Must new substances be formed when you observe a chemical property? explain

Yes. It is rather a natural phenomenon. If you search and understand the process of chemical change or reaction that results to its chemical property outcome, it has a new formed substance. Unlike physical change or reaction it only changes in shape or form but the substance, compound or element in the matter is till the same. Furthermore, when chemical change is undergoing you notice the change in shape, size and from like the physical change but there is a change in the element, compound or substance. When you lit a fire on a paper the fire disorients, disorganizes and disturb the former positions of the atoms and because of the fire as catalyzer the atoms did its behavior –changing both intrinsic and extrinsic.

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.