Acids, Bases, and Salts

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.

The governing concept is the chemical nature of common aqueous substances. Lemon juice contains citric acid and therefore has an acidic nature, so option B is correct. Soap solution is generally basic because of alkaline salts, lime water contains calcium hydroxide and is basic, and baking-soda solution is mildly basic because sodium hydrogen carbonate reacts with water to produce some hydroxide ions. Their household uses do not change these classifications.

The governing concept is the behaviour of bases in aqueous solution. Soap solution is generally basic because soap contains salts of weak acids and strong bases; it can turn red litmus blue. Therefore option C is correct. Vinegar contains acetic acid, lemon juice contains citric acid, and tamarind juice contains organic acids, so these three substances are acidic rather than basic. The classification refers to their usual aqueous chemical nature.

The governing concept is the colour change of acid-base indicators. Methyl orange appears red in an acidic solution and changes through orange near its transition range to yellow in a sufficiently basic solution. Hence option B is correct. Yellow is associated with the basic-side colour, while blue and green are not the standard methyl-orange colour for an acidic solution. The indicator itself does not make the solution acidic; it only signals the existing pH range.

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.

Initially, carbon dioxide reacts with limewater to form insoluble calcium carbonate, which causes the milky appearance: CO2 + Ca(OH)2 → CaCO3 + H2O. When excess CO2 is passed, the calcium carbonate reacts further with carbon dioxide and water to form soluble calcium hydrogen carbonate, Ca(HCO3)2. The solid precipitate dissolves, so the solution becomes clear again; option D is correct.

An exothermic process releases heat to the surroundings. During dilution, ions of a concentrated acid become hydrated as the acid mixes with water, and considerable heat may be released. This warms the glass container, so option A is correct. An endothermic process would absorb heat and produce cooling; evaporation and crystallisation do not describe the main heat change in this dilution. For safety, acid must be added slowly to water.

Diluting concentrated acid is strongly exothermic because hydration of acid particles releases a large amount of heat. If a small volume of water is poured into the acid, the water can heat rapidly, boil locally and eject corrosive acid droplets. Therefore option B is correct. Water does not become a base, and the acid does not instantly freeze or become neutral. The safer rule is to add acid slowly to a larger volume of water while stirring.