* No new substances are formed: When you juice a lemon, you are simply separating the juice from the pulp and rind. The chemical composition of the lemon juice remains the same as the original lemon. It still contains citric acid, sugars, and other compounds.

* Change in state: The change from whole lemon to lemon juice is a change in state (from solid to liquid) but not a change in the chemical makeup.

* Reversible: You could technically reconstitute the lemon juice back into something resembling a lemon (although it wouldn't be the original whole lemon).

While juicing itself is a physical change, there are chemical reactions happening within the lemon juice after it's extracted:

* Oxidation: The lemon juice will oxidize and begin to brown when exposed to air. This is a chemical reaction.

* Changes in taste and texture: The juice may become more bitter or sour over time as chemical compounds interact.

In summary:

Juicing a lemon is primarily a physical change, but there are accompanying chemical changes that happen within the juice after extraction.