I’m making an invisible ink experiment. I’m using baking soda, water and a heat source. How do I explain the chemical/physical reaction?

The physical reaction is...

Applying the baking soda/ water to the provided surface is a physical change, once a heat element is brought in, the releasing of heat is a chemical change; the baking soda changing color is in fact a chemical change because you are scorching it, it cant be undone.  

Explanation:

sodium (na), for example, has one electron in its valence shell. this is an unstable state because that valence shell needs eight electrons to be full. in order to fill its valence shell, find a way to add seven electrons to its valence shell

state the principle of separating two immiscible liquids by separating funnel. describe an activity with diagram to separate a mixture of water and kerosene oil. answer: immiscible layers separate out in layers depending on their densities in separating funne