Acids, Bases, and Salts

The governing concept is the logarithmic pH scale: pH = −log[H⁺]. A rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 1 to pH 4 is three units, so acidity changes by 10 × 10 × 10 = 1000 times downward. Thus option A is correct; B treats pH as a linear scale, while C and D give the wrong direction or ignore the change.

The governing concept is acid-base neutralisation. Slaked lime is calcium hydroxide, Ca(OH)₂, a basic substance. In acidic soil, its hydroxide ions react with excess hydrogen ions, reducing acidity and moving the pH upward toward a suitable range. Therefore option A is correct. B reverses the chemical effect, while C and D describe impossible changes and do not explain pH.

The governing concept is salt hydrolysis and the relative strengths of the acid and base that form a salt. A salt such as sodium chloride gives a nearly neutral solution, ammonium chloride gives an acidic solution, and sodium carbonate gives a basic solution. Hence option C is correct. Options A and B wrongly generalise one nature to every salt, and D confuses acidity with a fixed pH of zero.

The governing concept is the relationship between pH and alkalinity, together with laboratory safety. A pH of 13 indicates a strongly basic solution with a high hydroxide-ion concentration; it may irritate or damage skin and eyes. Gloves and goggles reduce exposure and protect these tissues, so option A is correct. Tasting, direct touching, and storing it in a drink container are unsafe practices, not precautions.

The governing concept is the ecological effect of acid-base change. Acid rain contains acidic oxides and dissolved acids that can lower the pH of lake water. Aquatic organisms function within particular pH ranges; excessive acidity can disturb enzyme activity, respiration, reproduction, and survival. Therefore option A is correct. B reverses the effect, while C and D ignore the biological and chemical consequences of lower pH.

The governing concept is acid-base identification followed by neutralisation. A solution that turns red litmus blue is basic, while one that turns blue litmus red is acidic. When suitable amounts react completely, hydrogen ions combine with hydroxide ions to form water, and the remaining ions form a salt. Thus option A is correct; the other choices are not general products of neutralisation.

The governing concept is the basic nature of many metal oxides and their neutralisation reaction with acids. For example, copper(II) oxide reacts with hydrochloric acid to form copper(II) chloride and water. This acid-neutralising behaviour supports option A. Option B describes an acidic response and is not generally true; C is false because many metal oxides are solids, and D has no chemical basis.

The governing concept is the acidic nature of many non-metal oxides in water. Oxides such as carbon dioxide and sulfur dioxide react or dissolve in water to produce acidic solutions, which provide hydrogen ions and turn blue litmus red. Therefore option A is correct. B is chemically unrelated, C reverses the usual behaviour, and D is too absolute because several non-metal oxides dissolve or react with water.

The governing concept is the reaction of acids with reactive metals and food-safety precaution. Curd contains lactic acid and has an acidic character. During prolonged contact, the acid may react with copper and produce copper compounds that can contaminate the food and may be harmful in excess. Therefore option A is correct; B and C are impossible, while D falsely denies the acidity of curd.

The governing concept is the colour range of a universal indicator and neutralisation. Violet indicates a strongly basic medium, while blue represents a less strongly basic medium. Adding acid consumes some hydroxide ions and lowers the pH, but the remaining solution is still on the basic side of pH 7. Therefore option A is correct; it is not acidic, completely neutral, or unchanged.

The governing reaction is acid–carbonate reaction: acid + carbonate produces a salt, water, and carbon dioxide. For example, sodium carbonate with hydrochloric acid gives sodium chloride, water, and CO₂. Hydrogen is associated with the reaction of suitable active metals with acids, not carbonates. Carbon dioxide can be confirmed because it turns limewater milky, so option A is correct.

Neutralisation is governed by the ionic reaction H⁺ + OH⁻ → H₂O. An acid supplies hydrogen ions, while sodium hydroxide dissociates in water to supply hydroxide ions. These two ions combine directly to form water; sodium and the acid’s negative ion remain as spectator ions in solution. Thus option A is correct, while the other pairs do not form water in this reaction.

A base is a substance that reacts with an acid to form salt and water, usually through neutralisation. Solubility is the key distinction here: an alkali is specifically a base that dissolves in water, whereas an insoluble base is still a base but not an alkali. Since the substance is insoluble yet neutralises an acid, option A is correct; it is neither an acid nor an indicator.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)₂. The process produces a chlorine-containing bleaching material commonly represented as CaOCl₂ at school level. The word “dry” matters because moisture can interfere with the preparation. Sodium chloride, gypsum, and washing soda are not the required starting substance, so option A is correct.

The governing idea is dilution. Adding water increases the solution’s volume, so the concentration of hydrogen ions decreases and the pH moves toward 7. However, dilution does not automatically remove every hydrogen ion or create hydroxide ions in excess. Therefore the solution can remain acidic, although weaker. Option A is correct; water does not turn every acid basic, destroy all ions, or leave pH unchanged.

The governing concept is chemical identification using indicators rather than appearance. Many common acids and bases, such as dilute hydrochloric acid and sodium hydroxide solution, are colourless. Litmus, universal indicator, or pH paper undergoes a characteristic colour change that provides evidence about acidic or basic nature. Thus option A is correct; no fixed visible colour applies to every acid or base, and colour alone cannot give exact pH.

Litmus is an acid–base indicator with a characteristic colour change. Blue litmus turns red in an acidic medium because the indicator responds to the presence of excess hydrogen ions. A basic solution generally turns red litmus blue, while a neutral solution does not produce this acidic change. Therefore the solution described is acidic, making option A correct; “salty” describes neither a definite indicator response nor a guaranteed pH.

The governing indicator rule is that a basic medium changes red litmus to blue. Bases produce hydroxide ions in aqueous solution, and the litmus dye responds with this characteristic colour change. Acids show the opposite effect on blue litmus, while a neutral solution normally does not turn red litmus blue. Hence option B is correct. The terms acidic, neutral, and oily do not explain the observed indicator response.

The pH scale expresses the acidic or basic nature of an aqueous solution. At approximately 25°C, pH 7 represents neutrality, where hydrogen-ion and hydroxide-ion concentrations are equal. Values below 7 are acidic and values above 7 are basic. Therefore option C, neutral, is correct. The answer assumes the usual school-level aqueous pH scale and conditions; pH 7 is not highly acidic or basic.

The pH scale indicates the acidic or basic nature of an aqueous solution. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic. Since pH 3 is below 7, the solution contains a relatively higher concentration of hydrogen ions and is therefore acidic. It is not neutral or basic; sweetness is unrelated to pH.