Question

How do I answer this ?

Answer 1

`67.5 " minutes"`

Explanation:

Look at the 10 minutes to bring the water to boiling. That required a temperature change of `80^@ K`.
The specific heat is `4.2 kJ*kg^-1*K^-1`
So let's multiply those 2 factors together

`(4.2 kJ*kg^-1*cancel(K^-1))*(80^@ cancel(K)) = 336 kJ*kg^-1`

That is a result that gives us an idea of the immersion heater's rate of producing heat and the affect of this amount of water. How big a job is getting the water to evaporate compared to heating it to the boiling point?

Notice that the `336 kJ*kg^-1` result we just got has the same units as `2268 kJ*kg^-1`, the specific latent heat of steam. Therefore, we can calculate the ratio by which getting the water to evaporate is a larger job than heating it to the boiling point.

`(2268 kJ * kg^-1)/(336 kJ * kg^-1) = 6.75`

Since the job that gave us the `336 kJ*kg^-1` result required 10 minutes, we need to multiply that last number by 10 minutes.

`10 "minutes" * 6.75 = 67.5 "minutes"`

I hope this helps,
Steve