Two beakers contain water at room temperature. One contains 200ml, the other 400ml. If both beakers are heated above identical Bunsen burners, which of the two will take longer to boil?

The equation dQ = mcdT tells us how the temperature of an object changes when heat energy dQ is added. In this case the specific heat capacity c is the same for the contents of both beakers since we are heating the same substance, and the change in temperature dT is also the same since water in both beakers start at the same temperature and have the same boiling point. However, the 400ml beaker contains double the mass of the 200ml beaker, so double the heat energy is needed to raise the temperature of the 400ml of water to the boiling point, and hence it will have to be heated twice as long as the 200ml beaker.