This is because the energy from the sun is __________ the water up.
Is this heating?
That depends upon what "This" is.
this is the blank word
I'm sorry, but you misunderstood my question. The first word of your sentence is This. To what does This refer?
To determine the missing word, we need to understand what the sun does to the water. In this context, the sun's energy causes some effect in the water. One common effect is that it increases the temperature of the water, which is known as "heating."
So, to fill in the blank appropriately, we can say that "the energy from the sun is heating the water up." The heating here refers to the process of increasing the temperature of the water.