If I add 1 liter of boiling water to an aquarium with 140 liters of water at 25°C, what is the temperature?

Answer

The concept you need here is the specific heat coefficient c . This is expressed in

Joule / kilogram / Kelvin

and indicates how much energy you need to raise the temperature of 1 kg of a given matter by 1 degree. For water this is 4186 J/kg/K. In other words, if the temperature of a mass of m kilogram increases by ΔT, the energy has increased by :

ΔE = cm ΔT

So suppose your aquarium first contains 140 at 25 K, and you put in a liter of 100 K. So that liter has 4186 . 1 . 75 Joule = 313 950 Joule more heat energy than if it had a temperature of 75 K. That is also the energy with which the aquarium is increased. If we divide that amount of energy over the currently present 141 liters, the temperature will rise by:

Δt = 313 950 / ( 4186 * 141) = 0.53 K

So a good half degree is added.