Invariant System:
An invariant system is one in which no variation of conditions is possible without one phase disappearing. In other words, it is a system where the presence of a particular phase is essential for the system to exist. Without that phase, the system cannot be maintained.

Explanation:
In chemistry, a phase refers to a physically distinct and homogeneous part of a system. A system can consist of one or more phases, such as solid, liquid, and gas. The behavior and properties of a substance can vary depending on the phase it is in.

When we have an invariant system, it means that the presence of a specific phase is crucial for the system to exist. Any change in conditions that results in the disappearance of that phase will cause the entire system to collapse.

Example:
Let's consider an example of water. At normal atmospheric pressure and temperature, water exists in three phases: solid (ice), liquid (water), and gas (water vapor). Each phase has its own distinct properties and characteristics.

If we start with a system containing all three phases of water and decrease the temperature below the freezing point, the solid phase (ice) will remain stable while the liquid and gas phases will disappear. This is an example of an invariant system because without the presence of the solid phase, the system cannot be maintained.

Similarly, if we increase the temperature above the boiling point, the liquid phase will disappear, leaving only the solid (ice) and gas (water vapor) phases. Again, the system is invariant because without the presence of the liquid phase, the system cannot exist.

Conclusion:
An invariant system is one in which no variation of conditions is possible without one phase disappearing. It is a concept used in chemistry to describe systems where the presence of a specific phase is essential for the system to exist. Understanding the concept of invariant systems is important in various fields of chemistry, as it helps in analyzing phase changes and predicting the behavior of substances under different conditions.