Acids, Bases, and Salts

Blue copper sulphate crystals are hydrated copper sulphate, CuSO₄·5H₂O, and their blue colour is associated with water of crystallisation. On heating, this water is removed and anhydrous copper sulphate, CuSO₄, is formed, which is white. The change is reversible when water is added again. The crystals do not become red, green or black under the stated school-level condition, so option A is correct.

Anhydrous copper sulphate, CuSO₄, is white because it contains no water of crystallisation. When water is added, hydration occurs and blue hydrated copper sulphate, CuSO₄·5H₂O, is produced. This is the reverse of the colour change seen on heating blue crystals. The substance does not become red, yellow or black in this standard reaction, so option A, blue, is correct.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Thus, carbon dioxide is released as the gas. The water formed is vapour, while sodium carbonate remains as a solid; no hydrogen, oxygen or nitrogen is produced in this reaction. Therefore, option A is correct.

Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid, such as tartaric acid. On heating or when water is added, the acid reacts with the soda to release carbon dioxide, making cakes and bread rise. Iron, chlorine and copper are not added for this purpose. Exam tip: Remember the difference between baking soda and baking powder—baking powder also contains an acid.

Sodium hydrogen carbonate in baking powder produces carbon dioxide on heating or when it reacts with an acid. Bubbles of this gas expand the batter or dough, making cakes and bread rise. Oxygen supports combustion, but it does not cause the rising. Exam tip: Remember that baking powder and baking soda release carbon dioxide.

In a soda-acid fire extinguisher, sodium hydrogen carbonate reacts with an acid to produce carbon dioxide. This gas forms a blanket over the fire and cuts off its contact with oxygen, thereby stopping combustion. Oxygen is a close distractor, but it supports burning rather than putting out a fire. Exam tip: Remember carbon dioxide as the gas that smothers fire in this extinguisher.

In the chlor-alkali process, concentrated aqueous sodium chloride solution, called brine, is electrolysed. Chlorine is produced at the anode, hydrogen at the cathode, and sodium hydroxide remains in solution. A sucrose solution is not an electrolyte, so it is not used in this process. Exam tip: Remember that brine means concentrated aqueous NaCl solution.

In the chlor-alkali process, aqueous sodium chloride (brine) is electrolysed. At the anode, chloride ions lose electrons to form chlorine gas: \(2Cl^- \rightarrow Cl_2 + 2e^-\). Therefore, chlorine is correct. Hydrogen is produced at the cathode, not at the anode, by reduction of water. Exam tip: oxidation occurs at the anode; in the chlor-alkali process, remember it by chlorine formation.

Na₂CO₃·10H₂O is washing soda, or sodium carbonate decahydrate. Its carbonate ions react with dissolved Ca²⁺ and Mg²⁺ ions responsible for permanent hardness, producing insoluble carbonates that can be removed. NaHCO₃ is baking soda, CaOCl₂ is bleaching powder, and CaSO₄·½H₂O is plaster of Paris. Therefore, the salt used for washing and softening hard water is option A.

Acidic soil contains excess acidic substances, so it is improved by adding a suitable base through neutralisation. Slaked lime, Ca(OH)₂, is a basic substance and reacts with excess acidity, helping raise the soil pH toward a suitable value for crops. Vinegar and lemon juice contain acids, and hydrochloric acid would make the soil even more acidic. Hence, slaked lime in option A is the correct choice.

The pH scale indicates the acidic or basic nature of a solution: pH 7 is neutral, values below 7 are acidic, and values above 7 are basic. Since pH 3 is below 7, this solution is acidic. Acids change blue litmus to red, so option A is correct. Options B, C, and D do not describe the standard blue-litmus response to an acidic solution.

A pH greater than 7 indicates a basic or alkaline solution. Because the given pH is 10, the solution is basic. A base changes red litmus paper to blue, so option A is correct. Red litmus remaining red would suggest no basic change, while yellow or colourless are not the characteristic observations for this indicator test. The conclusion follows directly from the pH scale and litmus behaviour.

On the pH scale, acidity increases as the pH value decreases. Since 2 is lower than 5, the pH 2 solution is more acidic. In terms of hydrogen-ion concentration, a one-unit pH difference represents a tenfold difference; therefore pH 2 has a much greater hydrogen-ion concentration than pH 5. Thus option A is correct, not B, C, or D.

A solution with pH above 7 is basic, and basicity increases as the pH value rises. Both pH 8 and pH 12 are basic, but 12 is higher than 8, so the pH 12 solution is more basic. The difference also corresponds to a much lower hydrogen-ion concentration at pH 12. Therefore option A is correct; options C and D misclassify both solutions.

Phenolphthalein is an acid-base indicator. It is colourless in acidic and neutral conditions, but it develops a pink colour in a basic solution, especially when the pH is sufficiently alkaline. The observed pink colour therefore indicates that the solution is basic, making option A correct. Acidity would keep the indicator colourless, and being a salt alone does not determine the observed colour without knowing its solution properties.

Methyl orange is an acid-base indicator whose colour depends on the medium. It appears red in an acidic solution and changes through orange toward yellow as the medium becomes less acidic or basic. Since the observation is red, the solution is acidic, so option A is correct. A basic solution would produce yellow, while neutral and metallic are not supported by the indicator’s red colour.

Turmeric contains curcumin, which acts as a natural indicator. In an acidic or neutral medium it generally remains yellow, but a basic substance changes it to a reddish-brown colour. Therefore the observed reddish-brown turmeric paper shows that the solution is basic, so option A is correct. The other options do not explain the characteristic colour change produced by an alkaline medium.

Onion is an olfactory indicator because its characteristic smell changes in different chemical media. In a basic medium, the onion odour is reduced or becomes difficult to detect, whereas it generally persists in an acidic medium. Thus a decrease in smell indicates a basic solution, making option A correct. This observation is not a test for insolubility, and neutral or acidic descriptions do not match the usual indicator response.

Hydrogen chloride shows acidic behaviour when it dissolves in water and ionises, producing hydronium ions, H₃O⁺, which are responsible for the acidic reaction. Dry HCl gas in contact with dry blue litmus has no water available for this ionisation, so the ions are not produced and the litmus remains unchanged. Hence option A is correct; colourlessness or basicity is unrelated to the result.

Acids display their characteristic properties in aqueous solution because water allows their molecules to ionise and produce hydrogen ions, more accurately represented as hydronium ions, H₃O⁺. These ions cause acidic behaviour, including litmus change and reaction with suitable substances. Without water, the ions are not produced freely, so the acid does not show the usual aqueous properties. Therefore option A is correct.