Question

It's possible to boil water by adding hot rocks to it, a technique that has been used in many societies over time. If you heat a rock in the fire, you can easily get it to a temperature of 500∘C. If you use granite or other similar stones, the specific heat is about 800 J/(kg⋅K).

If 5.0 kg of water at 10∘C is in a leak-proof vessel, what minimum number of 1.0 kg stones must be added to bring the water to a boil?

Answer 1

Heat required to boil water

Q = mc dT

= 5 x 4180 x (100-10) = 1881000 J

Mass of stones

1881000 = m x 800 x (500-100)

m = 5.87kg

ABout 6 stones required