Changes of stateIB MYP Chemistry: Mind map

Exam questions on Changes of state

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.
   Explain why the temperature stays constant while the ice is melting, even though the flame is still heating the beaker.2 marks
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.
   Describe, in terms of particles, what happens when dry ice sublimes.2 marks
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.
   Describe the pattern in the temperature results.3 marks