The key difference between physical and chemical digestion is that chemical digestion involves:the formation of new substancesgrinding, mashing or churningbile produced by the livermixing substances together

The key difference between physical and chemical digestion is that chemical digestion involves the formation of new substances.

Here's a step-by-step breakdown:

1. Physical Digestion: This is the process where food is broken down into smaller pieces through mechanical processes. This includes grinding, mashing, or churning, which happens when we chew food or when the stomach muscles contract to mix food. However, the food's chemical structure doesn't change.

2. Chemical Digestion: This is where food is broken down into simpler nutrients that can be used by the cells. It involves the formation of new substances. This happens through the action of enzymes and other substances like bile produced by the liver. The enzymes break down complex food molecules like proteins, fats, and carbohydrates into simpler substances that can be absorbed and used by the body.

So, the key difference is that physical digestion involves mechanical breakdown of food without changing its chemical structure, while chemical digestion involves breaking down food into simpler substances through chemical reactions.