Acids, Bases, and Salts

A basic solution can feel slippery or soapy because alkaline substances react with oils on the skin, although an unknown chemical must never be touched directly. The decisive test here is that a base changes red litmus to blue. Therefore, option B is correct. Acids generally turn blue litmus red, neutral solutions show no such characteristic change, and carbonate gas is not a suitable category for a solution displaying this indicator response.

A universal indicator is a mixture of indicators that gives different colours over the pH scale. Orange generally corresponds to an acidic pH, usually more acidic than yellow but less strongly acidic than red. Therefore, option A is the appropriate conclusion. Green is associated with a nearly neutral solution around pH 7, while violet or purple suggests a strongly basic solution. The colour indicates the solution’s acid-base nature, not whether it contains a metal.

Universal-indicator colours are related to pH. Violet or purple is found at the high-pH end of the indicator scale, so it indicates a strongly basic or alkaline solution. Hence option C is correct. Strong acids are represented by red shades, neutrality near pH 7 by green, and the colour change itself requires an aqueous ionic environment; it does not mean that no ions are present. The exact pH depends on the indicator chart and concentration.

pH is defined as the negative logarithm of hydrogen-ion concentration: pH = −log[H⁺]. Adding water dilutes the acid, so the hydrogen-ion concentration per unit volume decreases. A smaller [H⁺] gives a larger pH value, although the solution may still remain acidic if its pH is below 7. Therefore, option A is correct. Dilution does not automatically make the pH zero or leave it unchanged.

For a basic solution, dilution lowers the hydroxide-ion concentration and reduces its basic strength. Because the ionic product of water relates hydrogen and hydroxide ions, the pH moves downward toward 7 as the solution is diluted, though it generally remains above 7 unless sufficient dilution or other conditions make it neutral. Thus option B is correct. Dilution cannot make the pH exceed 14, become zero, or immediately produce an acidic solution.

An acid reacts with a metal carbonate to produce a salt, water and carbon dioxide. For the pair in option A, the balanced equation is 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. This exactly gives all three products named in the question. Sodium hydroxide and water do not provide an acid-carbonate reaction, sodium chloride is already a salt, and sugar with water is not the required reaction. Therefore, option A is correct.

An acid-base reaction is called neutralisation. In a typical neutralisation, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt; for example, HCl + NaOH → NaCl + H₂O. Thus option C is correct. Acid with a metal commonly releases hydrogen, and acid with a carbonate or hydrogencarbonate releases carbon dioxide, so those pairs can produce gas.

A sufficiently reactive metal can displace hydrogen from a dilute acid. The products are a salt and hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Therefore, option A is correct. Acid and base usually undergo neutralisation to form salt and water, while acid and metal carbonate form salt, water and carbon dioxide. Mixing water with common salt does not normally produce hydrogen gas.

The governing concept is the characteristic reaction of acids with carbonates. A pH below 7 indicates that the solution is acidic, meaning it has a higher hydrogen-ion concentration than a neutral solution. Acids react with metal carbonates to form a salt, water and carbon dioxide gas, causing effervescence. Therefore option A is correct; a base or neutral solution does not give this combined evidence, and solid salt is not a solution.

The governing concept is the identification of bases by indicators and the pH scale. A base changes red litmus to blue, while a neutral substance generally leaves it unchanged. Also, pH 10 is greater than 7, so the solution has alkaline character. Both observations lead to option C. Option A contradicts both tests, option B would fit pH 7, and the test gives no evidence that the sample is a gas.

Blue litmus turning red shows that the solution is acidic. An acidic solution contains hydrogen ions, \(\mathrm{H^+}\). Zinc reacts with these ions to release colourless hydrogen gas: \(\mathrm{Zn + 2H^+ \rightarrow Zn^{2+} + H_2}\). Hence, \(\mathrm{H^+}\) is the correct option. \(\mathrm{OH^-}\) is associated with basic solutions; \(\mathrm{Cl^-}\) can occur in HCl but does not produce the acidic behaviour. Exam tip: Blue litmus changing to red and \(\mathrm{H_2}\) evolution with a metal are key tests for an acid.

The governing safety principle is that dilution of concentrated acid is highly exothermic: considerable heat is released when acid mixes with water. If a small amount of water is poured into acid, that water can heat and boil suddenly at the surface, throwing corrosive acid droplets outward. Therefore option A is correct. The safe procedure is to add acid slowly to a larger volume of water with stirring and protection; the other options describe no relevant chemical effect.

The governing concept is the logarithmic nature of pH: pH = −log10[H+]. An increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 3 to pH 6 is three units, so [H+] becomes 10 × 10 × 10 = 1000 times smaller. Thus acidity decreases by a factor of 1000, making option A correct; it is not merely three times lower.

The governing concept is that acidic behaviour requires ions in an aqueous medium. Hydrogen chloride molecules do not release mobile hydrogen ions merely because they are present as dry gas; in water they ionise, represented as HCl + H2O → H3O+ + Cl−. Dry litmus therefore shows no acidic colour change. Option A is correct. Litmus can respond to acids as well as bases, HCl is not basic, and the important missing condition is water rather than total chemical inactivity of litmus.

The governing reactions are carbonate–acid reaction and the limewater test for carbon dioxide. Acid plus sodium carbonate releases CO2. Initially, CO2 reacts with limewater: CO2 + Ca(OH)2 → CaCO3↓ + H2O, and insoluble CaCO3 causes milkiness. With excess CO2, CaCO3 + CO2 + H2O → Ca(HCO3)2, which is soluble, so the milkiness disappears. Hence option A is correct.

The governing concept is salt hydrolysis and the relative strengths of the parent acid and base. A salt made from a strong acid and a weak base contains a cation that hydrolyses water and produces hydronium ions; for example, ammonium chloride gives an acidic solution because NH4+ reacts with water. Therefore option A is correct. A strong-acid/strong-base salt is generally neutral, while a weak-acid/strong-base salt is generally basic. Colour cannot determine acidity.

Baking soda is sodium hydrogen carbonate, with the formula \\(\mathrm{NaHCO_3}\\); therefore, option A is correct. Washing soda is sodium carbonate decahydrate, \\(\mathrm{Na_2CO_3\cdot 10H_2O}\\), not calcium sulphate. Bleaching powder is calcium oxychloride, whereas plaster of Paris is calcium sulphate hemihydrate. Exam tip: memorise common salt names together with their chemical formulae.

The governing concept is electrical conduction by mobile ions and the distinction between strong and weak acids. At equal concentration, hydrochloric acid ionises almost completely in water, producing many H3O+ and Cl− ions. Ethanoic acid is weak and only partially ionises, so it supplies fewer charge carriers. Consequently HCl has higher conductivity, making option A correct. Equal labels as acids do not imply equal ionisation, and option B contradicts the stated equal concentration.

The governing reaction pattern is acid plus carbonate or hydrogencarbonate. In both cases, the acid supplies H+ to form carbonic acid, which decomposes into water and carbon dioxide: CO3^2− + 2H+ → H2O + CO2, while HCO3− + H+ → H2O + CO2. The remaining ions form a salt. Thus option A lists all main products; hydrogen, oxygen and nitrogen are not produced in these reactions.

The governing concept is neutralisation. A bee sting is commonly treated in school chemistry as introducing an acidic substance, so a mild, weakly basic material can partially neutralise it and reduce irritation. Baking soda, sodium hydrogen carbonate, is mildly basic and is therefore the suitable choice in option A. Lemon juice and vinegar are acidic, so they would not neutralise an acidic sting, while hydrochloric acid would add corrosive acidity rather than provide relief.