Acids, Bases, and Salts

An ant bite commonly injects formic acid, which contributes to the burning or irritation. A mild baking soda solution is weakly basic and can help neutralise some of this acid at the skin surface, so option A is the suitable school-level answer. Vinegar, lemon juice, and another dilute acid are acidic and would not provide the intended neutralising effect. The solution should be mild and applied safely; severe reactions require medical attention.

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.

Sodium chloride is produced by neutralisation of hydrochloric acid, a strong acid, with sodium hydroxide, a strong base. Both ions remain substantially unhydrolysed in water, so neither an excess of hydrogen ions nor hydroxide ions is generated. The solution is therefore generally neutral. Option B gives an incomplete and incorrect origin, option C is chemically meaningless, and option D is false because sodium chloride dissociates into Na+ and Cl− ions.

The governing reaction is the acid–carbonate reaction. An acid first forms carbonic acid from carbonate or hydrogen carbonate ions; unstable carbonic acid then decomposes into carbon dioxide and water. For example, carbonate + acid gives salt + water + CO2, while hydrogen carbonate follows the same overall pattern. Therefore carbon dioxide is the common gas. Hydrogen is associated with many metal–acid reactions, not this characteristic test.

The governing concept is the displacement reaction between an active metal and a dilute acid. Magnesium is above hydrogen in the reactivity series, so it displaces hydrogen from the acid. The general reaction is magnesium + dilute hydrochloric acid → magnesium chloride + hydrogen, or Mg + 2HCl → MgCl2 + H2. The released hydrogen forms visible bubbles. Oxygen, nitrogen, and chlorine are not the expected products.

The governing concept is the reactivity series and displacement of hydrogen from acids. A metal can liberate hydrogen from a dilute non-oxidising acid only when it is more reactive than hydrogen. Copper lies below hydrogen in the reactivity series, so it cannot displace hydrogen ions from dilute hydrochloric acid under ordinary conditions. The other statements are chemically false: copper is a solid metal, the dilute acid contains water, and hydrogen is a non-metal.

Two independent observations identify the solution as acidic. First, an acid changes blue litmus to red. Second, an acid reacts with a metal carbonate to produce carbon dioxide: acid + carbonate → salt + water + CO2. The gas turns limewater milky because insoluble calcium carbonate forms. A basic or neutral solution would not normally give both of these characteristic tests, and “without nature” is not a scientifically meaningful classification.

The governing relation is pH = −log[H+]. A tenfold decrease in hydrogen-ion concentration increases pH by one unit. Becoming one hundred times less acidic means the hydrogen-ion concentration falls by 100 = 10², so the pH rises by two units. Starting from pH 2, the approximate new pH is 2 + 2 = 4. Dilution does not make the solution strongly basic, so six and eight are unsuitable.

The governing concept is the indicator colour scale and dilution. Orange indicates an acidic solution, while yellow represents a less strongly acidic region on the universal-indicator scale. Adding water lowers the concentration of hydrogen ions, so the pH increases and the acidity decreases. However, a yellow colour does not prove that all acid has disappeared or that the solution has become basic. Ions remain present after dilution, so option A is correct.

The governing concept is electrolytic conductivity, which depends mainly on the concentration and mobility of ions in solution. Hydrochloric acid is a strong acid and ionises almost completely in water, producing a much larger concentration of mobile H+ and Cl− ions. Ethanoic acid is weak and ionises only partially at the same formal concentration, so fewer charge carriers are available. Therefore hydrochloric acid conducts better; option A is correct.

Blue litmus turning red indicates that the salt solution is acidic. This generally occurs when a salt is formed from a strong acid and a weak base, because the cation from the weak base undergoes hydrolysis and produces excess hydronium ions. Therefore option A is correct. A weak-acid/strong-base salt is usually basic, a strong-acid/strong-base salt is nearly neutral, and a weak-acid/weak-base salt is not always neutral.

Red litmus turning blue shows that the salt solution is basic. A salt formed from a weak acid and a strong base usually gives a basic solution because the anion of the weak acid reacts with water and generates hydroxide ions. Thus option A is correct. A strong-acid/weak-base salt is generally acidic, while a strong-acid/strong-base salt is approximately neutral; water and a metal do not describe a normal salt-forming combination here.

The gas that turns limewater milky is carbon dioxide. Acids react with carbonates and hydrogen carbonates to release carbon dioxide: carbonate or hydrogen carbonate plus acid produces a salt, water, and CO2. The CO2 then reacts with limewater to form insoluble calcium carbonate, causing milkiness. Therefore option A is correct; ordinary chlorides, sulphates, and nitrates do not give this characteristic gas merely with acid.

Initially, carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, which causes milkiness: Ca(OH)2 + CO2 → CaCO3 + H2O. When excess CO2 is passed, the calcium carbonate reacts further and forms soluble calcium hydrogen carbonate: CaCO3 + CO2 + H2O → Ca(HCO3)2. Thus option A is correct; calcium carbonate is the first, insoluble product.

Magnesium reacts with a dilute acid by displacement, producing a salt and hydrogen gas; for example, Mg + 2HCl → MgCl2 + H2. Calcium carbonate reacts with acid to produce a salt, water, and carbon dioxide; CaCO3 + 2HCl → CaCl2 + H2O + CO2. Therefore, in the stated order, the gases are hydrogen and carbon dioxide, so option A is correct.

Dilution of a concentrated acid is highly exothermic because hydration of ions releases heat. Adding acid slowly to a larger volume of water allows the heat to spread through the water and be dissipated more safely. If water is poured into concentrated acid, the small amount of water may heat suddenly, boil, and eject acid droplets. Hence option A expresses the laboratory safety rule and is correct.

An acid shows its characteristic behavior in aqueous medium, where it ionizes and produces hydronium ions, H3O+. Dry blue litmus contains insufficient water for the acidic gas to ionize effectively, so no colour change is observed. Moist litmus provides water; the gas dissolves, forms hydronium ions, and changes blue litmus to red. Therefore option A is correct, while the other statements confuse the role of moisture.

Ammonia itself is a covalent gas, but in the presence of water it dissolves and reacts reversibly to form ammonium and hydroxide ions: NH3 + H2O ⇌ NH4+ + OH−. The hydroxide ions turn moist red litmus blue. Dry litmus lacks the water needed for this ion formation, so it shows no change. Thus option A is correct; the observation does not mean ammonia is a strong acid or always neutral.

Sodium hydroxide is deliquescent, so it absorbs moisture from air. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O. Absorbed water changes the sample’s concentration, while carbonate formation changes its composition; both effects reduce purity. Therefore option A is correct. The other options describe chemically impossible transformations under ordinary storage conditions.