(a) (i) Five solutions A, B, C, D and E when tested with pH paper showed pH as 4, 1, 13, 7 and 10 respectively. Which solution is: (1) Strongly acidic (2) Strongly alkaline (3) Weakly acidic (4) Neutral and (5) Weakly alkaline? Arrange the solutions in increasing order of ion concentration. (ii) Write the name and formula of (1) an acidic salt and (2) a basic salt giving the name of the parent acid and parent base used to form the salt in each case. OR (b) Name and state in brief the process which is used to prepare sodium hydroxide from sodium chloride. In this process along with the main product two gases 'X' and 'Y' are also given off at the two electrodes. Name 'X' and 'Y' specifying the name of their respective electrode at which each gas is obtained. One of these gases when reacts with dry calcium hydroxide produces a compound 'Z' which is widely used in water treatment plants and textile industries. Name Z and write chemical equation for the reaction involved in its formation.
(a) (i) Five solutions A, B, C, D and E when tested with pH paper showed pH as 4, 1, 13, 7 and 10 respectively. Which solution is: (1) Strongly acidic (2) Strongly alkaline (3) Weakly acidic (4) Neutral and (5) Weakly alkaline? Arrange the solutions in increasing order of ion concentration. (ii) Write the name and formula of (1) an acidic salt and (2) a basic salt giving the name of the parent acid and parent base used to form the salt in each case. OR (b) Name and state in brief the process which is used to prepare sodium hydroxide from sodium chloride. In this process along with the main product two gases 'X' and 'Y' are also given off at the two electrodes. Name 'X' and 'Y' specifying the name of their respective electrode at which each gas is obtained. One of these gases when reacts with dry calcium hydroxide produces a compound 'Z' which is widely used in water treatment plants and textile industries. Name Z and write chemical equation for the reaction involved in its formation.
Detailed answers for both parts (a) and (b) are provided below.
Marking Scheme
- 1Part (a)(i): 1.5 Marks (0.3 marks for each correct classification of A, B, C, D, E).
- 2Part (a)(i): 0.5 Marks for correct increasing order of H+ ion concentration (C < E < D < A < B).
- 3Part (a)(ii): 1 Mark for correct name, formula, parent acid, and parent base of an acidic salt.
- 4Part (a)(ii): 1 Mark for correct name, formula, parent acid, and parent base of a basic salt.
- 5OR
- 6Part (b): 1 Mark for naming and describing the Chlor-alkali process.
- 7Part (b): 1 Mark for identifying Gas X (Chlorine at anode) and Gas Y (Hydrogen at cathode).
- 8Part (b): 1 Mark for identifying Compound Z as Bleaching powder (Calcium oxychloride).
- 9Part (b): 2 Marks for writing the balanced chemical equation: Ca(OH)2 + Cl2 -> CaOCl2 + H2O.
Hint
For (a), remember that lower pH means higher concentration. For (b), the process of electrolysis of brine is called the chlor-alkali process, and the gas reacting with dry slaked lime to form a bleaching agent is chlorine.
Quick Oral Answer
In the Chlor-alkali process, chlorine gas is released at the anode, hydrogen gas at the cathode, and sodium hydroxide is formed near the cathode. Chlorine reacts with dry slaked lime to form bleaching powder.
Analysis & Explanation
Part (a) tests the understanding of the pH scale where pH is inversely proportional to the concentration of hydronium () ions. A pH of 1 represents a very high concentration of ions (strongly acidic), while a pH of 13 represents a very low concentration of ions (strongly alkaline). Salts formed from a strong acid and a weak base are acidic (pH < 7), while salts formed from a weak acid and a strong base are basic (pH > 7).
Part (b) focuses on the Chlor-alkali process, named so because of the products formed: 'chlor' for chlorine and 'alkali' for sodium hydroxide. Chlorine gas is produced at the anode (positive electrode) because chloride ions () are negatively charged and migrate towards the positive electrode to get oxidized. Hydrogen gas is produced at the cathode (negative electrode) because hydrogen ions () migrate towards the negative electrode to get reduced. Chlorine gas reacts with dry calcium hydroxide to form bleaching powder (), which is widely used as a disinfectant in water treatment and as a bleaching agent in textile industries.
Common Mistakes
- 1In part (a), writing the increasing order of H+ concentration in reverse (confusing high pH with high H+ concentration).
- 2In part (b), swapping the electrodes for chlorine and hydrogen gases (remember: Chlorine is negative Cl- so it goes to positive Anode; Hydrogen is positive H+ so it goes to negative Cathode).
Interesting Facts
The Chlor-alkali process is one of the most important industrial chemical processes, producing chemicals that are key building blocks for PVC, soaps, paper, and disinfectants.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2024?
This question carries 5 marks in the CBSE Class 10 Science 2024 examination.
Which chapter does this question come from in Science?
This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "pH Scale and Chlor-Alkali Process" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2024 paper?
This is a Long Answer question from Section D in the CBSE Class 10 Science 2024 paper.