Acids, Bases, and Salts

Burning coal, petroleum, and other fossil fuels releases sulphur dioxide and nitrogen oxides. In the atmosphere, these gases undergo oxidation and react with water droplets, producing sulphuric acid and nitric acid or related acidic substances. The acids are then carried to the ground with rain. Oxygen does not directly become salt, sodium chloride does not melt into acids, and limewater evaporation is unrelated; therefore A is correct.

Litmus is a natural acid-base indicator. In an acidic medium, blue litmus changes to red because the indicator responds to the presence of acidic ions. This red colour is a characteristic test for acidity, although it does not by itself measure the exact pH or strength of the acid. A basic solution generally turns red litmus blue, while green or no change is not the standard result for blue litmus in acid. Thus A is correct.

Red litmus paper is an indicator used to test for basicity. When placed in a basic or alkaline solution, it changes from red to blue because the indicator has a different colour in an alkaline medium. Keeping it red would not show the characteristic basic response, and yellow or black are not the standard litmus colours for this test. Therefore option B correctly describes the observation and identifies the solution as basic.

At about 25 °C, the pH scale classifies aqueous solutions: pH 7 is neutral, values below 7 are acidic, and values above 7 are basic or alkaline. A pH below 7 indicates a relatively higher concentration of hydrogen ions than in a neutral solution. Therefore the solution is acidic. Option B describes pH above 7, option C corresponds approximately to pH 7, and option D is not a pH classification.

The pH scale describes the acidic or basic nature of an aqueous solution. A pH of 7 is conventionally neutral at about room temperature, while values greater than 7 indicate a lower hydrogen-ion concentration and a higher relative hydroxide-ion concentration. Therefore, the solution is basic, so option B is correct. Acidic means pH below 7, neutral means pH 7, and “salt-free” is not a pH classification.

The governing concept is the pH scale, which expresses the hydrogen-ion concentration of an aqueous solution. Under usual school-level conditions, a neutral solution has balanced acidic and basic effects and is assigned pH 7. Thus option B, seven, is correct. A pH near 1 is strongly acidic, pH 10 is basic, and pH 14 represents a very strongly basic condition rather than neutrality.

A universal indicator is a mixture of indicators that displays different colours over a broad pH range. A neutral solution has a pH of approximately 7, and at this central point the standard colour shown by the universal indicator is green. Therefore option C is correct. Red generally corresponds to acidic conditions, while yellow is acidic to weakly acidic and violet indicates a strongly basic region.

Universal-indicator colour is governed by the pH of the tested solution. Red is found at the low-pH end of the indicator scale, where the concentration of hydrogen ions is relatively high. A low pH means acidic nature, so option A is correct. A neutral solution is represented near green, basic solutions move through blue shades, and “soapy” describes a possible substance rather than a precise indicator classification.

The universal indicator scale changes towards violet as pH becomes very high. A high pH means the solution is strongly basic, with a relatively high hydroxide-ion concentration compared with hydrogen ions. Therefore option C is correct. Acidic solutions occur at the low-pH, red end of the scale, and a neutral solution is near pH 7 and green, so neither A, B, nor D fits violet.

Turmeric contains curcumin, which acts as a natural acid-base indicator. It normally appears yellow, but in contact with a basic substance it changes to a reddish-brown colour because the alkaline medium changes the indicator form of curcumin. Hence option B is correct. The unchanged yellow colour is not the positive test for a base, and blue or green are not the characteristic turmeric response.

Phenolphthalein is an acid-base indicator whose visible colour depends on the acidity of the solution. In an acidic medium, and also in the nearly neutral range used in school tests, phenolphthalein remains colourless because its coloured alkaline form is not produced. Therefore option B is correct. Pink is observed in a sufficiently basic solution, while blue and green are not its characteristic colours.

Phenolphthalein changes colour because its indicator form is different in an alkaline medium. It is colourless in acidic or insufficiently basic conditions, but in a basic solution it develops a pink colour. Thus option B is correct. Red is commonly associated with acidity for some other indicators, yellow may describe methyl orange in base, and colourless is the acidic response of phenolphthalein rather than its basic response.

Methyl orange is an acid-base indicator with a characteristic colour transition. In an acidic solution, its observed colour is red; as the medium becomes less acidic and then basic, the colour changes through orange toward yellow. Therefore option A is correct. Yellow corresponds to the basic-side response of methyl orange, while green and blue are not its standard acidic colour in this test.

Methyl orange responds to the acidity of the medium by changing its molecular form and colour. It is red in acidic solution and becomes yellow in a basic solution. Consequently, option B is correct. The red option describes its acidic response, whereas black and white are not characteristic colours in the methyl-orange acid-base test. The colour change therefore helps distinguish acidic and basic conditions.

According to the Arrhenius concept, an acid increases the concentration of hydrogen ions in an aqueous solution. In water, these ions are commonly associated with water molecules as hydronium ions, H₃O⁺. Thus the expected school-level answer is hydrogen ion. Hydroxide ions are characteristic of bases, while carbonate and sodium ions are not produced by every acid.

By the Arrhenius definition, a base produces hydroxide ions, OH⁻, when it dissolves or dissociates in water. For example, sodium hydroxide separates as NaOH → Na⁺ + OH⁻. Therefore option B is correct. Hydrogen ions indicate acidity, whereas chloride and nitrate ions may occur in particular salts but are not the defining ions of all bases.

An alkali is specifically a base that dissolves in water and produces hydroxide ions in the resulting solution. Sodium hydroxide and potassium hydroxide are common examples. Therefore option B is correct. An acid has acidic properties, a salt is an ionic product of reactions, and an indicator changes colour to show acidity or basicity; none of these terms means a water-soluble base.

An alkali is not simply any base; it is a base that is soluble in water. Some bases, such as copper(II) oxide, do not dissolve appreciably in water, so they are bases but not alkalis. Hence option A gives the correct reason. Bases can react with acids to form salts, and they do contain particles, so options C and D are false; option B incorrectly says bases are acids.

The reaction in which an acid reacts with a base is called neutralisation because the acidic and basic effects are reduced. In a typical example, HCl + NaOH → NaCl + H₂O: the hydrogen and hydroxide ions combine to form water, while the remaining ions form a salt. Filtration, evaporation, and melting are physical processes, not this chemical reaction.

Neutralisation is the reaction of an acid with a base. Its general pattern is acid + base → salt + water. For example, HCl + NaOH → NaCl + H₂O. The water forms when H⁺ combines with OH⁻, and the remaining ions produce the salt. Therefore option A is correct; the other options describe products unrelated to ordinary acid-base neutralisation.