Both figures are mixtures,
Figure II is a heterogenous mixture
Figure I is a homogenous mixture
Mentos reacts to coca-cola because of the sugars and then they expand which causes the soda to explode out of the open container. this is a chemical reaction because it cause the sugars to expand which you can't see happen.
2Mg+ O₂= 2MgO
The charge of Mg is+2 and the charge of Oxygen is -2. 2 oxygen atoms will have a charge of -4 so the Mg atoms have to equal +4. A coefficient of 2 for the Mg atoms will balance it out