States of matter and particle modelIB MYP Sciences: Subtopic test

1. 1
   At a school science fair in Dubai, a student shows a block of dry ice (solid carbon dioxide) in a bowl. The block gets smaller and a cold gas pours over the edge of the bowl, but no liquid collects in the bowl. The student explains the demonstration using the particle model, in which each particle is drawn as a small solid sphere.

   (a)

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

   [1 mark]

   • AMelting
   • BEvaporation
   • CSublimation
   • DCondensation

   (b)

   Which statement describes a limitation of drawing particles as small solid spheres?

   [1 mark]

   • AIt does not show the forces between particles, and real atoms and molecules are not hard spheres
   • BIt cannot show that gas particles are far apart
   • CIt cannot show that solid particles are close together
   • DIt cannot show that particles can be in different arrangements

   (c)

   Explain, using the particle model, why a gas can be compressed into a smaller volume but a solid cannot.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In Dubai in summer, wet laundry hung on a balcony dries within a couple of hours, even though the temperature is far below 100 °C and the water never boils. A student wants to explain why the water leaves the clothes and why the air near the laundry feels slightly cooler.

   (a)

   Which statement correctly describes evaporation?

   [1 mark]

   • AIt happens throughout the liquid and forms bubbles
   • BIt happens only when the liquid reaches its boiling point
   • CIt happens when a gas changes into a liquid
   • DIt happens at the surface of a liquid at temperatures below the boiling point

   (b)

   Which change would make the laundry dry more quickly?

   [1 mark]

   • AFolding the clothes into a thick pile
   • BHanging the clothes on a windy day
   • CHanging the clothes in a very humid room
   • DMoving the clothes to a cooler place

   (c)

   Explain, in terms of particles, why evaporation makes the remaining liquid cooler.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats a solid sample of pure stearic acid in a water bath and records its temperature every minute. The temperature rises steadily from 20 °C, stays at 69 °C for several minutes while the solid melts, and then rises again.

   (a)

   State the melting point of stearic acid and explain, in terms of particles and energy, why the temperature stays constant while it melts.

   [3 marks]

   (b)

   The student's hypothesis was: 'A pure substance melts at one fixed temperature.' A second sample of stearic acid mixed with a little wax melted gradually between 60 °C and 66 °C. Evaluate whether the results support the hypothesis and suggest one improvement to the investigation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the United Arab Emirates, much of the drinking water comes from desalination plants. These plants heat seawater until it turns to steam, then cool the steam so that it condenses into fresh water. The salt is left behind as a concentrated salty solution.

   (a)

   Describe and explain, using the particle model, what happens to the water particles as liquid water at 20 °C is heated until it becomes steam, including why the temperature stays constant while it boils.

   [6 marks]

   (b)

   Evaluate the use of desalination plants to supply drinking water in the United Arab Emirates. Discuss the benefits and drawbacks for people and the environment, and reach a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions