5 is less than 7, the solution is acidic; option A is correct. It may contain a salt, so “salt-free” cannot be inferred from pH. It is neither neutral nor basic, making options B, C and D incorrect.

The pH scale indicates the acidic or basic nature of an aqueous solution: pH below 7 is acidic, pH 7 is neutral, and pH above 7 is basic. Since the given pH is 9, it is greater than 7, so the solution is basic and option B is correct. Option A describes a pH below 7, option C applies near pH 7, and volatility is a physical property unrelated to pH.

Neutrality in an aqueous solution means that acidic and basic effects are balanced; at about 25°C, the concentrations of hydrogen ions and hydroxide ions are approximately equal. On the usual school-level pH scale, this condition corresponds to pH 7. Therefore option B, seven, is correct. A pH of 3 is acidic, while 11 and 14 indicate basic solutions, so those distractors do not describe neutrality.

A universal indicator is a mixture of indicators that displays different colours over a range of pH values. A neutral solution has a pH of approximately 7, and the indicator colour associated with this middle point is green. Thus option B is correct. Red generally indicates strong acidity, while blue and violet indicate increasingly basic conditions; therefore those colours are not expected for a neutral solution.

Universal indicator changes colour according to the pH of a solution. Red occurs at the low-pH end of its colour range, showing a high concentration of hydrogen ions and therefore a strongly acidic nature. Hence option A is correct. Green is associated with near-neutral conditions, while blue or violet indicates basic conditions. The word salty describes taste or composition and is not a pH classification.

The universal-indicator colour scale progresses from red at low pH through green near pH 7 to blue and violet at high pH. Violet therefore represents a very high pH and a strongly basic solution. Option C is correct. Acidic and slightly acidic solutions have pH below 7 and appear toward the red or orange side, while a neutral solution is represented near green, so options A, B, and D are unsuitable.

The reaction between an acid and a base is called neutralisation. Hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions combine to form a salt. For example, HCl + NaOH → NaCl + H₂O. Therefore option A, salt and water, is correct. The other pairs are not the general products of an acid-base neutralisation reaction.

Baking soda is the common name for sodium hydrogen carbonate, whose formula is NaHCO₃. It is a mild basic salt and reacts with acids to release carbon dioxide, which explains its use in baking and some antacid preparations. Hence option A is correct. Sodium carbonate decahydrate is washing soda, calcium sulphate is a different salt, and sodium chloride is common table salt, so none of those alternatives identifies baking soda.

The governing concept is the colour change of an acid-base indicator. Litmus is a natural indicator: blue litmus changes to red when it is placed in an acidic solution, because the indicator responds to the acidic conditions. Hence option A is correct. A basic solution would turn red litmus blue, while a neutral solution normally causes no such characteristic change. “Metallic” is not a solution nature identified by litmus.

The governing concept is the litmus test for bases. Red litmus turns blue when it contacts a basic or alkaline solution, so the observed colour change identifies the solution as basic. Therefore option B is correct. An acidic solution turns blue litmus red, and a neutral solution generally produces no characteristic litmus change. Being salt-free is unrelated to this indicator result and cannot be concluded from the observation, so A, C, and D are incorrect.

The governing concept is the pH scale. At approximately 25°C, a solution with pH 7 is considered neutral, values below 7 indicate increasing hydrogen-ion concentration and acidic nature, and values above 7 indicate basic nature. Thus a pH less than 7 identifies an acidic solution, making option A correct. Basic and neutral describe the other pH ranges, while insolubility concerns dissolving and is unrelated to pH.

The reaction in which an acid reacts with a base is called neutralisation. The acidic effect of hydrogen ions and the basic effect of hydroxide ions are reduced because H+ and OH− combine to form water; the remaining ions generally form a salt. For example, HCl + NaOH → NaCl + H2O. Evaporation, filtration, and melting are physical processes, not acid-base reactions, so option A is correct.

Neutralisation is an acid-base reaction. The hydrogen ions supplied by the acid combine with hydroxide ions supplied by the base to produce water: H+ + OH− → H2O. The positive and negative ions left from the acid and base combine to form a salt. Thus, the principal products are salt and water, as in HCl + NaOH → NaCl + H2O. The other choices are unrelated to ordinary neutralisation.

The essential ionic step in neutralisation is H+ + OH− → H2O. Acids furnish hydrogen ions in aqueous solution, while soluble bases furnish hydroxide ions. These ions combine to form water; spectator ions such as Na+ and Cl− remain in solution and can form sodium chloride. Calcium-carbonate and copper-sulphate pairs do not represent the general water-forming step, so option A is correct.

Sodium hydroxide is a soluble base and dissociates in water: NaOH(aq) → Na+(aq) + OH−(aq). The hydroxide ions increase the concentration of OH− in the solution and are responsible for its basic properties, such as changing indicators and neutralising acids. Sodium ions do not cause basicity, while hydrogen, carbonate, and chloride ions are not the characteristic ions released by NaOH in this process. Therefore option B is correct.