(1) Seawater and sand are two different substances. In order to study the specific heat of water and sand, the initial temperature and mass of water and sand should be controlled equal in the experiment.

(2) The same alcohol lamp emits the same amount of heat per unit time, and sand and water absorb the same amount of heat when heated for the same time.

(3) The heat absorbed by the object cannot be directly reflected, but indirectly reflected by the increase of thermometer reading.

So the answer is: (1) the type of substance; Mass and initial temperature; (2) The absorbed heat is equal; (3) no.