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Thermal Energy Transfers · Paper 2 — Free Physics SL Practice Questions

1MasterySAQ-SHeat and temperature5 marksPaper 2~8 min
The graph below shows the temperature of a pure substance as it is heated at a constant rate. The substance starts as a solid and is heated until it becomes a gas. The graph contains two horizontal sections.
(a)
State what is happening to the temperature of the substance during a horizontal section of the graph. [1 mark]
(b)
Explain, in terms of molecular energy, why the temperature does not change during a horizontal section even though heat is being added. [2 marks]
(c)
The specific latent heat of fusion of the substance is 3.34×105 J kg−13.34 \times 10^5 \ \text{J kg}^{-1} and the specific latent heat of vaporisation is 7.54×105 J kg−17.54 \times 10^5 \ \text{J kg}^{-1}. Calculate the ratio of the thermal energy required to completely vaporise 0.200 kg0.200 \ \text{kg} of the substance (at its boiling point) to the thermal energy required to completely melt the same mass (at its melting point). [2 marks]
diagram

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