Acids, Bases, and Salts

The governing concept is the behaviour of acids in aqueous solution. Turning blue litmus red is the characteristic test for an acidic solution, which contains hydrogen ions, represented as H+ or hydronium ions in water. These mobile ions also carry electric charge, explaining the good conductivity. Hydroxide ions indicate a base, while chloride and sodium ions alone do not establish acidity.

The governing concept is ionisation in water. Hydrochloric acid dissociates almost completely into H+ (more precisely H3O+) and Cl− ions, and these mobile charged particles conduct electricity. Sugar and alcohol may dissolve, but their molecules remain largely uncharged, so their solutions conduct very poorly. Distilled water contains only a tiny concentration of ions and is therefore a very poor conductor compared with hydrochloric acid.

The governing concept is the colour response of litmus indicators. Acids change blue litmus to red because they provide H+ ions in water, whereas bases change red litmus to blue because they provide OH− ions or accept protons. Therefore option A states both changes in the correct direction. Option B reverses them, while C and D incorrectly deny indicator effects and the differing pH ranges.

The governing concept is the pH scale: lower pH means greater acidity, pH 7 is neutral at ordinary school-level conditions, and values above 7 indicate increasing basicity. Thus moving from pH 2 to pH 7 reduces acidity to neutrality, and continuing to pH 10 increases basicity. Option B reverses the direction; C and D do not show a continuous decrease in acidity with increasing basicity.

The governing concept is the acid–base reaction. Acids react with bases to produce salt and water, and many non-metal oxides show the same behaviour when they react with bases. For example, carbon dioxide reacts with sodium hydroxide to form sodium carbonate and water. Thus the oxide has acidic nature; it is not basic, metallic, or completely neutral.

The governing idea is that colour alone is not a reliable chemical test. Many acids and bases, such as dilute hydrochloric acid and sodium hydroxide solution, are colourless, while the colour of a substance may come from another dissolved chemical. An indicator, universal indicator, or pH paper should be used because its colour change is linked to acidity or basicity. Therefore option A is correct.

The governing principle is laboratory safety. Unknown acids and bases may be corrosive, poisonous, or harmful even in small amounts, so tasting them can cause burns, poisoning, or internal injury. Their identity and pH must instead be tested with suitable indicators, pH paper, or approved laboratory procedures. Therefore option A is correct; the other choices make false claims about sweetness, colour, or the accuracy of taste.

Blue litmus turning red is the standard indication of an acidic solution. Deep red on pH paper generally corresponds to a very low pH, indicating strong acidity rather than neutrality or basicity. Taken together, the observations support the conclusion that the solution is strongly acidic. The result does not prove that it contains only one substance or only salt, so option A is the safest and best answer.

A base changes red litmus paper to blue, so option A is correct. Litmus is a natural indicator that shows different colours in acidic and basic substances. Red litmus turns blue in a basic medium; it does not turn green or yellow. Exam tip: bases turn red litmus blue, whereas acids turn blue litmus red.

A pH greater than 7 generally indicates a basic (alkaline) solution. A pH of 7 is neutral, whereas a pH below 7 indicates an acidic solution. “Salty” only suggests the possible presence of a salt; it does not necessarily mean that the pH is greater than 7. Exam tip: Use 7 as the reference point on the pH scale—below it is acidic and above it is basic.

A pH below 7 indicates a relatively higher concentration of hydrogen ions
(H^+
), so the solution is acidic. A pH of 7 is neutral, whereas a pH above 7 is basic. “Saline” may indicate dissolved salts, but it does not by itself determine whether a solution is acidic or basic. Exam tip: remember 7 as the midpoint of the pH scale—below it is acidic and above it is basic.

On the pH scale, 7 is neutral. A solution with pH below 7 has acidic properties, so it is an acidic solution. Basic solutions have pH above 7, whereas a neutral solution has pH 7. Exam tip: remember pH < 7 as acidic, pH = 7 as neutral, and pH > 7 as basic.

On the pH scale, a value of 7 indicates a neutral solution, so neutral is correct. Pure water has a pH of about 7; it is neither acidic nor basic. Basic solutions have pH values greater than 7, whereas acidic solutions have pH values below 7. Exam tip: remember that pH 7 is neutral, below 7 is acidic, and above 7 is basic.

Litmus is an acid–base indicator. In an acidic solution, blue litmus changes to red, so option A is correct. This colour change occurs because the indicator responds to the acidic conditions. A base generally changes red litmus to blue, while green and yellow are not the standard litmus result for this test. Thus the blue-to-red rule identifies an acid.

Bases change red litmus paper to blue, so option A is correct. Litmus is a natural indicator: acids turn blue litmus red, whereas bases turn red litmus blue. Green is not the usual litmus result for a neutral solution. Exam tip: red litmus turning blue indicates a base.

The governing concept is identifying acids by their chemical formula and composition. Hydrochloric acid consists of hydrogen and chlorine in a 1:1 ratio, so its formula is HCl. When dissolved in water, it ionises to form hydrogen ions and chloride ions. H₂SO₄ is sulphuric acid, HNO₃ is nitric acid, and CH₃COOH is acetic acid. Therefore, option A is unambiguously correct.

A solution with a pH below 7 is acidic, so ‘Acidic’ is correct. A pH of 7 indicates a neutral solution, while a pH above 7 indicates a basic solution. A pH below 7 does not necessarily mean strongly acidic; the strength depends on the actual pH value. Exam tip: Remember 7 as the neutral point on the pH scale.

The governing concept is identifying an ionic compound from its ions and chemical formula. Common salt is sodium chloride, formed from sodium ions, Na⁺, and chloride ions, Cl⁻, in a 1:1 ratio; therefore its neutral formula is NaCl. NaOH is sodium hydroxide, Na₂CO₃ is sodium carbonate, and HCl is hydrochloric acid. Since Na⁺ and Cl⁻ balance each other’s charges, option A is correct.

Citric acid is the main organic acid in lemon juice and is chiefly responsible for its sour taste. Acetic acid is mainly found in vinegar, lactic acid in curd, and formic acid in ant stings. Exam tip: Associate lemons, oranges, and other citrus fruits with citric acid.

Sodium hydroxide contains sodium ions, Na⁺, and hydroxide ions, OH⁻. Their charges balance in a 1:1 ratio, giving the formula NaOH. NaCl is sodium chloride, while Na₂CO₃ is sodium carbonate. Exam tip: check a compound’s formula by balancing the charges of its positive and negative ions.