The Langmuir isotherm is a mathematical model used to describe the adsorption of gas molecules onto a solid surface. It was proposed by Irving Langmuir in 1918 and is widely used in various fields of science and engineering. The statement is true for the following reasons:

Monolayer coverage:
- The Langmuir isotherm assumes that adsorption occurs in a monolayer, meaning that only one layer of gas molecules can be adsorbed onto the surface of the solid.
- This assumption is valid when the surface is not overcrowded with adsorbate molecules and there is enough space for the gas molecules to occupy.

Equivalent adsorption sites:
- The Langmuir isotherm assumes that all adsorption sites on the surface are equivalent, meaning that they have the same energy and affinity for adsorbate molecules.
- This assumption is reasonable when the solid surface is homogeneous and there are no variations in the surface properties.

Dynamic equilibrium:
- The Langmuir isotherm assumes that there is a dynamic equilibrium between the free gas molecules in the gas phase and the adsorbed gas molecules on the solid surface.
- This means that the rate of adsorption is equal to the rate of desorption, resulting in a constant concentration of adsorbed gas molecules.
- This assumption is valid when the system has reached equilibrium and there is no net change in the concentration of adsorbed gas molecules over time.

Independent adsorption probability:
- The Langmuir isotherm assumes that the adsorption probability is independent of the occupancy at the neighboring sites.
- This means that the probability of a gas molecule being adsorbed onto a particular site is not affected by the presence or absence of adsorbate molecules on the neighboring sites.
- This assumption is reasonable when the interactions between the gas molecules and the solid surface are weak and do not depend on the presence of other adsorbate molecules.

In summary, the Langmuir isotherm is valid for monolayer coverage, assumes equivalent adsorption sites, describes a dynamic equilibrium between free gas and adsorbed gas, and assumes that the adsorption probability is independent of occupancy at the neighboring sites.