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Q1.
A substance is heated at a constant rate and its temperature is taken every minute. During the heating, the substance undergoes one change of state. The results are shown on the graph in Figure 5.
Explain the shape of the graph in terms of the changes in the movement and arrangement of the particles as the substance is heated.
(4)
From point B to C, the line on the graph is flat. This is because the substance is changing state from a solid to a liquid. The heat energy being supplied is not increasing the temperature but is instead being used to break the forces between the particles (intermolecular bonds) that hold them in a solid structure. The particles start to escape from their fixed positions in the solid lattice and become randomly arranged, forming a liquid. This process is called melting. The temperature stays constant during this time because all the energy goes into changing the state, not raising the temperature.
From point A to B, the graph shows a rise in temperature. This is when the substance is still a solid. The particles are arranged in a regular lattice structure and are closely packed together in fixed positions. As the substance is heated, the particles gain energy and begin to vibrate more rapidly around their fixed positions. The rise in temperature means the average kinetic energy of the particles is increasing.
From point C to D, the graph also shows a temperature increase. At this point, the substance is in the liquid state. The particles have broken out of the lattice and are now free to move past each other randomly. As more heat is added, the particles gain more energy and begin to move faster. This increase in particle movement causes the temperature to continue rising.
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