1. 1
   A chef at a restaurant places lumps of solid carbon dioxide (dry ice) onto a serving tray at room temperature. The chef notices that the dry ice slowly disappears, producing a visible white vapour cloud, without ever forming a puddle of liquid.

   (a)

   What name is given to the process by which the dry ice turns directly into a gas without forming a liquid?

   [1 mark]

   • AEvaporation
   • BMelting
   • CSublimation
   • DCondensation

   (b)

   Compared with the particles in solid carbon dioxide, how do the particles in the gas produced move?

   [1 mark]

   • AThey vibrate about a fixed position only
   • BThey move randomly at high speed in all directions
   • CThey flow over each other but stay close together
   • DThey stop moving completely

   (c)

   Explain, in terms of the arrangement, movement and energy of the particles, what happens as the solid carbon dioxide changes into a gas.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student places a single drop of purple potassium manganate(VII) solution into a beaker of still water, without stirring, and watches the purple colour gradually spread throughout the beaker over the next hour.

   (a)

   What is the name of the process by which the purple colour spreads through the water?

   [1 mark]

   • ADiffusion
   • BOsmosis
   • CFiltration
   • DDistillation

   (b)

   If the experiment were repeated with warmer water, what effect would this have on the rate at which the colour spreads?

   [1 mark]

   • AThe colour would spread more slowly
   • BThe rate would stay exactly the same
   • CThe colour would not spread at all
   • DThe colour would spread more quickly

   (c)

   Explain, in terms of particle movement and energy, why the purple colour spreads through the water even though the beaker is never stirred.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the solubility of potassium nitrate in water at different temperatures by dissolving the solid in a fixed mass of water and recording data. The student's results show that at 20°C the solubility of potassium nitrate is 32 g per 100 g of water, and at 60°C the solubility is 110 g per 100 g of water.

   (a)

   Describe how a student could use this apparatus and method to determine the solubility of potassium nitrate in water at 40°C, in units of g per 100 g of water.

   [3 marks]

   (b)

   A saturated solution of potassium nitrate is prepared at 60°C using 50 g of water. The solution is then cooled to 20°C. Calculate the mass of potassium nitrate crystals that will form as the solution cools, showing your working.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A desalination plant produces drinking water from seawater by heating the seawater until it boils, collecting the steam produced, and then cooling the steam so that it condenses back into pure liquid water, leaving the dissolved salts behind.

   (a)

   A desalination plant heats seawater until it boils, forming steam, then cools the steam so that it turns back into pure liquid water. Explain, in terms of the arrangement, movement and energy of the particles, what happens to the water particles during (i) boiling and (ii) the subsequent cooling of the steam back to liquid water.

   [6 marks]

   (b)

   An ice-making machine cools liquid water until it freezes solid. Explain why the temperature of the water stops falling for a period of time while it is freezing, even though the machine continues to remove energy from it, and describe the changes in particle arrangement, movement and energy that occur during freezing.

   [6 marks]

   Total for question 4: 12 marks

End of questions