1. 1
   During the electrolysis of concentrated sodium chloride solution, a pale green-yellow gas with a sharp, choking smell is given off at the positive electrode (anode). A student holds a piece of damp blue litmus paper at the mouth of the collecting tube.

   (a)

   Which observation confirms the gas is chlorine?

   [1 mark]

   • AThe paper turns red, then bleaches white
   • BThe paper turns blue, then stays blue
   • CA squeaky pop is heard
   • DThe paper is unaffected

   (b)

   Why is chlorine gas collected at the anode (positive electrode) rather than the cathode during this electrolysis?

   [1 mark]

   • AChlorine gas is less dense than air
   • BChloride ions are positively charged and are attracted to the positive electrode
   • CChloride ions are negatively charged and are attracted to the positive electrode
   • DChlorine gas dissolves at the cathode

   (c)

   Describe the full sequence of colour changes observed on the damp blue litmus paper when it is exposed to the chlorine gas, and explain, in terms of the properties of chlorine, why this particular sequence occurs.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A water treatment worker adds a small amount of chlorine gas to a sample of drinking water to disinfect it, and notices a sharp, choking smell escaping.

   (a)

   Which property of chlorine gas does this smell relate to?

   [1 mark]

   • AIts density
   • BIts characteristic odour
   • CIts solubility in water
   • DIts colour

   (b)

   To confirm the escaping gas is chlorine, the worker holds damp blue litmus paper near the water sample. Which sequence of colour changes would confirm chlorine is present?

   [1 mark]

   • ABlue stays blue, no bleaching
   • BBlue turns green
   • CRed turns blue
   • DBlue turns red, then bleaches white

   (c)

   Explain why chlorine's bleaching effect makes it useful for disinfecting drinking water, and suggest one safety precaution the worker should take when handling chlorine gas.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A school laboratory technician needs to safely prepare and test a small sample of chlorine gas for a demonstration, generated by reacting concentrated hydrochloric acid with manganese(IV) oxide under gentle heating.

   (a)

   Describe the procedure the technician should follow to generate the gas and confirm its identity, including one safety precaution.

   [3 marks]

   (b)

   A student watching the demonstration suggests that dry blue litmus paper could be used instead of damp litmus paper to save preparation time. Explain why this substitution could give an unreliable result, referring to how chlorine's bleaching action actually occurs.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A swimming pool manager needs to confirm whether a delivery of gas cylinders, labelled as chlorine for pool disinfection, actually contains chlorine before it is connected to the pool's dosing system, since using the wrong gas could be dangerous or ineffective.

   (a)

   Describe, in detail, a safe and thorough procedure the manager could follow to test a small sample of the gas, and explain the reasoning behind each step.

   [6 marks]

   (b)

   The manager observes that the damp blue litmus paper turns red but is not bleached, even after some time. Evaluate what this partial result might indicate, and discuss what further action the manager should take before deciding whether the cylinder is safe to connect to the pool system.

   [6 marks]

   Total for question 4: 12 marks

End of questions