Changes of stateIB MYP Chemistry: Subtopic test

1. 1
   A student in Reykjavik heats a beaker of crushed pure ice at −10 °C with a gentle flame, stirring and recording the temperature every minute. The temperature rises, then stays constant for several minutes while the ice melts, even though heating continues. It then rises again until the water boils.

   (a)

   What is the temperature of the mixture while the ice is melting?

   [1 mark]

   • A−10 °C
   • B100 °C
   • C0 °C
   • DIt keeps rising steadily

   (b)

   Which statement describes what happens to the particles as the ice melts?

   [1 mark]

   • AThey gain enough energy to overcome the forces holding them in fixed positions and begin to move past each other
   • BThey stop vibrating completely
   • CThey move far apart and travel randomly
   • DThey are destroyed and new particles are formed

   (c)

   Explain why the temperature stays constant while the ice is melting, even though the flame is still heating the beaker.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A delivery company in Dubai packs ice cream in insulated boxes with blocks of dry ice (solid carbon dioxide). The dry ice changes straight from a solid to a gas, leaving no liquid behind. When a box is taken out into the warm, humid air, droplets of water form on the cold outside of the box.

   (a)

   What is the name of the change of state of the dry ice?

   [1 mark]

   • AMelting
   • BEvaporation
   • CCondensation
   • DSublimation

   (b)

   What is the name of the process that forms the droplets of water on the outside of the box?

   [1 mark]

   • AFreezing
   • BCondensation
   • CEvaporation
   • DSublimation

   (c)

   Describe, in terms of particles, what happens when dry ice sublimes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Students in Accra heat solid stearic acid in a test tube in a hot water bath until it is completely liquid at 80 °C. They then stand the test tube in a beaker of cool water and record its temperature every minute for 13 minutes. The temperature falls from 80 °C to 69 °C by minute 3, stays at 69 °C until minute 8 while the liquid turns into a solid, and then falls to 55 °C by minute 13.

   (a)

   Describe the pattern in the temperature results.

   [3 marks]

   (b)

   Explain the pattern in the results and suggest why the students used a water bath rather than heating the stearic acid directly with a Bunsen burner.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A coastal city in the United Arab Emirates uses a thermal desalination plant to make drinking water from seawater. Seawater is heated until it boils. The steam passes into a cooler pipe, where it turns back into liquid water. The salt stays behind in a very salty liquid called brine, which is returned to the sea. The plant uses a large amount of energy, usually from burning natural gas.

   (a)

   Explain how the plant uses changes of state to obtain pure water from seawater, describing what happens to the water particles at each stage.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of using thermal desalination to supply drinking water to the city, and reach a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions