7.both slicing an apple and a chemical change such as cooking an egg cannot be reversed. however, why is slicing an apple still considered only a physical change?

In order, consider a change as chemical change, the change must produce a new product. If the change doesn't produces a new product it is considered as physical change.

As far as the given concept, slicing of apple is not considered as chemical change because in this activity a new product is not formed and apart from this once an apple is sliced it cannot be reversed into a full apple again. Due to all these reason, slicing of apple is considered as physical change.

photosynthesis makes the glucose that is used in cellular respiration to make atp. the glucose is then turned back into carbon dioxide, which is used in photosynthesis. while water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.