Assertion & Reason Test: Acids, Bases & Salts - 1 - Class 10 MCQ

• Assertion (A): After white washing the walls, a shiny white finish on walls is obtained after two to three days.

  • This statement is true. After applying whitewash (which typically contains calcium hydroxide, not calcium oxide directly), the walls initially appear wet and chalky. Over time, as the whitewash dries and carbon dioxide from the air reacts with calcium hydroxide, a shiny white finish develops.

• Reason (R): Calcium Oxide reacts with Carbon dioxide to form Calcium Hydrogen Carbonate which gives shiny white finish.

  • This statement is partly incorrect:
    Calcium oxide (CaO) reacts with carbon dioxide (CO2) to form calcium carbonate (CaCO3), not calcium hydrogen carbonate. Calcium carbonate can contribute to the shiny white appearance after whitewashing, but it does not form calcium hydrogen carbonate as stated.

• Assertion (A): Gas bubbles are observed when sodium carbonate is added to dilute hydrochloric acid.

  • This statement is true. When sodium carbonate (Na2CO3) reacts with dilute hydrochloric acid (HCl), carbon dioxide gas (CO2) is produced. The evolution of gas bubbles is observed as a result of this reaction.

• Reason (R): Carbon dioxide is given off in the reaction.

  • This statement is true and correctly explains why gas bubbles are observed. The reaction between sodium carbonate and hydrochloric acid produces carbon dioxide gas, which escapes as bubbles in the solution.

• Assertion (A): Ammonium hydroxide (NH₄OH) solution is an alkali because it produces hydroxide ions (OH^-) in an aqueous solution. Hence, the assertion is true.

• Reason (R): Ammonium hydroxide solution does not turn blue litmus paper red; instead, it turns red litmus paper blue because it is basic in nature. Therefore, the reason is false.

• Assertion (A): Baking soda creates acidity in the stomach.

  • This statement is false. Baking soda (sodium bicarbonate, NaHCO3) is alkaline in nature, not acidic. It acts as a base and can neutralize acids in the stomach, helping to alleviate symptoms of acid reflux or indigestion by raising the pH level.

• Reason (R): Baking soda is alkaline.

  • This statement is true. Baking soda is indeed alkaline. When ingested, it reacts with stomach acid (hydrochloric acid, HCl) to form carbon dioxide gas, water, and salt, which can help to neutralize excess stomach acid.

• Assertion (A): Plaster of Paris is used by doctors for setting fractured bones.

  • This statement is true. Plaster of Paris (calcium sulfate hemihydrate, CaSO4·0.5H2O) is commonly used in medicine for immobilizing broken bones. It is mixed with water to form a paste that sets into a hard mass, providing support and stability to the fractured limb.

• Reason (R): When Plaster of Paris is mixed with water and applied around the fractured limbs, it sets into a hard mass.

  • This statement is true and correctly explains why plaster of Paris is effective in setting fractured bones. When mixed with water, plaster of Paris undergoes an exothermic reaction, forming gypsum (calcium sulfate dihydrate, CaSO4·2H2O), which hardens rapidly into a solid mass.

• Assertion (A): While dissolving an acid or base in water, the acids must always be added slowly to water with constant stirring.

  • This statement is true. When dissolving acids or bases in water, especially concentrated solutions, adding them slowly to water with stirring helps to control the reaction and prevents splashing or excessive heat generation.

• Reason (R): Dissolving an acid or base in water is a highly exothermic reaction.

  • This statement is true. The process of dissolving strong acids or bases in water is exothermic, meaning it releases heat.

• Assertion (A): On adding H2SO4 to water, the resulting aqueous solution gets corrosive.

  • This statement is true. Sulfuric acid (H2SO4) is a strong acid that dissociates completely in water to form hydronium ions (H3O+) and sulfate ions (SO4^2-). The resulting solution is highly acidic and corrosive to many materials.

• Reason (R): Hydronium ions are responsible for corrosive action.

  • This statement is true and correctly explains why the solution becomes corrosive. Hydronium ions (H3O+) in acidic solutions have a high affinity for reacting with materials, causing corrosion or damage to surfaces they come into contact with.

• Assertion (A): Phenolphthalein gives a pink color in basic solution.

  • This statement is true. Phenolphthalein is an indicator that turns pink in basic or alkaline solutions with a pH greater than 8.2.

• Reason (R): Phenolphthalein is a natural indicator.

  • This statement is false. Phenolphthalein is a synthetic indicator, not a natural one. It is synthesized in the laboratory and is widely used in acid-base titrations and as a pH indicator.

• Assertion (A): HCl produces hydronium ions (H3O+) and chloride ions (Cl-) in aqueous solution.

  • This statement is true. When hydrogen chloride (HCl) dissolves in water, it dissociates completely into hydronium ions (H3O+) and chloride ions (Cl-). The reaction can be represented as: 

• Reason (R): In presence of water, bases give H+ ions.

  • This statement is false. Bases do not give H+ ions in water; they give OH- ions instead. The presence of OH- ions characterizes a basic solution, not H+ ions.

Assertion (A): Sodium hydroxide reacts with zinc to produce hydrogen gas.

• This statement is true. When sodium hydroxide (NaOH) reacts with zinc (Zn), it produces hydrogen gas (H2).

Zn (s) + 2NaOH (aq) → Na₂ZmO₂ (aq)+ H₂ (g)

Reason (R): Acids react with active metals to produce hydrogen gas.

• This statement is also true. Acids such as hydrochloric acid (HCl) or sulfuric acid (H2SO4) react with active metals like zinc (Zn) to produce hydrogen gas.

- Magnesium Hydroxide: Known as Milk of Magnesia, it is a mild base used to neutralize stomach acid.
- Stomach Acid: The stomach produces hydrochloric acid for digestion. Excess acid can cause pain and irritation.
- Antacids: Bases like magnesium hydroxide are used to neutralize excess stomach acid, providing relief from discomfort.
- Correct Answer: Both the assertion and reasoning are true, and the reason correctly explains the assertion. Thus, the choice is A.

- The assertion is true; rubbing dock leaves on nettle stings is a traditional remedy.
- The reasoning is also true; nettle stings are caused by methanoic acid.
- Dock leaves are believed to help due to their soothing properties, but there's no definitive scientific evidence that they neutralize methanoic acid specifically.
- Therefore, both the assertion and reasoning are true, and the reasoning correctly explains the assertion.
- The answer is A: Both assertion and reasoning are true, and the reasoning is the correct explanation for the assertion.

- Common salt (sodium chloride) is a crucial raw material in the chemical industry.
- It is used to produce sodium hydroxide, baking soda, washing soda, and bleaching powder through various chemical processes.
- Sodium hydroxide is produced via the chlor-alkali process, which also yields chlorine and hydrogen.
- Baking soda, washing soda, and bleaching powder are derived from further reactions involving sodium chloride and its derivatives.
- The assertion and reasoning are true, and the reasoning accurately explains the assertion. Hence, option A is correct.

Assertion: "HCl gas does not change the colour of dry blue litmus paper.: This statement is true. Dry HCl gas cannot release hydrogen ions (H⁺) because it needs to be dissolved in water to form hydronium ions (H₃O⁺), which cause the acidic effect. Therefore, HCl gas will not change the colour of dry blue litmus paper.

Reason: "HCl gas dissolves in the water present in wet litmus paper to form H⁺ ions.": This statement is also true. When HCl gas dissolves in the water present on wet litmus paper, it dissociates to form H⁺ ions, which can then change the colour of blue litmus to red, indicating an acidic environment.

Both the assertion and reason are true, but the reason does not correctly explain the assertion since the assertion pertains to dry litmus paper, while the reason refers to wet litmus paper.

Thus, the correct answer is:

Option 2: Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

- Assertion (A): True. Acid should be added to water with stirring to prevent exothermic reactions from causing splashes or eruptions.
- Reason (R): True. Adding acid to water decreases the concentration of H⁺ ions per unit volume as the solution becomes more diluted.
- The reason (R) correctly explains the safety aspect of dilution rather than the procedural necessity of adding acid to water.
- Correct choice: B. Both A and R are true, but R is not the correct explanation of A.