1. 1
   A school laboratory technician is preparing a set of gas jars for a demonstration on identifying gases produced during industrial reactions. One jar contains a gas collected from a fermentation vessel used to make bioethanol.

   (a)

   The technician bubbles the gas through limewater in a test tube. Which observation confirms the gas is carbon dioxide?

   [1 mark]

   • AThe limewater turns milky (cloudy)
   • BThe limewater turns from colourless to pink
   • CA squeaky pop is heard
   • DThe limewater bubbles vigorously but stays clear

   (b)

   The technician collects a second gas sample from the headspace above a beaker where zinc metal is reacting with dilute hydrochloric acid. A lit splint is held at the open end of a test tube containing this gas. What result would confirm the gas is hydrogen?

   [1 mark]

   • AThe splint relights
   • BA squeaky pop
   • CThe splint is extinguished immediately
   • DWhite fumes form around the splint

   (c)

   The technician also collects a gas produced when hydrogen peroxide solution decomposes in the presence of manganese dioxide catalyst. Describe the test used to identify this gas and state the positive result.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A water treatment plant uses chlorine gas to sterilise drinking water. An engineer wants to confirm that a cylinder genuinely contains chlorine before it is connected to the system.

   (a)

   She holds a piece of damp litmus paper in a sample of the gas taken from the cylinder. What happens to the litmus paper if the gas is chlorine?

   [1 mark]

   • AIt turns milky and cloudy
   • BIt turns from red to blue
   • CIt is bleached and turns white
   • DIt chars and turns black

   (b)

   Before testing for chlorine, the engineer first checks a nearby cylinder that the delivery team suspects may actually contain carbon dioxide instead. She bubbles a sample of this gas through limewater. Which observation would confirm the gas is carbon dioxide rather than chlorine?

   [1 mark]

   • AA lit splint produces a squeaky pop
   • BDamp litmus paper placed in the gas is bleached
   • CA glowing splint relights
   • DThe limewater turns milky (cloudy)

   (c)

   The delivery team also brings a cylinder labelled only 'unknown gas'. Describe two different gas tests the engineer could carry out, one after another, to help narrow down whether the gas could be chlorine or hydrogen, and state what result would rule out hydrogen.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A greenhouse grower pumps extra carbon dioxide into a sealed polytunnel to boost plant growth, and wants to check the gas concentration has not fallen due to a suspected leak.

   (a)

   Describe, in full, how the grower could carry out a simple chemical test to confirm carbon dioxide is still present in a sample of air taken from the polytunnel, including the apparatus used and the expected positive result.

   [3 marks]

   (b)

   The grower also wants to identify the gas released by a compost heap at the edge of the polytunnel, since compost heaps can produce several different gases. Explain how the grower could use a combination of tests to distinguish between hydrogen, oxygen and carbon dioxide if any of these were present in a sample of gas collected from the compost heap.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A museum is restoring a Victorian-era laboratory exhibit and wants to recreate a historical demonstration in which four unlabelled test tubes of gas (hydrogen, oxygen, carbon dioxide and chlorine) are identified one at a time in front of visitors, using only simple chemical tests, without any modern instruments.

   (a)

   Describe, in a logical sequence, how each of the four gases could be identified, explaining the apparatus used, the procedure followed, and the specific observation that confirms each gas. Explain why the order in which the tests are carried out matters for visitor safety.

   [6 marks]

   (b)

   During rehearsal for the demonstration, a museum volunteer accidentally mixes up the labels on two of the test tubes, one containing oxygen and one containing carbon dioxide, and cannot remember which test tube is which. Explain how the curator could design a single side-by-side comparative experiment using both a lit splint and a glowing splint to correctly re-identify the two gases, describing what would be observed for each gas and why the tests give different results based on the chemical properties of the two gases.

   [6 marks]

   Total for question 4: 12 marks

End of questions