In the International System of Units (SI), what is the fundamental base unit used to measure the amount of substance in chemistry and physics?

Introduction / Context:
This general knowledge question checks whether you know the SI base units, in particular the unit for amount of substance. In chemistry and physics, we often talk about how many particles, atoms or molecules take part in a reaction. Directly counting them is impossible, so scientists use a convenient counting unit. Recognising this unit is essential for understanding chemical equations, gas laws and many physical calculations.

Given Data / Assumptions:

• The context is the SI system of units.
• We are concerned with the base quantity called amount of substance.
• Options given are Mole, Candela, Kelvin and Meter.
• We assume standard scientific definitions, not informal usage.

Concept / Approach:
The SI base unit for amount of substance is the mole, symbol mol. One mole corresponds to a fixed number of specified elementary entities, such as atoms or molecules, equal to the Avogadro number. Other quantities listed in the options are SI base units for different physical quantities: candela for luminous intensity, kelvin for temperature and metre for length. Only mole corresponds to amount of substance.

Step-by-Step Solution:
Step 1: Recall the seven SI base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. Step 2: Match each quantity with its base unit: metre for length, kilogram for mass, second for time, ampere for current, kelvin for temperature, mole for amount of substance and candela for luminous intensity. Step 3: Identify that the question asks specifically for the unit of amount of substance. Step 4: From the list above, the unit for amount of substance is mole. Step 5: Check the options and see that Mole appears as option A. Step 6: Confirm that candela, kelvin and metre correspond to other base quantities, not amount of substance. Step 7: Conclude that Mole is the correct answer.

Verification / Alternative check:
In chemical equations, you often read statements such as one mole of hydrogen reacts with one mole of chlorine to produce two moles of hydrogen chloride. Molar mass, molar concentration and molar volume all use the unit mole. Standard references and periodic tables quote atomic and molecular masses in grams per mole. All these examples consistently use mole as the base counting unit for amount of substance, confirming that mole is the correct SI base unit.

Why Other Options Are Wrong:
Candela is the SI base unit for luminous intensity, describing how bright a light source is in a given direction. Kelvin is the SI base unit for thermodynamic temperature, used in temperature scales and thermodynamic relations. Meter is the SI base unit for length, used to measure distance, not the amount of substance.

Common Pitfalls:
Sometimes learners mix up the meanings of SI base units because they memorise the list without linking each unit to the corresponding physical quantity. It is helpful to associate each unit with a clear example: metre with distance, kelvin with absolute temperature, candela with light from a lamp and mole with counting atoms and molecules. This association makes it easier to recall that the amount of substance uses the unit mole, a central idea in chemistry and physics.

Final Answer:
The fundamental SI base unit for amount of substance is the Mole (mol).