The molecule that is split during the light reactions of photosynthesis to release oxygen is water (H₂O).

Here's how it works:

1. Light Absorption: Light energy is absorbed by chlorophyll in the chloroplasts of plant cells.

2. Water Splitting: This absorbed light energy is used to split water molecules (H₂O) into:

* Oxygen (O₂): This is released as a byproduct into the atmosphere.

* Hydrogen ions (H+): These are used in the electron transport chain.

* Electrons: These are also used in the electron transport chain to generate ATP (energy) and NADPH (a reducing agent).

So, the splitting of water is a crucial step in photosynthesis, providing the oxygen we breathe and the energy needed for the Calvin cycle to produce sugars.