Acids, Bases, and Salts

Sodium carbonate, Na₂CO₃, is formed from the strong base NaOH and the weak acid H₂CO₃. In water, the carbonate ion undergoes hydrolysis and generates OH⁻ ions, making the solution basic. A basic solution turns red litmus blue, so option C is correct. NaCl is nearly neutral, NH₄Cl is acidic, and CuSO₄ is not the expected basic salt in this comparison.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. Carbonate ions react with dissolved calcium and magnesium ions in hard water to form insoluble carbonates, removing them from solution. Thus washing soda softens water and can remove permanent hardness; option A is correct. The other choices describe neither its standard composition nor its principal household use.

Water of crystallisation is the definite number of water molecules chemically associated with the formula unit of certain crystalline salts. It is not merely moisture on the outside. For example, CuSO₄·5H₂O contains five water molecules per formula unit. This water can influence crystal colour, shape, and stability, so option A gives the correct meaning.

Hydrated copper sulphate crystals, CuSO₄·5H₂O, are blue because of their water of crystallisation. On heating, this water is removed and anhydrous copper sulphate, CuSO₄, is formed, which is white. The change is therefore due to loss of bound water, not addition of oxygen or conversion into common salt or an acid. Adding water can restore the blue hydrated form, so option B is correct.

Ammonium chloride, NH₄Cl, is produced by the neutralisation reaction HCl + NH₄OH → NH₄Cl + H₂O. Hydrochloric acid is strong, whereas ammonium hydroxide is a weak base, so option D matches the stated condition. NaCl comes from a strong acid and strong base; CH₃COONa comes from a weak acid and strong base; Na₂CO₃ is also associated with a strong base and weak acid.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When mixed with water, it rehydrates and changes into gypsum, CaSO₄·2H₂O, forming a hard solid: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Its rapid setting and rigid support help immobilise a fractured bone, so option A is correct.

The governing concept is the extent of ionisation in water. A strong acid ionises almost completely and produces a high concentration of hydrogen or hydronium ions. Hydrochloric acid, HCl, is therefore a strong acid, so option A is correct. Acetic acid in vinegar, carbonic acid, and citric acid ionise only partially and are commonly classified as weak acids. Strength refers to ionisation, not merely to the amount of acid present.

A weak acid ionises only partially in water, so it produces fewer hydrogen ions than a strong acid of comparable concentration. Acetic acid, the principal acid in vinegar, is a weak acid; therefore option D is correct. Hydrochloric acid and nitric acid are strong acids, and sulphuric acid is also treated as a strong acid at the school level. The distinction is based on ionisation, not on whether the substance tastes sour.

The governing concept is concentration: it describes the amount of solute present in a given amount of solution. A concentrated acid solution contains a relatively larger amount of acid, whereas a dilute solution contains a smaller amount, so option A is correct. Concentration must not be confused with strength. Strength concerns the extent to which an acid ionises in water; a weak acid may be concentrated, and a strong acid may be dilute.

The governing concept is exothermic dilution. When an acid dissolves and its particles become hydrated in water, heat is released; consequently, the temperature of the solution generally increases. Thus option B is correct. The temperature does not necessarily fall to zero or remain unchanged. Because rapid heat production can cause splashing, the safe laboratory rule is to add acid slowly to water with stirring, never to pour water quickly into concentrated acid.

The governing concept is that acids show their characteristic behaviour in aqueous solution. Dry hydrogen chloride gas does not dissociate to produce hydrogen ions without water, so dry litmus has no acidic ions with which to react. Therefore option A is correct. Litmus can identify acids, and hydrogen chloride itself is not basic. When the litmus is moist, HCl dissolves and ionises, allowing the expected colour change to occur.

The governing relationship is pH = −log[H⁺]. Because the logarithm is preceded by a minus sign, increasing the hydrogen-ion concentration makes the pH value decrease. Therefore option B is correct. A lower pH indicates a more acidic solution. The value does not automatically become 7, since pH 7 represents neutrality at standard conditions, and it cannot remain unchanged when [H⁺] changes.

The governing concept is the inverse relationship between pH and hydrogen-ion concentration. A lower pH means a larger effective concentration of hydrogen ions and therefore greater acidity. Since the two solutions already have the same stated concentration, the acid with the lower pH is more acidic, making option A correct. It is not basic or neutral, and lower pH alone does not mean that the substance must be less reactive in every reaction.

Phenolphthalein is an acid–base indicator whose colour depends on the pH of the solution. In an acidic or neutral medium, it is generally colourless, but in a basic medium it changes to pink because its molecular form changes in the presence of hydroxide ions. Therefore, option A is correct. Red and yellow are not the characteristic colours of phenolphthalein, while colourless describes its acidic form.

In an acid–base neutralisation reaction, hydrogen ions from the acid combine with hydroxide ions from the base to form water. This process commonly releases heat to the surroundings, so the measured temperature rises. Such a reaction is called exothermic. Therefore, option A is correct. The rise is not evidence of light absorption, salt vapour formation, or a complete absence of heat; in fact, heat is being liberated.

Most metal oxides are basic oxides, so they react with acids in a neutralisation reaction. The products are a salt and water; for example, copper(II) oxide reacts with hydrochloric acid to form copper(II) chloride and water. Therefore, option A is correct. Hydrogen gas is usually associated with the reaction of an acid with a reactive metal, not with a metal oxide, and oxygen gas is not the general product.

Non-metal oxides are generally acidic in nature because many of them react with water to form acids. For example, carbon dioxide dissolves in water to produce carbonic acid, and sulphur dioxide can form sulphurous acid. Therefore, option B is correct. The statement is a general trend, not an absolute rule: some oxides may be neutral or show other behaviour. Metallic is not a chemical nature classification for these oxides.

The pH scale indicates the concentration of hydrogen ions: values below 7 represent acidic solutions, 7 is neutral, and values above 7 are basic. A pH of 2 is far below 7 and indicates a high hydrogen-ion concentration, so the solution is strongly acidic. Therefore, option A is correct. It cannot be neutral or basic, and calling it weakly basic reverses the meaning of the pH scale.

Sodium carbonate, Na₂CO₃, is considered a salt of a strong base, sodium hydroxide, and a weak acid, carbonic acid. In water, the carbonate ion undergoes hydrolysis and produces hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. The OH⁻ ions make the solution basic, so option C is correct. NaCl is approximately neutral, NH₄Cl is acidic, and CuSO₄ is not the expected basic salt here.

A base changes red litmus paper to blue because the solution provides a basic medium. Turmeric is also a natural indicator: it turns reddish brown in the presence of a base, while it generally remains yellow in acidic or neutral conditions. Since both independent tests indicate alkalinity, option B is correct. An acidic or neutral solution would not produce both changes, and being salty does not automatically mean that a solution is neutral.

Magnesium reacts with dilute hydrochloric acid according to the equation Mg + 2HCl → MgCl₂ + H₂. The metal forms the positive part of the salt, while the chloride ion from hydrochloric acid forms the negative part; hydrogen gas is released separately. Thus the salt is magnesium chloride, so option C is correct. Sulphate and carbonate would require different acids, and sodium chloride contains sodium rather than magnesium.

Zinc reacts with dilute sulphuric acid in a single-displacement reaction: Zn + H₂SO₄ → ZnSO₄ + H₂. The zinc replaces hydrogen in the acid, forming zinc sulphate, while hydrogen gas is liberated. Therefore, option D is correct. Zinc chloride would require hydrochloric acid, zinc carbonate would involve a carbonate source, and sodium sulphate contains sodium rather than zinc. The acid radical helps identify the salt’s name.

An electric current in a solution requires mobile charged particles. An acid dissolved in water ionises, producing ions such as H₃O⁺ and a negative counter-ion that can move through the solution and carry charge, so the bulb glows. Dry hydrogen chloride gas does not provide freely moving ions in the same way, so it does not conduct sufficiently. Hence option A is correct; the other statements contradict ionic conduction.

The governing rule is that an acid reacts with a carbonate to produce a salt, water and carbon dioxide. Here, hydrochloric acid reacts with sodium carbonate: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. Thus sodium chloride, water and carbon dioxide are formed, so option B is correct. Option A omits carbon dioxide, while C and D contain substances not supplied by the reactants.

The limewater test is based on the reaction of carbon dioxide with calcium hydroxide. When CO2 is passed through limewater, insoluble calcium carbonate forms: CO2 + Ca(OH)2 → CaCO3 + H2O. The suspended calcium carbonate makes the solution milky, so option C is correct. Hydrogen is tested with a pop sound, oxygen relights a glowing splint, and chlorine has different characteristic tests.