A beaker containing a liquid is placed on a balance, as shown in the figure below.
A heater of power 
is immersed in the liquid. The heater is switched on and, when the liquid is boiling, balance readings 
are taken at corresponding times 
. A graph of the variation with time of the balance reading is shown in the graph below.
State the feature of the graph above which suggests that the liquid is boiling at a steady rate.
Use data from the graph above to determine a value for the specific latent heat 
of vaporisation
of the liquid.
.
