(a) (i) State what is meant by melting point. (ii) The temperature of a liquid is between its melting point and its boiling point. Using ideas about molecules, describe the changes that occur when the liquid evaporates. (b) The boiling point of nitrogen is –196 °C. Liquid nitrogen, below its boiling point, is stored in a vacuum flask, as shown in Fig. 10.1. [Figure 10.1] The flask has two glass walls with a vacuum between them.

1. (a)(i)State what is meant by melting point.[2]
2. (a)(ii)Using ideas about molecules, describe the changes that occur when the liquid evaporates.[3]
3. (b)(i)Suggest where, in the apparatus shown in Fig. 10.1, evaporation occurs.[1]
4. (b)(ii)State and explain the advantage of having a vacuum between the two glass walls.[3]
5. (c)(i)The liquid nitrogen reaches –196 °C, its boiling point. A small piece of metal at 20 °C is lowered slowly into the liquid nitrogen. Bubbles form within the liquid nitrogen as it boils. State what is inside the bubbles.[1]
6. (c)(ii)1The small piece of metal has a mass of 50 g. When it is lowered into the liquid nitrogen, the metal cools to -196°C. The specific heat capacity of the metal is 0.39J/(g°C). The specific latent heat of vaporisation of nitrogen is 200J/g. Calculate 1. the thermal energy (heat) lost from the metal as it cools,[3]
7. (c)(ii)22. the mass of nitrogen that boils away.[2]

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