Acids, Bases, and Salts

An acidic solution contains hydrogen ions, H⁺, while sodium hydroxide dissociates in water to provide hydroxide ions, OH⁻. The central ionic neutralisation reaction is H⁺ + OH⁻ → H₂O. Sodium ions and any accompanying anions generally remain as spectator ions and may form a dissolved salt. Therefore option A correctly states the main change; the other options involve chemically unsupported transformations.

The governing concept is dilution and hydrogen-ion concentration. Adding water increases the volume, so the concentration of hydrogen ions decreases and the pH rises toward 7. However, dilution alone does not automatically remove every hydrogen ion or create an excess of hydroxide ions. Therefore, while the acid becomes weaker, it remains acidic as long as hydrogen ions still determine the solution’s behaviour. Option B is false because water does not convert every acid into a base, and D is false because pH does change.

Neutralisation depends on the amounts of reacting hydrogen and hydroxide ions, not merely on the volumes of the two solutions. Equal volumes can contain very different numbers of particles when their concentrations differ; their acids or bases may also differ in strength. If the acid supplies a greater effective amount of hydrogen ions than the base can neutralise, some acidity remains and the final pH stays below 7. Thus A is correct. Equal volume does not guarantee neutrality, and salt formation does not produce hydrogen gas here.

The governing idea is that neutralisation compares reacting amounts, not just liquid volumes. Equal volumes of an acid and a base may contain unequal concentrations of ions. A more concentrated or stronger base can provide more hydroxide ions than the acid can neutralise. After the reaction, excess hydroxide ions remain, so the mixture has a pH above 7 and is basic. Option C is wrong because equal volume has no fixed pH, while D is wrong because acid-base neutralisation commonly forms water along with salt.

A base is a substance that reacts with an acid in a neutralisation reaction to produce salt and water. Solubility is the key distinction in this question: a base may be soluble or insoluble in water, whereas an alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Since the substance is stated to be insoluble, it cannot be called an alkali. It is therefore correctly identified as a base, making A the only suitable answer.

The governing property of a base is its ability to neutralise an acid. Many metal oxides show this behaviour: they react with acids to produce a salt and water, for example, copper oxide reacts with hydrochloric acid to form copper chloride and water. This neutralisation reaction supports their generally basic classification. Option B describes acidic behaviour and is not universally true; metal oxides are not always gases, and option D has no chemical basis. The word generally also allows recognized exceptions such as amphoteric oxides.

An acidic oxide generally forms an acidic solution with water or reacts with a base to form salt and water. Many non-metal oxides show one or both of these behaviours; for example, carbon dioxide dissolves in water to form carbonic acid and reacts with bases to produce carbonates. Hence they are generally classified as acidic oxides. Option C describes a basic effect, while B and D make absolute claims that are not chemically valid for all non-metal oxides.

Copper oxide is a metal oxide, and its reaction with hydrochloric acid demonstrates the basic nature of many metal oxides. The reaction is CuO + 2HCl → CuCl₂ + H₂O. Copper(II) chloride dissolves in the solution and gives it a blue-green colour, while water is also formed. Thus the observation supports option A: copper oxide reacts with an acid to produce a salt and water. The other options contradict the reaction or the observed chemistry.

Sodium carbonate reacts with an acid to release carbon dioxide: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Carbon dioxide is identified by passing it through limewater, where it reacts with calcium hydroxide to form insoluble calcium carbonate, making the limewater milky. A pop sound is the test for hydrogen, not carbon dioxide. Litmus and turmeric describe general acid-base behaviour and are not the specific gas test required here.

Hard water contains dissolved calcium and magnesium ions, which interfere with soap and form scum. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions. These combine with calcium and magnesium ions to form insoluble calcium carbonate and magnesium carbonate precipitates, removing the troublesome ions from the water. Consequently, soap can lather more readily. It does not produce hydrogen, increase acidity, or destroy sodium ions, so option A correctly explains the softening process.

The pH scale shows that a value of 4.5 represents acidic soil. Slaked lime, calcium hydroxide [Ca(OH)₂], is a basic substance, so adding an appropriate quantity helps neutralise excess acidity and raise the soil pH toward a suitable range. The amount must be based on soil testing because excessive lime can also disturb soil conditions. Vinegar, lemon juice, and dilute acid are acidic materials; adding them would intensify rather than correct the problem. Therefore A is correct.

The governing concept is that pH measures the acidic or basic nature of water. Acid rain adds acidic substances and may lower the lake’s pH. Aquatic organisms often need a limited pH range for enzyme activity, respiration, reproduction, and maintaining body chemistry. Therefore, very low pH can disrupt life processes and reduce survival. The other options are scientifically unrelated or use absolute claims.

Universal indicator gives different colours over a pH range: blue generally indicates a basic solution, while green indicates a value close to neutral. When acid is added, hydrogen ions react with hydroxide ions in the basic solution, reducing its alkalinity. If the colour becomes green, the mixture has reached approximately neutral pH. It has not become more basic, and the observation does not prove that it is a strong acid.

The governing principle is neutralisation. A red universal-indicator colour represents a strongly acidic region, with a relatively high hydrogen-ion concentration. Adding a base supplies hydroxide ions, which react with hydrogen ions to form water. This decreases acidity and moves the pH toward 7. Green indicates a nearly neutral mixture, so option A is correct. The result does not show that the solution became strongly basic or that the base had no effect.

pH is related inversely to hydrogen-ion concentration: when hydrogen-ion concentration decreases, pH rises. An acidic solution contains excess H⁺ ions. Added hydroxide ions, OH⁻, react with them according to H⁺ + OH⁻ → H₂O. This neutralisation removes free hydrogen ions from the solution, so its acidity falls and its pH increases. The other choices describe neither the relevant reaction nor the reason for the pH change.

The relevant principle is the metal reactivity series. A metal above hydrogen can displace hydrogen from a dilute acid, producing hydrogen gas; for example, metal + acid can form a salt and H₂. Copper is below hydrogen in the usual school-level reactivity series and generally does not release hydrogen from dilute non-oxidising acids. Thus A follows. The gas is not always carbon dioxide, and the observation does not make copper highly reactive.

Appearance alone is not a reliable chemical test. Many common acids, bases, and neutral solutions are colourless, so a person cannot determine their nature merely by looking at them. An appropriate indicator, pH paper, or universal indicator gives evidence through a controlled colour change; a pH meter can provide a numerical value. Options B and C make false universal claims, while D incorrectly says colour always gives an exact pH.

The governing principle is laboratory safety: an unknown chemical must never be tasted. Acids and bases may be corrosive, poisonous, contaminated, or dangerous even in small amounts, and taste cannot reliably identify concentration or pH. Indicators, pH paper, or suitable instruments provide safer evidence without direct contact. Therefore A is correct. Taste does not give an exact pH, not all acids or bases are edible, and indicators are specifically useful for testing acidic or basic nature.

A blue indicator paper turning red is evidence that the solution is acidic. Many acids react with carbonates to release carbon dioxide, so bubbling would normally be expected when the reagent and conditions are suitable. However, no bubbles can result from an unsuitable or insufficient carbonate, low acid concentration, poor contact, or an experimental fault. Thus the cautious conclusion in A is best; the observation does not prove that the solution is basic, neutral, or only sugar.

The experiment demonstrates the role of water in producing acidic ions. Dry hydrogen chloride gas does not provide mobile H⁺ or hydronium ions to the dry indicator, so no red colour appears. On moist paper, HCl dissolves in water and ionises, forming hydronium ions; these give the acidic indicator colour. Therefore A is correct. The result does not show that HCl is basic, that dry paper is defective, or that water alone is an acid.