Acids, Bases, and Salts

The governing concept is neutralisation. In a typical acid-base reaction, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions combine to form a salt. For example, HCl + NaOH → NaCl + H₂O. Thus option A is correct because both salt and water are products; a metal is not always produced, and the reaction does occur.

Zinc reacts with dilute sulphuric acid according to Zn + H₂SO₄ → ZnSO₄ + H₂. The evolved gas is hydrogen. When a burning splint is brought near hydrogen, it burns rapidly with a characteristic ‘pop’ sound, so option A is the suitable identification test. Limewater turning milky identifies carbon dioxide, while litmus colour changes test acidity or basicity rather than confirming hydrogen.

Dilution of a concentrated acid with water is an exothermic process, so heat is released. If a small quantity of water is poured into concentrated acid, the water may heat suddenly, boil, and produce acidic splashes. These splashes can cause severe burns. Therefore, the safe laboratory rule is to add concentrated acid slowly to a larger volume of water while stirring. Option A correctly describes the danger; the other choices contradict the heat-producing process.

A carbonate reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. Carbon dioxide passed through limewater forms insoluble calcium carbonate, making the limewater milky. Thus option A is the confirmatory observation. A pop sound is the test for hydrogen, while litmus change and a yellow precipitate are not the standard identification tests for carbon dioxide.

The acidic character of an acid appears when it is dissolved in water because the acid ionises or dissociates to produce hydrogen ions, more accurately represented in water as hydronium ions, H3O+. These ions cause indicator changes and other acidic properties. A dry solid acid cannot produce these aqueous ions, and oil or vacuum does not provide the necessary medium. Therefore, option A is correct.

Hydrogen chloride shows acidic behavior only after dissolving in water. In moist conditions, HCl reacts with water to form hydronium ions and chloride ions: HCl + H2O → H3O+ + Cl−. Hydronium ions cause blue litmus to turn red. Dry HCl gas and dry litmus provide no water for this ionisation, so no color change occurs. Hence option A is correct; HCl is not a base and can form an acid in water.

Bacteria in the mouth can use sugars to produce acids. As the pH of the mouth falls below about 5.5, the mineral component of tooth enamel begins to dissolve more readily, increasing the risk of decay. Therefore, option A is correct. A pH of 7 is neutral, while 8.5 and 14 represent alkaline conditions and do not describe the critical acidic level for enamel damage.

Sodium hydroxide is hygroscopic and can absorb moisture, but it also absorbs carbon dioxide from air. The absorbed carbon dioxide reacts with sodium hydroxide, for example: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Therefore option A is correct. Nitrogen, oxygen, and noble gases do not explain the gradual formation of sodium carbonate under ordinary storage conditions.

Cold, dilute chlorine reacts with sodium hydroxide to produce sodium hypochlorite: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O. Hypochlorite is a bleaching and disinfecting substance, so it is used to remove colour from materials such as textiles and paper. Option A is correct. It does not sweeten food, convert metals into gold, or acidify drinking water.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Thus sodium carbonate, water, and carbon dioxide are formed, making option A correct. The reaction does not produce sodium metal, oxygen, or only hydrogen, and the substance does not remain unchanged.

Baking powder generally contains sodium hydrogen carbonate and a suitable edible acid. When the mixture becomes moist and is heated, an acid-base reaction produces carbon dioxide; the gas bubbles expand and become trapped in the batter, making the cake rise. Option A is correct. The process does not convert oxygen into gold, make the dough completely acidic, or freeze water.

The governing concept is the removal of hardness-causing ions. Hard water contains dissolved calcium and magnesium salts. Washing soda, sodium carbonate (Na2CO3), reacts with these ions and forms insoluble carbonates such as calcium carbonate and magnesium carbonate. These precipitates can be separated, leaving softer water. Therefore, option A is correct; the other options do not describe the chemical process of softening.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO4·5H2O. The governing concept is dehydration: on heating, the water of crystallisation is driven out, producing white anhydrous copper sulphate, CuSO4. If water is added again, the blue hydrated form can return. Therefore option A is correct. The other choices incorrectly suggest a change into gold, oxygen, or additional water absorption.

White anhydrous copper sulphate is CuSO4 and lacks water of crystallisation. When water is added, it becomes hydrated copper sulphate, commonly represented as CuSO4·5H2O, whose crystals are blue. The governing concept is hydration and restoration of the crystal structure. Hence option A is correct. Copper does not burn, sulphur gas is not produced, and the salt does not become an acid.

Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. Its governing property is that it reacts with water and changes into gypsum, CaSO4·2H2O. This hydration causes the powder to set into a hard mass, so moisture during storage can spoil its usefulness. Therefore option A is correct; the other options describe chemically impossible effects.

Bleaching powder is a reactive salt whose available chlorine can decrease during prolonged exposure to air. Moisture and carbon dioxide promote decomposition or conversion of active constituents into less effective substances. Consequently, an open sample gradually loses bleaching and disinfecting power. Option A is correct. The alternatives involving gold, pure water or iron have no chemical basis and do not explain loss of activity.

The chlor-alkali process is the electrolysis of concentrated sodium chloride solution, or brine. At the cathode, water produces hydrogen gas and hydroxide ions; at the anode, chloride ions are oxidised to chlorine gas. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. Thus option A is correct, while the other gas pairs are not products of brine electrolysis.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes wet and is heated, the acid reacts with sodium hydrogen carbonate and produces carbon dioxide. The gas forms bubbles that make baked food soft and spongy; the acid also helps prevent an unpleasant alkaline taste. Thus option A is correct. The other choices contradict the reaction because gas is formed and baking soda does not become a metal.

Crystals of sodium carbonate commonly contain water of crystallisation; washing soda is represented as Na₂CO₃·10H₂O. On heating, some or all of this water is driven out of the crystal lattice as water vapour, so the measured mass decreases. Sodium atoms do not disappear, carbon does not turn into gold, and the crystal does not consume air. Therefore, option A correctly explains the mass loss.

Acids react with metal carbonates to form a salt, water, and carbon dioxide: acid + carbonate → salt + H₂O + CO₂. This reaction makes option A correct. Acids generally turn blue litmus red, so option B reverses the indicator change. In aqueous solution acids produce ions, especially hydrogen or hydronium ions, which rejects option C. They do not produce oxygen with all metals, so D is also false.

An acidic solution has a pH below 7. When a base is added, hydroxide ions react with hydrogen or hydronium ions, reducing the effective acidity. Consequently, the pH rises toward 7 at the neutralisation point and may become greater than 7 if excess base is added. It does not always remain zero or continually decrease. Thus option A is the correct general trend during gradual neutralisation.

The strength of an acid describes its degree of ionisation in water, not simply the amount of acid present. A strong acid, such as hydrochloric acid in an aqueous solution, ionises almost completely, whereas a weak acid ionises only partially and establishes an equilibrium. Therefore option A is correct. A weak acid remains an acid, strong acids can dissolve in water, and both types produce ions to some extent, so B, C, and D are incorrect.

Concentration refers to the amount of solute present in a given amount of solution. A concentrated acid solution contains a relatively large amount of dissolved acid, while a dilute solution contains a smaller amount because more solvent is present. This idea is different from acid strength, which depends on ionisation. Colour and name do not define concentration, and dilution does not turn an acid into a base. Therefore option A is correct.

The governing concept is salt hydrolysis. Sodium carbonate is formed from sodium hydroxide, a strong base, and carbonic acid, a weak acid. In water, the carbonate ion reacts partly with water and produces hydroxide ions, so the solution becomes basic. Sodium chloride and potassium nitrate are generally neutral because they come from strong acids and strong bases. Ammonium chloride is acidic because ammonium ions hydrolyse to produce hydrogen ions.

The governing idea is hydrolysis of ions from a weak base and a strong acid. Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonia or ammonium hydroxide, a weak base. In water, ammonium ions hydrolyse and release hydrogen ions, making the solution acidic. Sodium chloride is generally neutral, sodium carbonate is basic, and potassium sulphate is not the expected acidic salt in this school-level comparison.