Acids, Bases, and Salts

The governing concept is the pH colour scale of a universal indicator. Blue generally represents a basic solution, while green is near neutral, around pH 7. When acid is added, hydrogen-ion concentration increases and the basic character is reduced. Thus the solution moves from basic toward neutral, making option A correct. Option B predicts the opposite change, and C would require a strongly acidic colour rather than green.

The governing reaction pattern is acid plus carbonate producing salt, water and carbon dioxide: carbonate + acid → salt + H₂O + CO₂. For example, sodium carbonate reacting with hydrochloric acid releases CO₂, not hydrogen. The gas can turn limewater milky, providing a useful confirmation. Hydrogen is associated with many acid–active-metal reactions, so option A correctly fixes the student’s misconception; B, C and D do not fit the reaction.

Neutralisation is governed by the net ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l). An acidic solution contains hydrogen or hydronium ions, and sodium hydroxide dissociates to supply OH⁻ ions. These ions combine to form water; Na⁺ and the acid’s negative ion remain as dissolved salt ions. Therefore A is correct. The other choices describe no valid ionic process in this neutralisation.

A base is a substance that reacts with an acid to form salt and water, usually in a neutralisation reaction. Solubility is the key distinction here: an alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Since the stated substance is insoluble, it can be called a base but not an alkali. Therefore option A is correct; C is the reacting counterpart and D only detects chemical nature.

Bleaching powder, commonly represented as CaOCl₂ in school-level chemistry, is prepared by passing chlorine gas over dry slaked lime, Ca(OH)₂. The word “dry” matters because excess moisture can interfere with obtaining the intended product. Sodium chloride is a raw material in chlorine manufacture but is not the direct absorbing substance here. Gypsum and washing soda have different uses and do not give bleaching powder in this preparation, so A is correct.

Dilution increases the volume without adding a basic substance. Consequently, the hydrogen-ion concentration decreases, so the pH rises toward 7, but the remaining H⁺ ions can still keep the solution acidic. Water does not automatically convert an acid into a base, nor does it destroy all ions. The exact pH change depends on concentration and acid strength, but the concept described in A explains why strong dilution does not necessarily produce basicity.

Colour alone is not a reliable chemical test because many common acids and bases, such as dilute hydrochloric acid and sodium hydroxide solution, are colourless. Their nature must be tested safely with an indicator, pH paper, or a calibrated pH meter. Litmus gives an acid–base indication, while a universal indicator gives an approximate pH range. Therefore A is correct; B and C make false universal claims, and D overstates what appearance can show.

Acidic behaviour in aqueous solution depends on the formation of hydrogen or hydronium ions. Water allows an acid to ionise or dissociate, and these ions change blue litmus to red. In the dry state, ions are not freely available to move and interact with the indicator, so the colour change may not occur. Thus A gives the appropriate reason. The other choices contradict the definition and use of acid–base indicators.

The governing relation is pH = −log₁₀[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 2 to pH 5 is 5 − 2 = 3 units, so [H⁺] decreases by 10³ = 1000 times. Equivalently, the concentration changes from 10⁻² to 10⁻⁵ mol L⁻¹. Hence option C is correct; A and B count too few units, while D reverses the direction.

A pH of 10 indicates that the salt solution is basic. In the usual school-level classification, a salt formed from a weak acid and a strong base undergoes hydrolysis of its anion, producing OH⁻ ions and hence a basic solution; sodium acetate is a familiar example. A strong acid–weak base salt is generally acidic, a strong acid–strong base salt is approximately neutral, and option D does not describe salt formation. Therefore B is correct.

The gas is carbon dioxide. First, carbon dioxide reacts with lime water, calcium hydroxide, to form insoluble calcium carbonate, which produces the milky appearance. When carbon dioxide is passed in excess, the precipitate changes into soluble calcium hydrogen carbonate. Acids reacting with carbonates release carbon dioxide, so option A is correct; the other reactions do not normally produce this gas.

Electrical conductivity in an aqueous acid solution depends mainly on the concentration and mobility of ions. Since both acids have the same stated concentration, the one showing greater conductivity is producing more mobile ions, indicating a greater degree of ionisation. Thus option A is the best conclusion. Conductivity alone should not be used to claim that an acid has become a base, and option C contradicts the observation.

Dilution of a concentrated acid is highly exothermic because hydration of its ions releases considerable heat. Adding acid slowly to a larger quantity of water allows the heat to disperse gradually and reduces the chance of local boiling, splashing, and burns. Therefore option A is correct. The acid does dissolve in water, is not made solid, and dilution does not itself neutralise it.

A red litmus paper turns blue when a basic medium is present. The gas shows this effect only when moisture is available, because water enables the gas to dissolve and produce ions responsible for basicity. Ammonia is the familiar example: it dissolves in water and forms an alkaline solution. Hence option A is correct; the dry paper lacks the aqueous medium needed for the colour change.

In the chlor-alkali process, concentrated aqueous sodium chloride is electrolysed. Water is reduced at the cathode, producing hydrogen gas and hydroxide ions: 2H2O + 2e− → H2 + 2OH−. Hydrogen burns rapidly with a characteristic ‘pop’ when a burning splint is brought near it. Therefore option A is correct; chlorine is produced at the anode, not the cathode.

Gypsum is calcium sulfate dihydrate, CaSO4·2H2O. Controlled heating removes one and a half molecules of water to form plaster of Paris, calcium sulfate hemihydrate, CaSO4·1/2H2O. Excessive heating can remove the remaining water and produce anhydrous calcium sulfate, which does not set properly with water. Thus option A is correct.

Methyl orange is red in a strongly acidic medium, while phenolphthalein remains colourless in acidic and neutral solutions. Together these observations indicate that the unknown solution is acidic. Neutralisation requires an alkali to supply hydroxide ions that react with hydrogen ions to form water. Sodium hydroxide is therefore suitable, making option A correct. Vinegar, lemon juice, and dilute acid would add more acid rather than neutralise it.

Sodium hydroxide is deliquescent, so it absorbs water vapour from air and becomes wet. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O. Water absorption changes its physical condition, while carbonate formation lowers chemical purity. Therefore option A gives both reasons; the other choices describe no relevant atmospheric process.

A gas that turns lime water milky is carbon dioxide, so the unknown substance should contain a carbonate or hydrogencarbonate ion. Sodium carbonate reacts with an acid to release CO2 and forms a basic aqueous solution because carbonate ions hydrolyse water to produce hydroxide ions. Sodium chloride is nearly neutral, ammonium chloride is acidic, and copper sulfate is not the expected carbonate source. Hence option A is correct.

Tooth enamel is mainly a calcium-containing mineral that is vulnerable to acidic conditions. Oral bacteria can convert sugars into acids, lowering the pH at the tooth surface. When the pH falls below approximately 5.5, demineralisation of enamel becomes more likely and tooth decay risk increases. Therefore option A is correct. Values above neutral or strongly alkaline values do not represent this usual acid-dissolution threshold.