Acids, Bases, and Salts

Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The word decahydrate means that each formula unit of sodium carbonate is associated with ten molecules of crystallisation water. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and calcium sulphate hemihydrate is plaster of Paris.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of some hydrated salts. It is part of the crystal structure, not merely moisture or dissolved water. For example, CuSO4·5H2O contains five water molecules per formula unit, while washing soda is Na2CO3·10H2O. Thus option A gives the correct meaning; B, C and D describe unrelated substances or situations.

Plaster of Paris is used because it undergoes a controlled setting reaction with water. Calcium sulphate hemihydrate changes into gypsum, CaSO4·2H2O, forming a hard, mouldable support around the injured limb. The hardened cast restricts movement and helps protect the fracture while healing occurs. Thus option A is correct; it neither dissolves bone nor remains liquid or disappears.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, calcium hydroxide, Ca(OH)2. In the school-level representation, the product is written as CaOCl2, commonly called bleaching powder, and its bleaching action is associated with chlorine. Therefore chlorine is the correct gas in option A. Hydrogen, nitrogen and helium do not produce bleaching powder in this preparation.

Chlorine is one of the products of brine electrolysis in the chlor-alkali process. Because chlorine is a strong oxidising and disinfecting agent, it is used in controlled amounts to kill many disease-causing microorganisms in drinking water. It is also used for bleaching paper pulp and textiles. Therefore option A is correct; the other options are not recognised applications of chlorine.

Baking soda is the common name of sodium hydrogen carbonate, whose formula is NaHCO₃. It is a mild base and is used in baking because it can release carbon dioxide under suitable conditions, helping dough rise. Sodium chloride is common salt, sodium hydroxide is a strong base, and calcium carbonate is found in materials such as limestone and chalk; therefore, option A is correct.

The reaction between an acid and a base is called neutralisation. In the usual neutralisation reaction, the 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. Metal oxidation usually forms an oxide, hydrogen combustion forms water alone, and dissolving salt is not a new formation reaction. Hence A is correct.

Vinegar contains acetic acid, so its aqueous solution is acidic. Acids turn blue litmus paper red because the indicator changes colour in an acidic environment; therefore option A is correct. Sodium hydroxide, soap solution, and lime water are basic substances or solutions and generally turn red litmus blue rather than changing blue litmus to red. Identifying the nature of each solution before applying the indicator rule avoids confusion.

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 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.

This question uses indicator colour changes. Phenolphthalein is colourless in acidic or neutral solution but turns pink in a basic solution, so its pink colour is decisive. Methyl orange is yellow in a neutral or basic medium and red in an acidic medium, so yellow supports the same conclusion. Thus the solution is basic and option A is correct; it is not acidic, neutral, or insoluble.

Red methyl orange indicates an acidic medium, while colourless phenolphthalein is also consistent with an acidic solution. To neutralise an acid, a suitable base is added so that hydrogen ions react with hydroxide ions to form water: H⁺ + OH⁻ → H₂O. The quantity must be controlled to avoid making the mixture strongly basic. Therefore option A is correct; salt and sugar do not provide the required neutralisation.

Reddish-brown turmeric and blue red litmus are both tests for a basic medium. In aqueous school-level chemistry, basicity is associated with an excess of hydroxide ions, OH⁻, over hydrogen ions. Therefore hydroxide ion is the best answer. Hydrogen ions would indicate acidity, whereas chloride and sulphate ions may occur in salts but do not by themselves determine the observed indicator changes. Hence option A is correct.

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 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.

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.

The governing concept is the fixed composition of hydrated salts. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so ten water molecules are incorporated in the crystal structure in a definite ratio. This chemically associated water is called water of crystallisation, making option A correct. The other choices are not recognised chemical terms for water present in a salt crystal.

Indigestion associated with excess acidity involves an unnecessarily high concentration of hydrochloric acid in the stomach. A mild basic antacid reacts with and neutralises some of this acid, raising the pH toward a less irritating value. It must be mild because a strong alkali could damage tissues. More hydrochloric acid would worsen the condition, while sugar and carbon dioxide do not neutralise stomach acid. Therefore A is correct.

When the irritating substance from an ant sting is treated as acidic, a mild base is the appropriate chemical counteragent. Baking soda, sodium hydrogen carbonate (NaHCO₃), provides mild alkalinity and can neutralise part of the acid, reducing its irritating effect. Lemon juice and vinegar are acidic, so they would not neutralise the acid, and adding a stronger acid would worsen irritation. Therefore option A is the best answer.

Methyl orange is an acid-base indicator. Its molecular form and ionic form have different colours, so the observed colour changes with the acidity of the solution. Red indicates an acidic medium, while yellow indicates a basic or less acidic medium in the simplified school-level description. This makes it useful for identifying the nature of a solution and for titration observations. Therefore A is correct; the other choices are unrelated.

Turning red indicator paper blue shows that the solution is basic rather than acidic or neutral. Phenolphthalein becoming pink supports the same conclusion because it is pink in a sufficiently alkaline medium. Of the listed values, pH 11 is clearly basic, whereas pH 7 is neutral and pH 3 or 1 are acidic. Therefore option A is the only value consistent with both indicator observations.

Hardness of water is mainly caused by dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). Washing soda, sodium carbonate (Na₂CO₃), reacts with these ions and forms insoluble carbonates such as calcium carbonate and magnesium carbonate. These precipitates can be separated, so the hardness-producing ions are removed. The other options do not represent the ions responsible for ordinary hard water.

Baking soda is sodium hydrogen carbonate, while vinegar contains acetic acid. When they react, the acid first forms carbonic acid, which quickly decomposes into carbon dioxide, water, and a salt: NaHCO₃ + acid → CO₂ + H₂O + salt. The escaping CO₂ bubbles become trapped in the liquid and produce foam. No hydrogen metal, frozen oxygen, or burning sodium chloride is involved.