A chemical reaction means that the substrate is converted to product (new substance) over time, the amount of substrate is reduced. Concentration will increase eventually.
Chemical reaction means that the precursor to the products (new) Over time, the quantity of the precursor are reduced. The new substance, quantity is increased, until in the end.
This means that the chemical reactions substances begin to change to a product (new Substances) over time. quantity of substances will be reduced to commence will be increased by the amount of new substances and finally