1. 1
   A student adds dilute hydrochloric acid to a spatula of calcium carbonate powder in a test tube, and a colourless gas is given off. The student bubbles the gas through a test tube of limewater.

   (a)

   Which observation would confirm the gas is carbon dioxide?

   [1 mark]

   • AThe limewater turns cloudy/milky white
   • BA squeaky pop is heard
   • CThe limewater turns blue
   • DThe limewater bubbles vigorously but stays clear

   (b)

   What is limewater a solution of?

   [1 mark]

   • ASodium hydroxide
   • BCalcium hydroxide
   • CCalcium carbonate
   • DCalcium chloride

   (c)

   Explain, in terms of the substance formed, why limewater turns cloudy when carbon dioxide is bubbled through it.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A baker wants to check that the gas released when bicarbonate of soda reacts with vinegar during baking is carbon dioxide, and sets up a delivery tube leading from a flask of the reacting mixture into a test tube of limewater.

   (a)

   Which piece of apparatus connects the reaction flask to the limewater to allow the gas to be tested safely without escaping into the room?

   [1 mark]

   • AA thermometer
   • BA measuring cylinder
   • CA Bunsen burner
   • DA delivery tube

   (b)

   After a short time, the limewater turns from clear to cloudy white. What does this confirm about the gas being produced?

   [1 mark]

   • AThe gas is hydrogen
   • BThe gas is oxygen
   • CThe gas is carbon dioxide
   • DThe gas is chlorine

   (c)

   Explain why the delivery tube must be placed so that its end is under the surface of the limewater, and describe what might happen if the tube's end were left above the surface instead.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An environmental scientist wants to test whether the gas exhaled by a group of volunteers contains more carbon dioxide than the air they breathed in. The scientist plans to bubble a sample of inhaled air through one tube of limewater and a sample of exhaled air through a second, identical tube of limewater.

   (a)

   Describe how this comparative experiment should be carried out.

   [3 marks]

   (b)

   The scientist finds that the limewater tested with exhaled air turns cloudy within a few seconds, while the limewater tested with inhaled air remains clear even after a much longer time. Explain what this result shows about the concentration of carbon dioxide in exhaled air compared with inhaled air, and identify one variable the scientist should control to make the comparison fair.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A greenhouse manager wants to check whether a newly installed carbon dioxide generator, intended to boost plant growth, is actually releasing carbon dioxide as claimed by the supplier, rather than a different gas.

   (a)

   Describe, in detail, a full test procedure the manager could use to confirm the identity of the gas being released, including the apparatus needed and how the result should be interpreted.

   [6 marks]

   (b)

   The manager finds that the limewater does turn cloudy, but only very slowly compared with a known carbon dioxide source tested previously. Evaluate what this slower result might suggest, and describe further checks the manager could carry out to reach a confident conclusion about the generator's performance.

   [6 marks]

   Total for question 4: 12 marks

End of questions