(b) A beaker of boiling water is placed on the pan of a balance, as illustrated in Fig. 2.1. [Figure 2.1] The water is maintained at its boiling point by means of a heater. The change M in the balance reading in 300s is determined for two different input powers to the heater. The results are shown in Fig. 2.2. [Figure 2.2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [1] (a) (thermal) energy per (unit) mass (to cause change of state)
2. [1] (a) (energy required to cause/released in) change of state at constant temperature
3. [1] (b)(i)1. (work done on/against) the atmosphere
4. [1] (b)(i)2. water as it turns from liquid to vapour
5. [1] (b)(i)2. as potential energy of molecules increases
6. [1] (b)(i)2. or surroundings as its temperature rises
7. [1] (b)(i)2. as energy is lost/transferred to surroundings
8. [1] (b)(ii) VIt − h = M / t × L (where h = power loss) or L = (VIt – Q)/M (where Q = energy loss)
9. [1] (b)(ii) (14.2 × 6.4) - (11.5 × 5.2) = (9.1 – 5.0) × L/300 or L = [(14.2 × 6.4) − (11.5 × 5.2)] × 300/(9.1 – 5.0)
10. [1] (b)(ii) L = 2300 Jg⁻¹