Acids, Bases, and Salts

Baking soda is sodium hydrogen carbonate, while vinegar contains acetic acid. When they react, the acid first forms carbonic acid, which quickly decomposes into carbon dioxide, water, and a salt: NaHCO₃ + acid → CO₂ + H₂O + salt. The escaping CO₂ bubbles become trapped in the liquid and produce foam. No hydrogen metal, frozen oxygen, or burning sodium chloride is involved.

Bleaching powder, commonly represented as calcium oxychloride, releases chlorine or chlorine-containing disinfecting species in water. These substances oxidise and destroy many harmful microorganisms, making the water safer. Baking soda and washing soda are carbonates used for other purposes, while blue vitriol is copper sulfate and is not the standard school-level disinfectant for drinking-water treatment. Thus option A is correct.

Bleaching powder is used as a bleaching agent because it releases chlorine or active chlorine in suitable conditions. This active chlorine oxidises many coloured substances, reducing or removing their colour from cloth and wood pulp. Sodium chloride is common salt, ammonium chloride is used in several laboratory and industrial applications, and gypsum is a calcium sulfate mineral. Therefore only option A matches this use.

A salt solution need not always be neutral because the ions may react with water, a process called hydrolysis. Ammonium chloride is formed from a strong acid and a weak base; its ammonium ion reacts with water and produces hydronium ions, so its solution is acidic. Sodium chloride, from a strong acid and strong base, is approximately neutral. Pure water and dry sand are not counterexamples to the statement about salts.

The reaction between an acid and a base is called neutralisation. In a typical strong-acid and strong-base reaction, hydrogen ions combine with hydroxide ions to form water, and energy is released to the surroundings: H⁺ + OH⁻ → H₂O. This released energy raises the solution temperature, so the process is exothermic in this case. The words “always absorbs” and “never react” are incorrect.

Ions are electrically charged particles. When an ionic substance dissolves in water, its ions may become mobile; their movement between electrodes allows the solution to conduct electric current. Therefore electrical conductivity is strong experimental evidence for mobile ions, although the degree of conductivity depends on ion concentration and mobility. Colour, taste, or apparent temperature does not demonstrate the presence of ions and may be unrelated to composition.

Methyl orange is red in an acidic medium and yellow in a neutral or alkaline medium. Adding enough base consumes hydrogen ions through neutralisation, so the pH rises and the solution moves from the acidic indicator range toward the alkaline range. Consequently, the observed colour changes from red toward yellow. The other choices describe the reverse change or colours that do not match methyl orange in this situation.

Acids ionise or dissociate when dissolved in water, producing hydronium ions and corresponding negative ions. These charged particles are mobile in the aqueous solution and carry charge when an electric potential is applied, allowing current to flow. The strength of conduction depends on the number and mobility of ions. Colour, taste, or floating solid particles is not the reason for electrical conduction. Thus option A is correct.

The pH scale is defined by hydrogen-ion concentration: pH = −log[H⁺]. Thus a decrease of one pH unit represents a tenfold increase in hydrogen-ion concentration. Among pH 2, 7, 10, and 12, pH 2 is the lowest and therefore has the greatest [H⁺]. The pH 7 solution is neutral, while pH 10 and pH 12 are alkaline and have lower hydrogen-ion concentrations. Hence option A is correct.

The governing concept is that acids show their characteristic behaviour through hydrogen ions, more precisely hydronium ions in aqueous solution. Water allows the acid to ionise and permits the indicator molecules to respond with a colour change. A dry acid may therefore show little or no indicator effect. Indicators do not work only in metals, acids do not become bases without water, and dry acid still contains atoms.

The gas released from an acid–metal carbonate reaction is carbon dioxide. Initially, CO₂ reacts with calcium hydroxide in lime water: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, and insoluble calcium carbonate makes the solution milky. With excess CO₂, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂, which is soluble, so the milkiness disappears.

Zinc is above hydrogen in the reactivity series, so with a suitable dilute acid it displaces hydrogen: Zn + 2HCl → ZnCl₂ + H₂. Carbon dioxide would turn lime water milky by forming calcium carbonate, but hydrogen does not. Thus the absence of milkiness supports hydrogen rather than CO₂; the other statements contradict the reaction or the limewater test. A burning-splint pop test can provide confirmation.

Sodium hydrogen carbonate reacts with an acid in an acid–carbonate reaction. For example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. The carbon dioxide is released rapidly as gas bubbles, and these bubbles produce visible effervescence. No oxygen is absorbed, water does not freeze as the cause, and sodium metal is not formed because the sodium remains in the salt solution as ions.

A base is a substance that can neutralise an acid, but it need not dissolve in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Therefore, alkalis form a subset of bases: every alkali is a base, but every base is not necessarily an alkali. The other choices describe incorrect universal claims or reverse the behaviour of bases.

Hydrogen chloride gas itself does not release hydrogen ions effectively without water. When it dissolves, HCl + H₂O → H₃O⁺ + Cl⁻, and the hydronium ions give the solution acidic properties. Moist blue litmus contains the needed water and therefore turns red. Dry blue litmus does not show this reliable change, red litmus turning blue indicates a base, and limewater milkiness is a carbon-dioxide test.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonia, the weak base. In water, the ammonium ion hydrolyses and produces hydronium ions, so the solution is acidic: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. Sodium carbonate is generally basic, while sodium chloride and potassium nitrate are salts of strong acids and strong bases and are approximately neutral.

Sodium carbonate is the salt of a strong base, sodium hydroxide, and a weak acid, carbonic acid. The carbonate ion reacts with water and generates hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻, so its solution is basic. Sodium chloride and potassium chloride are approximately neutral salts, whereas ammonium chloride is acidic because NH₄⁺ produces hydronium ions.

Sodium chloride is produced by neutralisation of hydrochloric acid, a strong acid, and sodium hydroxide, a strong base: HCl + NaOH → NaCl + H₂O. In water, NaCl dissociates into Na⁺ and Cl⁻; neither ion hydrolyses appreciably, so the solution is approximately neutral at ordinary concentration. It does dissolve in water, is not made only of metal, and does not release carbon dioxide.

The governing concept is hydration of salts. Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The dot followed by 10H₂O shows that each formula unit of sodium carbonate is associated with ten water molecules of crystallisation. Therefore, option A is correct. Two, five, and half do not match the established formula of washing soda and refer to no correct hydration state here.

The governing concept is the distinction between hydrates and hemihydrates. Plaster of Paris is calcium sulphate hemihydrate, represented as CaSO₄·½H₂O. Thus, one formula unit of calcium sulphate is associated with half a water molecule on average. Option A is correct. Gypsum contains two water molecules per calcium sulphate unit, while the other choices describe different or incorrect hydration states.