Acids, Bases, and Salts

The relevant experiment compares dry hydrogen chloride with hydrogen chloride dissolved in water. Dry HCl gas does not change the colour of dry litmus because ions cannot move or form effectively without an aqueous medium. When HCl dissolves in water, it ionises to H₃O⁺ and Cl⁻, and the hydronium ions produce acidic behaviour. Thus the lack of effect on dry litmus demonstrates the need for water.

Indicator colours reveal the solution’s acid-base nature. Methyl orange is yellow in a basic or sufficiently less acidic medium, while phenolphthalein is pink in a basic medium, generally above its transition range. Since both observations are consistent with a basic solution, option C is correct. An acidic solution would make methyl orange red and phenolphthalein colourless; a neutral solution would not make phenolphthalein pink.

Both indicator tests identify an acidic medium. Methyl orange turns red in an acidic solution, and acids turn blue litmus red because hydrogen or hydronium ions are present. Therefore the observations independently support option A. A basic solution would turn methyl orange yellow and blue litmus would remain blue, while a neutral solution would not produce either red change. Being a salt solution alone does not guarantee basicity.

The governing concept is water of crystallisation, which is chemically bound water present in certain salt crystals. Blue copper sulphate is hydrated copper sulphate, CuSO4·5H2O. On heating, it loses this water and changes from blue to white anhydrous copper sulphate; adding water restores the blue colour. The other options do not show this characteristic reversible colour change in this test.

A hydrated salt contains a definite number of water molecules within its crystal structure. Washing soda is sodium carbonate decahydrate, Na2CO3·10H2O, so each formula unit is associated with ten molecules of water of crystallisation. Therefore, option A is correct. It is still an ionic salt; it is not merely water, ion-free, or a gas, as claimed by the other options.

Plaster of Paris is prepared by heating gypsum, CaSO4·2H2O, under controlled conditions so that part of its water of crystallisation is removed, producing CaSO4·½H2O. If gypsum is overheated, excessive water may be removed and anhydrous calcium sulphate can form. This material does not rehydrate and set in the desired way, so option C is correct; the other choices describe impossible changes.

The relevant property is the setting reaction of plaster of Paris. When water is added to calcium sulphate hemihydrate, it changes back into gypsum, CaSO4·2H2O, and forms a hard solid mass. It can therefore be shaped in a mould before setting. Option A is correct; immediate dissolution, acid formation, and hydrogen release are not properties responsible for making statues or moulds.

On heating, baking soda decomposes according to 2NaHCO3 → Na2CO3 + H2O + CO2. The sodium carbonate left in the food is alkaline and can impart an unpleasant bitter or soapy taste when present in excess. Thus option A correctly identifies its basic nature. It is not a strong acid, a gas, or sugar, so options B, C, and D are chemically unsuitable.

Baking powder contains sodium hydrogen carbonate and a mild edible acid. During mixing and heating, the acid reacts with sodium hydrogen carbonate and releases carbon dioxide: acid + NaHCO3 → salt + water + CO2. The gas expands and forms bubbles that remain in the batter, making the cake light and fluffy. Therefore option C is correct; the other gases are not produced.

Hard water contains Ca2+ and Mg2+ ions. These ions react with soap anions to form insoluble scum, so less soap remains available for lather. Washing soda, Na2CO3, supplies carbonate ions that precipitate calcium and magnesium as insoluble carbonates, softening the water. Hence option A is correct; the other choices do not describe softening or lather formation.

The aqueous solution of sodium chloride is called brine. During electrolysis, chloride ions are oxidised at the anode to form chlorine, while water is reduced at the cathode to form hydrogen and hydroxide ions. The sodium ions combine with hydroxide ions to produce sodium hydroxide in solution. Thus option A lists all three useful products; the other options are unrelated.

In electrolysis of brine, chloride ions lose electrons at the anode and form chlorine gas: 2Cl− → Cl2 + 2e−. Chlorine reacts with water to produce disinfecting species, so it is widely used in controlled water treatment. Therefore option A is correct. Making milk sour, setting plaster, and dissolving tooth enamel are not the intended uses of chlorine in this process.

At the cathode during brine electrolysis, water gains electrons and produces hydrogen gas: 2H2O + 2e− → H2 + 2OH−. Hydrogen is used to hydrogenate unsaturated vegetable oils, converting them into more saturated fats such as vanaspati. Hence option B is correct. Bleaching powder is associated with chlorine, while the remaining options do not describe hydrogen’s relevant industrial use.

The chlor-alkali process electrolyses brine, or concentrated sodium chloride solution, and produces chlorine, hydrogen, and sodium hydroxide (NaOH). The alkali left in solution is sodium hydroxide, which is used in making soaps and detergents, paper, and other chemicals. Therefore, option A is correct. The other choices describe no standard industrial use of sodium hydroxide in this context.

Bleaching powder, commonly represented as calcium oxychloride (CaOCl2) at school level, is prepared by passing chlorine gas over dry slaked lime, Ca(OH)2. The reaction produces the bleaching compound used for disinfecting water and bleaching materials. Thus option A gives the correct preparation method. The other mixtures do not provide the required chemical reaction or product.

Bleaching powder is useful because it contains available chlorine, which gives it bleaching and disinfecting action. Moisture and contact with air can gradually decompose or reduce the available chlorine, lowering the substance’s effectiveness. Keeping it in a dry, tightly closed container slows this deterioration. Therefore, option A is correct; the other options describe impossible changes unrelated to its storage.

Marble contains calcium carbonate, CaCO3, a carbonate salt. Acids react with carbonates to form a salt, water, and carbon dioxide; in general, acid + carbonate produces salt + H2O + CO2. Repeated acid-rain contact can therefore dissolve or roughen the marble surface and damage structures. Option A correctly states the governing reaction, while the other claims contradict it.

After meals, bacteria may break down sugars and produce acids. These acids lower the mouth’s pH and can gradually demineralise the calcium-containing enamel. Rinsing with water helps remove food residues and dilute or reduce the acidic material, limiting its contact with enamel. Hence option A is correct. The other choices either reverse the effect or make scientifically impossible claims.

Tooth enamel is damaged more readily when the mouth becomes acidic. A pH below about 5.5 is commonly taken as the critical level at which enamel minerals begin to dissolve more significantly, increasing the risk of decay. Therefore, option B is correct. A neutral pH near 7 and absence of acid are less favourable for decay, while pH 14 is strongly alkaline and not the stated condition.

Excess hydrochloric acid in the stomach can cause a burning sensation and indigestion. Antacids contain mild bases, such as magnesium hydroxide or other suitable basic compounds, which react with and neutralise part of the excess acid. This raises the pH toward a less acidic level and provides relief. Thus option A is correct; the other options do not describe neutralisation.

In school science, an ant sting is considered to contain formic acid. Baking soda, or sodium hydrogen carbonate, is mildly basic. It reacts with the acid in a neutralisation reaction and reduces its acidic effect, so option B is correct. Option A is wrong because baking soda is basic, not acidic. Exam tip: A suitable mild base is used to reduce the effect of an acid.

A very low pH indicates strongly acidic soil. Such acidity can change the availability of mineral nutrients, reduce the uptake of some essential elements, and sometimes increase the solubility of substances harmful to roots. Consequently, plant growth may be reduced or otherwise disturbed. Option A is therefore correct. The other options ignore the biological and chemical effects of soil acidity.

Soil pH indicates whether soil is acidic, neutral, or basic. Testing it helps farmers select suitable crops and decide whether an amendment is needed, such as lime for excessive acidity or suitable organic matter and management for other conditions. Therefore, option A is correct. Testing does not transform soil, supply water, or change plants into metals, as the distractors claim.

Turmeric contains the natural pigment curcumin, which acts as an acid-base indicator. It remains yellow in an acidic or neutral medium but changes to a reddish-brown colour in a basic medium, such as a soap solution. Therefore, turmeric in option A can identify a basic solution. Common salt, sugar, and pure water do not show this characteristic indicator colour change in the stated test.

Red and blue litmus provide only a broad classification and may show little or no change for a nearly neutral solution or a weak substance. A universal indicator gives a range of colours corresponding to approximate pH values, so it provides more information. Option A is correct. Tasting, rubbing, or putting an unknown solution in the eye is unsafe and scientifically invalid.