Isotherm
Definition and meaning of Isotherm in chemistry.
An isotherm is a curve or process that happens at one constant temperature. In chemistry, isotherms most often show how much of a substance sticks to a surface. This process is called adsorption, and it's measured as pressure or concentration changes while temperature stays fixed.
In more detail
The word isotherm comes from iso, meaning same, and therm, meaning temperature. In thermodynamics, an isothermal process keeps temperature constant while a gas expands, compresses, or changes some other way. In surface chemistry, an adsorption isotherm is a graph.
It plots how much gas or liquid sticks to a solid surface at a fixed temperature as pressure or concentration rises. The shape of this curve reveals useful details about a material. These include its surface area, how porous it is, and how strongly molecules stick to it.
Scientists use models such as the Langmuir, Freundlich, and BET equations to describe different isotherm shapes. These tools matter for designing filters, catalysts, and gas storage materials. A common misconception is that isotherms only apply to gases sticking to solids.
Liquids can also adsorb onto surfaces, and isotherms describe that process too, which matters in areas like water filtration and chromatography. The Langmuir model assumes every adsorption site on a surface is identical and can hold only one molecule, forming a single complete layer at most.
Real surfaces are often rougher and less uniform than this ideal. That is why the Freundlich and BET models were developed to describe more realistic surfaces. Chemists often plot several isotherms at different temperatures on the same graph to compare how a material behaves as conditions change.
Key facts
| Field | Physical Chemistry |
|---|---|
| Etymology | iso (same) + therm (heat or temperature) |
| Primary Use | Adsorption studies and surface characterization |
| Common Models | Langmuir, Freundlich, BET |
| Held Constant | Temperature |
Scientists measure how much nitrogen gas sticks to a material's surface at 77 Kelvin, the boiling point of liquid nitrogen. They use the Brunauer-Emmett-Teller (BET) method to turn this adsorption isotherm into a measurement of the material's total surface area.
Frequently asked questions
Why are adsorption isotherms important in chemistry?
They reveal key material properties such as surface area and pore structure, which matter for gas separation, water purification, and catalysis.
What is the difference between an isotherm and an isobar?
An isotherm represents constant temperature, while an isobar represents constant pressure. Each describes a different type of equilibrium condition.
What do the Langmuir and Freundlich models describe?
The Langmuir model assumes adsorption stops once a surface is fully covered in a single layer. The Freundlich model describes adsorption on surfaces with a range of different binding strengths instead.