The energy involved in change of state
3 marks · NormalDescribe a practical method to obtain a temperature–time graph showing the change of state from solid to liquid for a substance such as naphthalene. Include details on how to make the experiment as accurate as possible.
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Heat naphthalene in a boiling tube in a water bath. Record temperature every 30 seconds from before melting starts until all is melted. Stir gently for uniform temperature. Stop heating when all is melted. Plot temperature against time.
Must show: water bath heating, continuous temperature recording, stirring
Also accept: use thermometer; plot temperature-time graph
Common mistakes: Direct heating of solid; Not measuring temperature at short intervals
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Describes indirect heating (e.g., in a water bath) and correct apparatus (boiling tube, thermometer).
M2 · 1 mark · Mentions regular temperature measurements taken until all the solid has melted.
A1 · 1 mark · States at least one way to improve accuracy (e.g., stirring, good thermometer, read at eye level, heat gently).
Worked solution
Place naphthalene in a boiling tube and heat it gently in a water bath, using a thermometer to measure temperature. Record temperature every 30 seconds from before melting starts to after it has all melted. Stir the naphthalene (if possible) to ensure even heating. Stop heating once all the solid has melted. Plot a graph of temperature against time.