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:

smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. those muscles push materials like food or blood through organs. unlike skeletal muscle, smooth muscle can never be under your control.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.