Acids, Bases, and Salts

The governing concept is thermal decomposition of sodium hydrogen carbonate, commonly called baking soda. On heating, two formula units decompose as: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Thus sodium carbonate, carbon dioxide, and water are formed. Sodium chloride requires a chloride source and does not appear in this reaction, so it is not a product. Therefore option D is correct; the other three are explicitly present in the equation.

The governing concept is water of crystallisation in hydrated salts. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. When exposed to dry air, it can lose some or all of this bound water, a process called efflorescence. The crystal lattice changes and the crystals may become powdery or less transparent. This is not caused by absorbing oxygen, changing into nitrogen, or releasing hydrogen gas. Therefore option A is correct.

The governing concept is water of crystallisation in hydrated salts. Blue vitriol is hydrated copper sulphate, CuSO4·5H2O. Heating removes its crystallisation water and produces white anhydrous CuSO4. Adding water restores the hydrated form and its blue colour. Thus option A is correct; sodium chloride, basicity, or metallic character cannot explain this reversible colour change.

The governing concept is neutralisation. Bacteria acting on food residues in the mouth produce acids, and these acids can attack tooth enamel and begin decay. Basic toothpaste supplies alkaline substances that react with and reduce the acidity. Therefore option A is correct. Making teeth acidic or merely changing colour does not prevent acid damage, and saliva cannot be converted into a gas by toothpaste.

The governing concept is acid–base neutralisation. The irritating material associated with an ant sting is acidic, while baking soda, sodium hydrogen carbonate, is a mild base. Contact between them reduces the acidic effect and can lessen burning or irritation. Hence option A is correct. Increasing acidity, producing oxygen, or removing water is not the relevant chemical process responsible for relief.

The governing concept is neutralisation of an acidic irritant. Nettle hairs can inject an irritating substance with an acidic effect, so a mild basic material is the suitable opposite treatment in this school-level context. It can reduce the acidity without adding another strong irritant. Therefore option A is correct; a strong acid would worsen irritation, while neutral salt powder or sugar solution does not provide the needed neutralising action.

The governing concept is the reaction of a strong base with an acidic oxide. Sodium hydroxide absorbs carbon dioxide from air and reacts as 2NaOH + CO2 → Na2CO3 + H2O. Some hydroxide ions are therefore converted into carbonate, reducing the available OH− concentration and the solution’s effective basicity. Thus option A is correct; the other changes do not occur in this situation.

The governing concept is the nature of salts formed by neutralisation. Sodium chloride is produced by HCl, a strong acid, and NaOH, a strong base: HCl + NaOH → NaCl + H2O. Neither ion significantly hydrolyses water, so its aqueous solution is generally neutral, close to pH 7. Therefore option A is correct; the other statements are chemically false or irrelevant.

The governing indicator rule is that a basic solution changes red litmus to blue. Sodium carbonate dissolves in water and undergoes hydrolysis, producing a solution with alkaline character. The observation therefore identifies its aqueous solution as basic, so option B is correct. An acidic solution would turn blue litmus red, a neutral solution would show no such change, and “metallic” is not a solution nature indicated by litmus.

The governing concept is hydrolysis of salts. Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion hydrolyses and produces H3O+, making the solution acidic and giving pH below 7. Hence option A is correct. A strong-acid/strong-base salt is generally neutral, while the other choices do not describe NH4Cl.

The governing concept is neutralisation of a basic metal oxide by an acid. Calcium oxide reacts with sulphuric acid according to CaO + H2SO4 → CaSO4 + H2O, producing exactly calcium sulphate and water. Therefore option A is correct. A carbonate would also release carbon dioxide in an acid reaction, while the other listed pairs do not provide the stated products under the given conditions.

The governing reaction pattern is acid plus carbonate producing salt, water, and carbon dioxide. Sodium carbonate reacts with hydrochloric acid as Na2CO3 + 2HCl → 2NaCl + H2O + CO2↑, so option B is correct. Zinc with acid usually releases hydrogen, sodium hydroxide with acid gives neutralisation, and magnesium oxide with acid gives salt and water without carbon dioxide.

Red litmus turning blue demonstrates that the solution is basic, while phenolphthalein becoming pink confirms an alkaline medium. A pH greater than 7 is therefore required. Of the choices, pH 11 clearly represents a basic solution; pH 2 and 5 are acidic, and pH 7 is neutral, so they cannot explain both indicator changes.

Methyl orange is an acid-base indicator whose colour depends on the hydrogen-ion concentration of the solution. In an acidic medium it appears red, and as the medium becomes less acidic or basic it changes through orange toward yellow. Therefore red is correct. Yellow describes its alkaline range, while green and blue are not the standard colours for this indicator.

Phenolphthalein is pink only in a sufficiently basic solution. Adding hydrochloric acid supplies hydrogen ions, which neutralise hydroxide ions from sodium hydroxide: HCl + NaOH → NaCl + H2O. As the excess alkalinity is removed and the solution approaches neutral or becomes acidic, phenolphthalein loses its pink colour. The other choices do not describe the indicator transition.

Zinc is amphoteric, so it can react with a strong base such as sodium hydroxide. In aqueous solution, zinc forms a soluble zincate species while water is reduced and hydrogen gas is evolved; a simplified equation is Zn + 2NaOH + 2H2O → Na2[Zn(OH)4] + H2. Thus hydrogen is correct, not oxygen, carbon dioxide, or chlorine.

An amphoteric oxide shows both acidic and basic chemical behaviour. Aluminium oxide reacts with acids to form aluminium salts and water, while it reacts with strong bases to form soluble aluminate compounds. This ability to react with both kinds of substances gives the name amphoteric. The other choices incorrectly restrict its reactions or describe an impossible physical state.

Limewater turns milky when carbon dioxide is passed through it, because insoluble calcium carbonate forms: CO2 + Ca(OH)2 → CaCO3 + H2O. Dilute acids react with carbonate ions to release carbon dioxide, for example CO3^2− + 2H+ → CO2 + H2O. Therefore carbonate is the likely ion; chloride and nitrate do not normally give this gas test.

Bleaching powder contains hypochlorite and chloride species and is not completely stable in moist air. Moisture and carbon dioxide can promote decomposition or acidification of its chlorine-containing components, allowing a small amount of chlorine or related active chlorine to escape and produce the characteristic smell. It does not become sodium chloride, absorb oxygen as the main process, or freeze water.

Bleaching powder releases available chlorine when it contacts water. This chlorine forms disinfecting species such as hypochlorous acid, which damages essential cell components of many harmful bacteria and other microorganisms. Consequently, it helps make drinking water safer when used in a controlled amount. It is not added to sweeten water, force its pH to zero, or increase salt content.