Acids, Bases, and Salts

Litmus changes colour according to the acid-base nature of a solution. A basic or alkaline solution turns red litmus paper blue, so B is correct. An acidic solution shows the opposite change by turning blue litmus red. A neutral solution normally causes no characteristic litmus change, and “oily” describes a physical property rather than an acid-base category. The observation therefore identifies basicity, although it does not give the exact strength of the base.

At about room temperature, the pH scale classifies a solution with pH 7 as neutral, a value below 7 as acidic, and a value above 7 as basic. A pH below 7 indicates a comparatively higher hydrogen-ion concentration than a neutral solution. Therefore A is correct. The value does not indicate that the solution is metallic, and it cannot be called neutral or basic under this classification.

The pH scale describes the acidic or basic nature of an aqueous solution. A pH of 7 is generally neutral, values below 7 indicate acidity, and values above 7 indicate basicity or alkalinity. Since the question gives a pH greater than 7, the solution is basic. Option A describes pH below 7, option C describes pH 7, and insolubility is unrelated to pH. Therefore, option B is correct.

The pH scale is based on the concentration of hydrogen ions and is used to compare acidity and basicity. At approximately 25°C, a neutral aqueous solution has balanced hydrogen and hydroxide ion concentrations, so its pH is 7. A value such as 3 is acidic, 10 is basic, and 14 represents a strongly basic end of the usual scale. Therefore, option B, seven, is correct.

A universal indicator is a mixture of indicators that displays different colours over a range of pH values. A neutral solution has a pH close to 7, and the indicator colour associated with this neutral point is green. Red and yellow generally indicate acidic conditions, while violet or purple represents a strongly basic region. Therefore, option C, green, is the correct answer.

Universal indicator changes colour according to the pH of a solution. Red appears at the low-pH end of the indicator scale, where the concentration of hydrogen ions is relatively high and the solution is acidic. A basic solution generally gives blue to violet shades, while neutral pH gives green. A sugar solution is not identified by this colour alone. Thus, option A is correct.

The universal indicator colour progresses from red in strongly acidic conditions through green near neutral pH to blue and violet in alkaline conditions. Violet is associated with a high pH, meaning a high hydroxide-ion concentration and strong basic character. Acidic solutions are represented by red or orange shades, and neutral solutions by green. Therefore, option C is the best answer.

Turmeric contains a natural pigment that acts as an acid-base indicator. In its usual form it is yellow, but when it contacts a basic substance, the pigment changes to a reddish-brown colour. This change is useful for recognizing alkaline materials. The yellow colour is the original turmeric colour, while green and blue are not the characteristic basic reaction of turmeric. Hence, option B is correct.

Phenolphthalein is an acid-base indicator whose colour depends on the pH of the solution. In an acidic medium, its indicator form remains colourless; it becomes pink or magenta only in a sufficiently basic medium. Therefore, the absence of colour identifies the acidic condition in this test. The pink option belongs to the basic response, while blue and green are not the characteristic phenolphthalein colours. Option B is correct.

Phenolphthalein is colourless in acidic and nearly neutral conditions, but its structure changes in a sufficiently basic medium and produces a pink colour. Thus, when phenolphthalein is added to a basic solution, the appearance of pink indicates alkalinity. Colourless is associated with an acidic medium, while red and yellow are not the standard phenolphthalein result. Therefore, option C is correct.

Methyl orange is an acid-base indicator with a characteristic colour change over an acidic-to-basic pH range. In an acidic solution it appears red, while in a basic solution it appears yellow; during the transition it may show an intermediate orange shade. Since the question specifies an acidic medium, the expected colour is red. Yellow belongs to the basic condition, so option A is correct.

Methyl orange changes colour according to the acidity or basicity of the solution. It is red in an acidic medium and becomes yellow in a basic medium, so the yellow colour indicates that the solution is alkaline. The red option describes the acidic response, whereas black and white are not the characteristic colours of methyl orange in this test. Therefore, option B is correct.

According to the Arrhenius concept, an acid increases the concentration of hydrogen ions in an aqueous solution. For example, hydrochloric acid ionises in water to form hydrogen ions and chloride ions: HCl → H⁺ + Cl⁻. Therefore, option A is correct. Hydroxide ions are characteristic of bases, while sodium and carbonate ions are present only in particular salts, not in every acid.

By the Arrhenius definition, a base produces hydroxide ions in water or increases their concentration in an aqueous solution. For instance, sodium hydroxide dissociates as NaOH → Na⁺ + OH⁻, and the OH⁻ ions account for its basic nature. Hence option B is correct. Hydrogen ions are associated with acids, whereas chloride and nitrate ions are anions found in particular salts.

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. Every alkali is a base, but every base is not an alkali because some bases, such as copper(II) oxide, do not dissolve appreciably in water. An acid, salt, or indicator has a different chemical role.

The governing definition is that an alkali is a base soluble in water. Some bases are insoluble or only very slightly soluble, so they cannot form an appreciable alkaline aqueous solution and are not called alkalis. Thus option A is correct. The other statements are false: bases are not acids, bases can react with acids to form salt and water, and all bases consist of particles.

A reaction between an acid and a base is called neutralisation because the acidic and basic effects are reduced when hydrogen ions and hydroxide ions combine. The essential ionic change is H⁺ + OH⁻ → H₂O; the remaining ions usually form a salt. Therefore option A is correct. Filtration separates solids, evaporation removes solvent, and melting changes physical state rather than describing an acid–base reaction.

Neutralisation is the reaction of an acid with a base. In the simplest case, hydrogen ions from the acid combine with hydroxide ions from the base to form water: H⁺ + OH⁻ → H₂O. The other ions remain and combine to form a salt, so option A is correct. Metal and oxygen describe other reactions, while sugar, carbon, and nitrogen are not general neutralisation products.

During neutralisation, the characteristic ions of an acid and a base react directly. A hydrogen ion combines with a hydroxide ion to produce one water molecule: H⁺ + OH⁻ → H₂O. Therefore option A is correct. Sodium and chloride ions can form sodium chloride, while calcium–carbonate and copper–sulphate combinations represent salts or precipitates; they are not the ions that directly form water.

An active metal displaces hydrogen from a dilute acid, producing a salt and hydrogen gas. For example, zinc reacts with hydrochloric acid as Zn + 2HCl → ZnCl₂ + H₂↑. Hence option A is correct. Carbon dioxide is commonly released when an acid reacts with a carbonate, whereas oxygen and nitrogen are not the usual products of an acid–active-metal reaction.