Question 1: Chlorine is a halogen element that exists as a yellow-green gas at room temperature. It is commonly used in water treatment and disinfection processes due to its strong oxidising properties. In chemical tests, chlorine can be identified by its distinct reactions with various substances.

Answer: Chlorine is a halogen gas with a yellow-green colour, used for disinfection; it reacts with certain chemicals producing characteristic reactions.

Question 1: Define the term 'halogen' and explain why chlorine is classified as one.

Answer: A halogen is a group 17 element with seven electrons in its outer shell; chlorine is a halogen because it belongs to this group and shares similar chemical properties.

Question 2: Describe the mechanism by which chlorine reacts with sodium hydroxide during disinfection processes.

Answer: Chlorine reacts with sodium hydroxide to form sodium chloride and sodium chlorate, releasing hypochlorite ions which act as disinfectants.

Question 1: In an experiment, a student bubbles a gas through litmus paper. The paper turns red, and then bleaches a damp cloth. What does this indicate about the gas?

Answer: The gas is acidic and has bleaching properties, indicating it could be chlorine.

Question 2: A sample of gas causes a white precipitate when tested with silver nitrate solution. Which halogen does this suggest, and why?

Answer: It suggests the presence of chloride ions because silver chloride precipitate forms with chloride ions.

Question 3: A gas sample reacts with potassium iodide solution, turning brown in the presence of starch. What role does iodine play in this reaction, and what does it reveal about the gas?

Answer: Iodine is released when the gas oxidises iodide ions, indicating the gas is an oxidising agent like chlorine.

Question 1: Calculate the volume of chlorine gas (at room temperature and pressure) needed to react completely with 0.05 mol of sodium hydroxide. Use the reaction: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. (Molar volume of gases at RTP = 24 dm³/mol)

Answer: Volume = 0.025 dm³ (or 25 cm³)

Question 2: If a sample of chlorine gas has a mass of 1.27 g, calculate its molar quantity. (Atomic mass of Cl = 35.45 g/mol)

Answer: Moles = 0.0358 mol

Question 1: Design a simple experiment to test for the presence of chlorine gas in a sample. Include the key variables and safety precautions.

Answer: Introduce a damp litmus paper or a piece of moist blue litmus into the gas sample; observe colour change or bleaching. Variables include gas concentration and exposure time. Safety precautions: conduct in a fume cupboard, wear gloves and eye protection, avoid inhaling the gas directly.

Question 1: Explain how chlorine is used in water treatment and discuss the benefits and potential risks associated with its use.

Answer: Chlorine is added to drinking water to kill bacteria and viruses, ensuring safe water supplies. Benefits include effective disinfection; risks involve the formation of harmful chlorinated compounds and health concerns if not properly managed.