Acids, Bases, and Salts

Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from dilute sulphuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂. The gas evolved is hydrogen, and hydrogen burns rapidly with oxygen in air, producing the characteristic pop sound. Oxygen, nitrogen and chlorine are not products of this metal–acid reaction, so option A is correct.

Calcium oxide and water combine to form one product: CaO + H₂O → Ca(OH)₂. Formation of a single product from two reactants makes it a combination reaction. Because a considerable amount of heat is released, it is also exothermic. It is not decomposition, displacement, double displacement or precipitation, so option A gives both correct classifications.

Tooth enamel contains calcium phosphate minerals that can be attacked by acids. When the pH in the mouth falls below approximately 5.5, demineralisation of enamel becomes significant and decay may begin. A pH of 7 is neutral but is not the specific critical value asked here; pH 9 and 14 are alkaline. Thus the appropriate answer is 5.5, option A.

Atmospheric air contains carbon dioxide, an acidic oxide. Sodium hydroxide absorbs it and reacts to form sodium carbonate: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Continued exposure can therefore change the composition and reduce the purity of the sodium hydroxide solution. No source of nitrate, sulphide or phosphate is present, so sodium carbonate, option A, is correct.

Methyl orange is red in an acidic medium and yellow in a neutral or alkaline medium. The original colour therefore shows that the colourless solution is acidic. When sodium hydroxide, a base, is added gradually, hydroxide ions react with hydrogen ions and reduce the acidity: acid + base → salt + water. The indicator change consequently signals neutralisation, not salt decomposition, melting, or evaporation.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Thus sodium carbonate, carbon dioxide, and water are the three products formed. Sodium hydroxide does not appear in this balanced equation and is therefore the product that is not formed. This is a decomposition reaction, not a neutralisation reaction.

Metal carbonates react with acids to produce a salt, water, and carbon dioxide: carbonate + acid → salt + H₂O + CO₂. When the gas is passed through limewater, Ca(OH)₂, it reacts to form insoluble calcium carbonate, CaCO₃, which appears milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Therefore option A gives both the correct gas and the correct reason.

At 25°C, pH and pOH are related by pH + pOH = 14, and hydroxide concentration increases as pOH decreases. For pH 11, pOH is 3, so [OH⁻] is about 10⁻³ mol L⁻¹. The pH 7 solution is neutral, while pH 6 and pH 2 are acidic and have much lower hydroxide concentrations. Hence the pH 11 solution has the highest [OH⁻].

Sodium chloride is formed by the neutralisation of hydrochloric acid, a strong acid, with sodium hydroxide, a strong base: HCl + NaOH → NaCl + H₂O. In water, NaCl dissociates into Na⁺ and Cl⁻, but these ions do not significantly hydrolyse to produce excess H⁺ or OH⁻. Consequently, its aqueous solution is nearly neutral, with pH close to 7. Option D is false because the salt does ionise.

Acidity is reduced by a suitable basic substance because it reacts with excess acid in a neutralisation process. Milk of magnesia contains magnesium hydroxide, Mg(OH)₂, and is used as an antacid to neutralise excess stomach acid. Slaked lime, Ca(OH)₂, is basic and can be used to treat acidic soil. Vinegar, lemon juice, hydrochloric acid, and sulphuric acid are acidic, while salt and sugar do not provide the required neutralising action. Hence A is correct.

Blue vitriol is hydrated copper(II) sulphate, CuSO₄·5H₂O. Heating removes its water of crystallisation and produces anhydrous copper sulphate, CuSO₄, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Adding water rehydrates the salt and restores the blue hydrated form. No copper metal or sulphur dioxide is produced, and the observation is not simply complete dissolution. Thus option A explains both colour changes.

Baking soda is a basic salt. When a suitable edible acid is added and the mixture becomes moist or warm, an acid–carbonate reaction produces carbon dioxide, which raises the batter. The acid also neutralises excess sodium hydrogen carbonate, reducing the unpleasant alkaline or bitter taste and limiting unwanted residue. Oxygen, sodium chloride, and burning are not the intended results.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. Chloride ions are oxidised at the anode to chlorine gas, while water is reduced at the cathode to hydrogen gas and hydroxide ions. Sodium ions combine with hydroxide ions in solution to form sodium hydroxide. Thus chlorine and sodium hydroxide are in option A; the other pairs contain incorrect elements or products.

In the chlor-alkali process, brine is electrolysed. At the cathode, water gains electrons and is reduced: 2H2O + 2e− → H2 + 2OH−. Therefore hydrogen gas is evolved at the cathode. Chlorine is produced at the anode by oxidation of chloride ions, while oxygen and carbon dioxide are not the expected cathode products under these conditions.

A base is a substance that can neutralise an acid, whereas an alkali is specifically a base that dissolves in water to give hydroxide ions. Turning red litmus blue indicates basic character, but the statement says the substance is only slightly soluble. Therefore the broader and safer term is base. An alkali would require clear water solubility; a strong acid would show the opposite litmus change.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion undergoes hydrolysis and produces hydronium ions, making the solution acidic. Sodium carbonate gives a basic solution, while sodium chloride and potassium nitrate are generally neutral because they come from strong acid–strong base combinations. Thus A is the only acidic salt.

Carbon dioxide is a non-metal oxide and reacts with water to form carbonic acid, so it is acidic. Calcium oxide is a metal oxide and reacts with water to form calcium hydroxide, a base, so it is basic. Option B reverses the required order. Sodium oxide and magnesium oxide are basic metal oxides, while sulfur dioxide and carbon dioxide are both acidic non-metal oxides. Therefore A is correct.

The governing concept is neutralisation between an acidic oxide and a base. Carbon dioxide, CO₂, is an acidic oxide, while calcium hydroxide, Ca(OH)₂, is a base. They react as CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, producing calcium carbonate and water. Thus option A directly represents the required reaction. The other choices show metal–water, metal–acid, and combination reactions, not acidic oxide–base neutralisation.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of a hydrated salt. Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O, so it contains ten water molecules of crystallisation. Baking soda is NaHCO₃ and does not contain such water in its formula; sodium chloride and calcium oxide are also represented here as anhydrous substances. Therefore option A is correct.

Plaster of Paris is calcium sulphate hemihydrate. During its preparation, gypsum, CaSO₄·2H₂O, is heated carefully so that part of its water of crystallisation is removed, giving CaSO₄·½H₂O. Therefore option A is correct. Option B is gypsum, the starting material; option C is washing soda; and option D is baking soda. The half-water notation represents the average water content per calcium sulphate unit.

Hard water contains dissolved calcium and magnesium salts. Washing soda, Na₂CO₃·10H₂O, supplies carbonate ions that react with soluble calcium or magnesium ions to form insoluble carbonates, which can be removed. This decreases the ions responsible for hardness, so option A is correct. Vinegar and lemon juice are acids, while common salt does not generally precipitate the hardness-causing ions and therefore is not the standard softening substance.

Sodium hydroxide, NaOH, is a strong alkali. In concentrated form or even at significant laboratory exposure, it is highly corrosive and can damage living tissue, causing severe irritation or chemical burns. Therefore option A correctly explains the safety risk. It does not freeze skin, is not a neutral salt, and its effect is not limited to colour change. Gloves, eye protection, and proper handling are essential.

Using the stated classroom model, the bee sting is treated as acidic. Baking soda, sodium hydrogen carbonate (NaHCO₃), is a mild base, so a properly prepared mild paste may help neutralise some of the acidic effect. Therefore option A is the best answer among those given. Lemon juice, vinegar, and hydrochloric acid are acidic and would not provide the intended neutralisation. In real cases, medical guidance should be followed if symptoms are severe.

Under the stated school-level assumption, a wasp sting is basic in nature. Vinegar contains acetic acid and is a mild acid, so it can theoretically neutralise part of the basic effect; hence option A is the intended answer. Baking soda, slaked lime, and sodium hydroxide are basic or alkaline substances and would not provide the required acid–base neutralisation. Actual stings should be managed carefully, especially if allergic symptoms appear.

The governing concept is acid–base neutralisation. Excess hydrochloric acid in the stomach can cause acidity and irritation. Antacids contain mild bases, such as magnesium hydroxide or suitable carbonate compounds, which react with excess acid and reduce its acidity. Therefore option A is correct. An antacid is not intended to increase acid, convert food into acid, or fill the stomach with hydrogen gas; those statements do not describe its function.