Acids, Bases, and Salts

Carbon dioxide is an acidic oxide. With limited CO₂, sodium hydroxide can form sodium carbonate: 2NaOH + CO₂ → Na₂CO₃ + H₂O. When CO₂ is passed in excess, the carbonate reacts further with carbonic acid and forms soluble sodium hydrogen carbonate: Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃. Thus option A is correct.

Turning red litmus blue shows that the solution contains a base, such as sodium hydroxide. On heating with ammonium chloride, the base releases ammonia: NaOH + NH₄Cl → NH₃ + H₂O + NaCl. Ammonia has a characteristic pungent smell and turns moist red litmus blue. Hydrogen, carbon dioxide and sulphur dioxide do not fit these observations, so A is correct.

The pH relation is [H⁺] = 10⁻ᵖᴴ. Therefore, the ratio of the concentrations is 10⁻² ÷ 10⁻⁵ = 10³ = 1000. Equivalently, the pH difference is three units, and every one-unit decrease in pH means a tenfold increase in hydrogen-ion concentration. Hence the first solution has 1000 times more H⁺ and 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.

Sodium carbonate is a salt of a strong base, sodium hydroxide, and a weak acid, carbonic acid. Its carbonate ions hydrolyse in water and produce hydroxide ions, making the solution alkaline. Phenolphthalein is colourless in acidic or nearly neutral conditions but pink in a sufficiently basic solution. Therefore A is correct; not every salt gives a pink colour.

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.

Quicklime is calcium oxide. It reacts vigorously with water to form slaked lime and release heat: CaO + H₂O → Ca(OH)₂. The resulting limewater becomes milky when carbon dioxide is passed through it because insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. The combined observations identify quicklime, so 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.

According to the Arrhenius concept, a base is a substance that produces hydroxide ions, OH⁻, in aqueous solution. Many soluble bases also feel slippery or soapy because they react with oils on the skin; however, unknown substances should never be tested by touch. Acids produce H⁺ ions, a neutral salt is not necessarily hydroxide-producing, and a metal is a broader element class. Therefore option A is correct.

Neutralisation is the reaction of an acid with a base, usually producing salt and water. Hydrochloric acid supplies H⁺ ions and sodium hydroxide supplies OH⁻ ions; they combine as H⁺ + OH⁻ → H₂O, while Na⁺ and Cl⁻ form sodium chloride: HCl + NaOH → NaCl + H₂O. Mixing salt, sand, or sugar with water is dissolution or suspension, not an acid-base reaction. Thus A is certain.

Diluting a concentrated acid with water is strongly exothermic. If water is poured rapidly into acid, the small amount of water can heat suddenly, boil, and eject corrosive acid droplets. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring continuously and using suitable protection. This spreads the heat through the water. Therefore option A is correct; heating the acid or avoiding stirring increases danger.

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.

Plaster of Paris is calcium sulfate hemihydrate, CaSO4·1/2H2O. It reacts with water or atmospheric moisture and changes into gypsum, CaSO4·2H2O. This hydration causes the material to set and become hard, so it may lose its usefulness during storage. The other choices describe no relevant property: it neither forms a metal from oxygen, turns black in ordinary light, nor explodes with nitrogen.

Sodium hydrogen carbonate, or baking soda, decomposes on heating: 2NaHCO3 → Na2CO3 + H2O + CO2. The released carbon dioxide forms bubbles in dough or batter, and these bubbles expand during cooking, making the food rise and become porous. Oxygen is not the gas responsible, and water removal or hardening does not explain the leavening effect.