Acids, Bases, and Salts

The pH scale is logarithmic. A decrease from pH 13 to pH 10 is a change of three units, meaning the hydrogen-ion concentration increases by 10³, or 1,000 times. Equivalently, the hydroxide-ion concentration and the corresponding basic character decrease by about 1,000 times, assuming comparable conditions. The solution is still basic at pH 10, so option A is correct; it has not become acidic.

Sodium hydrogen carbonate, NaHCO₃, is a mild base. In excess stomach acidity, it reacts with hydrochloric acid and reduces the concentration of excess acid: NaHCO₃ + HCl → NaCl + H₂O + CO₂. Therefore, option A is correct because neutralisation can provide relief. Option B is opposite to its action, while C and D do not describe an acid–base reaction.

Dilution adds water and increases the solution’s volume, while the amount of dissolved acid is not instantly removed. Consequently, the number of hydrogen ions per unit volume, or [H⁺], decreases. Since pH = −log[H⁺], a decrease in hydrogen-ion concentration causes pH to rise, although the solution may remain acidic. Therefore option A is correct; dilution does not make it a strong base.

Baking soda is sodium hydrogen carbonate, a basic salt. In baking powder, an acidic component reacts with it and produces carbon dioxide, which helps the dough rise, while also preventing an excess of unreacted alkaline material. If the acid is absent, some baking soda can remain after heating and impart a bitter or soapy taste. Hence option A is correct; the other claims do not describe this reaction.

A strong acid supplies H⁺ ions and a strong base supplies OH⁻ ions. During complete neutralisation, they combine in equal effective amounts: H⁺ + OH⁻ → H₂O. The remaining ions form a neutral salt when the acid and base are suitably matched, so at about 25°C the final solution has pH close to 7. Thus option A is correct; pH 3 or 11 would indicate excess acid or base.

Blue litmus turning red shows that the salt solution is acidic. A salt formed from a strong acid and a weak base, such as ammonium chloride from HCl and NH₄OH, undergoes hydrolysis in water and produces relatively more H₃O⁺ ions. Therefore option A is the suitable combination. A weak acid–strong base salt is generally basic, while a strong acid–strong base salt is approximately neutral.

Both sodium carbonate, Na₂CO₃, and sodium hydrogen carbonate, NaHCO₃, react with a dilute acid to release carbon dioxide. The reactions differ in their salt and water products, but the gas test is the same: CO₂ turns limewater milky because insoluble calcium carbonate forms. Thus option A is correct. A pop sound identifies hydrogen, not carbon dioxide, and chlorine is not produced here.

Ammonia gas itself needs moisture to show its basic behaviour. On the wet litmus, NH₃ dissolves in water and reacts: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The hydroxide ions make the moist paper basic, so red litmus turns blue. Therefore option A is correct. Dry ammonia cannot ionise effectively without water, and the other options contradict the indicator test.

Blue copper sulphate crystals are hydrated copper sulphate, commonly represented as CuSO₄·5H₂O. Heating removes their water of crystallisation and produces white anhydrous CuSO₄. When water is added again, hydration occurs and the characteristic blue hydrated form is restored. Hence option A is correct. No copper metal or sulphur gas is formed, and the colour change is not caused by the solution becoming basic.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, whereas baking soda is sodium hydrogen carbonate, NaHCO₃. Thus they differ in both composition and properties: washing soda is used in cleaning and water softening, while baking soda is used in baking and some neutralisation applications. Option A is correct; the other choices assign incorrect names or formulas to these salts.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode, water gains electrons and produces hydrogen gas and hydroxide ions. Sodium hydroxide remains in the solution. Therefore, chlorine is formed at the anode and hydrogen at the cathode, making option A correct; the other choices reverse or misidentify the products.

Bleaching powder is useful because it contains available chlorine, which is released during suitable reactions and produces the bleaching and disinfecting effect. On prolonged exposure to air, moisture and carbon dioxide, some of this available chlorine may be lost and the material gradually becomes less effective. Thus option A gives the relevant storage-related reason. It does not completely become sodium chloride, gypsum, or washing soda, so options B, C and D are unsuitable.

Plaster of Paris is calcium sulphate hemihydrate, written as CaSO₄·½H₂O. When water is added, it combines with the required water and changes into calcium sulphate dihydrate, CaSO₄·2H₂O, which is gypsum. The formation of the harder solid explains its use for casts and moulds. Therefore option A is correct. Washing soda, baking soda and bleaching powder are different salts and are not produced in this hydration reaction.

Indicator colours must be interpreted together with their transition ranges. Phenolphthalein is colourless in acidic, neutral and mildly basic solutions, becoming pink only in a more strongly basic range. Methyl orange is yellow when the solution is not sufficiently acidic, including neutral and basic conditions. Hence the observations are consistent with a neutral or slightly basic solution, so A is the best answer. A strong base would generally turn phenolphthalein pink.

A pH of 8.5 indicates that the soil is alkaline because it is above 7. To lower basicity, the soil should receive a suitable mildly acidic amendment rather than another strong base. Organic matter can decompose and produce acidic substances gradually, helping improve alkaline soil when used appropriately. Slaked lime, sodium hydroxide and soap are basic materials and would tend to maintain or increase alkalinity. Therefore option A is the most suitable choice.

Marble consists mainly of calcium carbonate, CaCO₃. Acids present in acid rain react with carbonates to form a salt, water and carbon dioxide; in general, acid + carbonate → salt + H₂O + CO₂. Repeated exposure can dissolve or weaken the marble surface and damage carved details. Therefore option A correctly states the governing reaction. The other options either claim an impossible conversion or deny the chemical reaction involved.

Bacteria in the mouth can break down sugars and produce acids. These acids lower the local pH and may gradually demineralise tooth enamel, causing tooth decay. A mildly basic toothpaste can react with and neutralise some of the excess acid, helping protect the enamel. Thus option A expresses the correct acid-base principle. Toothpaste does not dissolve enamel immediately, convert sugar into acid, or make the mouth’s pH zero; those statements are chemically incorrect.

Turning blue litmus red identifies the first substance as an acid. The characteristic neutralisation reaction is acid + base → salt + water. Since the first substance is already identified as an acid and the products are salt and water, the other reacting substance must be a base, or alkaline substance. Therefore option A is correct. Another acid would not normally give this neutralisation pattern, while a neutral gas or an unreactive metal does not fit the stated evidence.

Electrical conduction through a solution requires mobile charged particles. An acid ionises or reacts with water to produce hydronium ions and corresponding negative ions; these ions move toward opposite electrodes and carry electric current. Solid sodium chloride is not necessary, because dissolved ions from the acid are sufficient for conduction. Therefore option A is correct. Colour, the glass container and low temperature do not provide the mobile charge carriers required for an electric current.

Sodium hydroxide is strongly hygroscopic and also deliquescent: it attracts water vapour from the atmosphere and can dissolve in the absorbed moisture. When its pellets are left exposed, they therefore become moist, sticky or apparently wet. This behaviour is a physical absorption process, not the creation of water or conversion into oxygen. Sodium hydroxide pellets do not normally release chlorine merely because they are exposed to air. Hence option A is correct.