MYP Chemistry sample question: The particle model and changes of state

Water in a beaker is heated until it boils. Its temperature stays at 100 °C while it boils, even though heating continues.

1. (a)Describe the arrangement and movement of particles in liquid water.[2]
   Show the mark scheme for (a)

   • • Particles are close together but arranged irregularly (1)
   • • They move past one another (1)
2. (b)Explain why the temperature stays constant while the water boils.[2]
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   • • The energy supplied is used to overcome the forces between the particles (1)
   • • rather than to increase their kinetic energy, so the temperature does not rise (1)
3. (c)Name the change of state when steam turns back into water.[1]
   Show the mark scheme for (c)

   • • Condensation (1)

Temperature measures the average kinetic energy of the particles. During a change of state the energy goes into breaking attractions instead.