1.0 kg of water is contained in a 1.25 kW kettle. Calculate the time taken for the temperature of water to rise from 25° C to it's boiling point 100° C. Specific heat capacity of water = 4.2 J g^-1 K^-1.

Given,

mass = 1 kg = 1000 g

Heat energy taken by water (Q) = mc△t
= 1000 x 4.2 x (100 – 25)
= 1000 x 4.2 x 75
= 315000 J

Now,

$Q = \text {Power} \times \text{time}$

Substituting the values in the formula above we get,

$315000 = 1250 \times t \\[0.5em] \Rightarrow t = \dfrac{315000}{1250} \\[0.5em] \Rightarrow t = 252 \text{ sec} = 4 \text{ min } 12 \text{ sec} \\[0.5em]$

Hence, time = 4 min 12 sec