Q.An electric kettle takes 20 minutes to heat a certain quantity of water from 0°C to its boiling point. It requires 90 minutes to turn all the water at 100°C into steam. Find the latent heat of vaporisation. (Specific heat of water = 1 cal/g°C)

Matter's three ordinary states -- solid, liquid, gas -- can transform into one another through heat exchange. Heating ice steadily and plotting temperature against time produces a heating curve with two flat plateaus: one at 0 °C (all the incoming heat is melting the ice, not raising its temperature) and one near 100 °C (all the incoming heat is boiling the water into vapour). Melting/solidification describes the solid-liquid transition; vaporisation/condensation describes the liquid-vapour transition; sublimation describes a direct solid-to-vapour transition without an intervening liquid phase (dry ice, iodine). Evaporation (surface-only, at any temperature, with a cooling effect on the liquid left behind) is distinct from boiling (throughout the whole liquid, at one fixed temperature that itself depends on pressure -- rising with increased pressure, falling with decreased pressure, which is why cooking is harder at altitude but faster in a pressure cooker). A pressure-temperature phase diagram, with its vaporisation, fusion and sublimation curves, shows which phase a substance occupies at any pressure/temperature combination, and the single point where all three curves meet -- where solid, liquid and vapour all coe …