The process you just described is part of a bigger process called photosynthesis. Here is a brief overview of photosynthesis, a process carried out by plants. Plants use this process to obtain food, or energy, and without it they would not be alive. Photosynthesis occurs in the chloroplast of the plant, an organelle in plant cells devoted to this process. Plants take in carbon dioxide (CO₂), water (H₂O), and light from the sun and convert it to sugar for themselves and oxygen for the environment.

Chlorophyll is the photoreceptive molecule that uses the light energy to break apart the hydrogen and oxygen atoms; that is the "something" you are talking about. There is a magnesium atom in the center of the porphyrin ring which makes chlorophyll and that is what absorbs the light energy. Chlorophyll is also the thing that gives plants their green color; it absorbs blue and red light and reflect green light, which is what we see. There are actually two types of chlorophyll: chlorophyll a and b. They only differ by their structure, and chlorophyll b acts as an accessory chlorophyll to help chlorophyll a. Chlorophyll a directly participates in the light reactions which are a part of photosynthesis.

You're talking about the reactions of photosynthesis. Photosynthesis is carried out by a complex series of proteins and other compounds operating inside of the chloroplast of a plant cell. The chemical that actually turns the light energy into chemical energy is chlorophyll, which is probably what you're being asked for, but chlorophyll isn't what actually breaks water apart into its component hydrogen and oxygen ions.

That "something" is referring to the complex photosynthetic pathways used in plants and other photosynthetic organisms. This animation shows where it occurs and how: