At least 2 of the reactants must change their chemical formula.
Cooking involve a chemical change.
A fire is a chemical change due to the irreversible changes that happen.
No chemical change happens, though a consequential change probably will.
Depending on the chemical change, sometimes color change, or appearance of bubbles.
the chemical shall be changed
no.
Cooking involves chemical changes.
The system remain unchanged.
The chemical properties cannot be changed; only after a chemical reaction the products have new chemical properties.
If I understand your question correctly, it would be a change in composition that signifies a chemical change. Physical changes are only appearance, while a chemical change alters the substances' entire composition.
Cooking an egg or burning logs on a fire.
Because explosions don't happen slowly