Acids, Bases, and Salts

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.

In an acid-base neutralisation, the essential ionic step is H⁺ + OH⁻ → H₂O. The hydrogen ion comes from the acid and the hydroxide ion comes from the base. Sodium and chloride ions, for example, remain as spectator ions and form sodium chloride rather than water. The other listed ion pairs can form salts or other substances, but they are not the direct ions that make water.

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. Thus option A is correct. Carbon dioxide is usually released when an acid reacts with a carbonate, while oxygen and nitrogen are not the normal gaseous products of this metal-acid reaction.

Hydrogen is a combustible gas. When a small amount is brought into contact with a burning splint, it burns rapidly and produces a characteristic ‘pop’ sound, so option B is the identification test. Limewater turning milky indicates carbon dioxide. Turmeric and phenolphthalein are indicators of basic conditions, not specific tests for hydrogen gas. The test must be performed carefully because hydrogen-air mixtures can be explosive.

Acids react with carbonates to form three products: a salt, water, and carbon dioxide gas. The general equation is acid + carbonate → salt + water + CO₂; for example, 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. Therefore option B is correct. Hydrogen is associated with acid-metal reactions, whereas chlorine and ammonia are not normal products of an acid-carbonate reaction.

The governing concept is the lime-water test for carbon dioxide. Lime water is an aqueous solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. The calcium carbonate formed is insoluble and appears as a white suspension, making the solution milky. Therefore, option A is correct; the other colours are not produced in this test.

The governing concept is precipitation during the carbon-dioxide test. Lime water contains dissolved calcium hydroxide. Carbon dioxide reacts with it according to Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is a fine, insoluble white precipitate, and its suspended particles scatter light, producing the observed milkiness. Thus option A is correct. Sodium chloride and sodium hydroxide do not cause this characteristic precipitate, while hydrogen gas is not formed.

The governing concept is neutralisation between a basic metal oxide and an acid. Copper oxide reacts with hydrochloric acid as CuO + 2HCl → CuCl₂ + H₂O. Formation of salt and water from an oxide and an acid is characteristic of a basic oxide. Therefore option B is correct. Copper oxide is not an acidic oxide, it is a solid compound rather than a neutral gas, and it is not itself common salt.

The governing concept is the classification of oxides according to their acid-base behaviour. Carbon is a non-metal, and its oxide, CO₂, reacts with water to form carbonic acid: CO₂ + H₂O → H₂CO₃. It also reacts with bases such as calcium hydroxide. Thus CO₂ is classified as an acidic oxide, so option B is correct. It is not a metal oxide, a basic oxide, or a salt.

The governing concept is the identification of common salts by their chemical names and formulae. Baking soda is sodium hydrogen carbonate, NaHCO₃. When it reacts with an acid, it produces carbon dioxide, water, and another salt; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is found in limestone and chalk.

The governing concept is water of crystallisation in hydrated salts. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O; the ten water molecules are part of its crystalline structure. This makes option C correct. Sodium chloride is common salt, sodium hydrogen carbonate is baking soda, and calcium sulphate hemihydrate is plaster of Paris. Heating washing soda can remove its crystallisation water and change its properties.

The governing concept is the relationship between a common name and the chemical name of a salt. Common salt is sodium chloride, with the formula NaCl. It consists of sodium and chloride ions in an ionic crystal and is widely used in food. Therefore option A is correct. Sodium carbonate is a different salt used in washing soda, sodium hydroxide is a strong base, and calcium carbonate is present in limestone and chalk.