In the SI unit system, the unit for the amount of substance is?

The unit for the amount of substance in the SI unit system is the mole. The mole is defined as the amount of substance that contains as many elementary entities (such as atoms, molecules, ions, etc.) as there are in 12 grams of carbon-12. This makes the mole a fundamental unit of measurement in chemistry, allowing for calculations involving the number of particles in a given sample of matter.

The mole is particularly useful because it links the macroscopic scale of substances we can measure (like grams or liters) to the microscopic scale of atoms and molecules. When chemists need to relate mass to the number of particles for chemical reactions, the mole provides a straightforward conversion factor, as it is directly related to the size of Avogadro's number, approximately (6.022 \times 10^{23}) entities per mole.

In contrast, the gram and kilogram are units of mass, while the liter is a unit of volume. These units are essential in various calculations in chemistry but do not specifically measure the amount of substance itself, which is why the mole is the appropriate answer.