Acids, Bases, and Salts

A major use of bleaching powder is disinfecting drinking water. It reacts with water and produces chlorine-containing oxidising species that destroy many disease-causing microorganisms. This makes it valuable in public sanitation, although the amount used must be carefully controlled. It is not a cooking material, it is not added to increase acidity, and it is not used for melting metals. Thus option A correctly states its main use.

Plaster of Paris, or calcium sulphate hemihydrate, has the formula CaSO₄·½H₂O. When mixed with water, it changes back into gypsum and sets into a hard, rigid mass. Because it can be moulded before setting and then provides firm support, doctors use it to make casts around fractured bones. The other options do not describe its characteristic application, so option A is correct.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses one and a half molecules of water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, known as Plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Baking soda, washing soda, and common salt are different salts, so option A is correct.

Water of crystallisation is the fixed number of water molecules chemically associated with the ions in a salt crystal. Blue vitriol, or hydrated copper sulphate, has the formula CuSO₄·5H₂O and is a standard example. On heating, it loses this water and becomes white anhydrous CuSO₄; adding water restores the blue hydrated form. Dry sand, coal, and petrol are not crystalline hydrated salts, so option A is correct.

White copper sulphate is anhydrous CuSO₄, formed after hydrated copper sulphate loses its water of crystallisation on heating. When water is added, CuSO₄ combines with five water molecules to form hydrated copper sulphate, CuSO₄·5H₂O, which is blue. The colour change is therefore evidence of hydration, not a change to red, green, or black. Hence option A is correct.

Litmus is a natural acid-base indicator. In an acidic solution, the concentration of hydrogen ions is sufficiently high to change blue litmus paper to red. This is the characteristic test used to identify acidity. Basic solutions generally turn red litmus blue, while a neutral solution causes no such litmus change; soap solution is usually basic rather than acidic. Therefore option A is the correct answer.

Red litmus turns blue in a basic or alkaline solution because the indicator responds to the hydroxide-rich chemical environment. This colour change is a standard qualitative test for bases. An acidic solution turns blue litmus red, while a neutral solution normally produces no change. A pure sugar solution is generally treated as neutral and therefore does not turn red litmus blue. Thus option B correctly identifies the solution type.

At about 25°C, pH is a logarithmic measure related to hydrogen-ion concentration: pH = −log[H⁺]. A pH of 7 indicates equal effective concentrations of hydrogen and hydroxide ions in a neutral solution, so option C is correct. Values below 7 are acidic and values above 7 are basic. The word “generally” is important because neutrality and the exact pH scale can depend on temperature, but the school-level classification remains pH 7 as neutral.

The pH scale describes the acidic or basic nature of an aqueous solution. A pH of 7 is considered neutral at ordinary conditions, while values below 7 indicate a greater concentration of hydrogen ions than hydroxide ions. Therefore, a solution with pH below 7 is acidic, so option A is correct. Basic solutions have pH above 7, and metallic is not a pH classification.

The pH scale distinguishes acidic, neutral, and basic solutions. A pH of 7 represents neutrality, whereas a value above 7 indicates that hydroxide ions are relatively more abundant than hydrogen ions. Thus, a solution with pH above 7 is basic or alkaline, making option B correct. An acidic solution has pH below 7, while salt-free does not describe pH nature.

A universal indicator is a mixture of indicators that displays different colours across the pH scale. A neutral aqueous solution has a pH of approximately 7, and the standard universal-indicator colour for this condition is green. Therefore option B is correct. Red is associated with stronger acidity, while yellow generally indicates an acidic range and violet indicates a strongly basic range.

Turmeric contains curcumin, which acts as a natural acid-base indicator. Its original yellow colour remains in acidic or neutral conditions, but in the presence of a basic substance it changes to a reddish-brown colour. Hence option B is correct. The unchanged yellow colour is not evidence of a base, and green or blue are not the characteristic turmeric response to alkalinity.

Phenolphthalein is an acid-base indicator whose colour depends on the pH of the solution. It remains colourless in acidic and nearly neutral conditions, but its indicator form becomes pink in a basic solution. Therefore option B is correct. Colourless corresponds to the acidic side of its test, whereas red and yellow are not the standard phenolphthalein colour for detecting a base.

Methyl orange is an acid-base indicator that changes colour over an acidic-to-neutral pH range. In an acidic solution, it appears red; as the solution becomes less acidic and then basic, the colour changes through orange toward yellow. Thus option A is correct. Yellow is associated with the basic-side response, while green and blue are not the characteristic colours used here for an acidic methyl-orange test.

Methyl orange changes colour according to the acidity or basicity of the medium. It is red in an acidic solution and becomes yellow in a basic solution because the indicator exists in a different colour form at higher pH. Therefore option B is correct. Red is the acidic response, while black and white are not characteristic colours of methyl orange in ordinary acid-base testing.

According to the Arrhenius concept, an acid produces hydrogen ions in aqueous solution; more precisely, these ions are present as hydronium ions, H3O+, after reacting with water. In school-level terminology, the expected answer is hydrogen ion, so option A is correct. Hydroxide ions are characteristic of bases, whereas sodium and carbonate ions come only from particular salts or compounds, not from all acids.

In the Arrhenius model, a base is a substance that produces hydroxide ions, OH−, when dissolved in water. These ions account for the characteristic basic properties of the solution, so option B is correct. Hydrogen ions are associated with acids. Chloride and nitrate ions may be released by particular salts, but they are not the defining ions produced by every base in water.

The reaction of an acid with a base is called neutralisation because the acidic and basic properties oppose and reduce each other. In a typical reaction, hydrogen ions combine with hydroxide ions to form water, while the remaining ions form a salt; for example, HCl + NaOH produces NaCl + H2O. Therefore option A is correct. Filtration, evaporation, and melting are physical processes, not acid-base reactions.

Neutralisation is the reaction between an acid and a base. The hydrogen ions supplied by the acid combine with hydroxide ions from the base to form water: H⁺ + OH⁻ → H₂O. The other ions remain and combine to produce a salt. Therefore, salt and water are the main products. Metal and oxygen describe neither neutralisation, while sugar and water or carbon and nitrogen are unrelated products.

The governing ionic concept of neutralisation is that hydrogen ions from an acid react with hydroxide ions from a base. They combine in a 1:1 ratio to form water: H⁺ + OH⁻ → H₂O. Sodium and chloride ions may remain in a sodium chloride solution, but they do not form water. The calcium-carbonate and copper-sulphate pairs also represent other ions, not the water-forming combination. Hence option A is correct.