In practice, the choice of a liquid dielectric for a given application is made primarily on the basis of its: 

Explanation:

Liquid Dielectric

Definition: A liquid dielectric is a liquid that is used as an insulating material in electrical systems to prevent electrical discharges and to provide cooling. These liquids are often used in high-voltage applications such as transformers, capacitors, and switchgear.

Correct Option Analysis:

The correct option is:

Option 4: Chemical stability

This option is chosen because chemical stability is one of the most critical properties for liquid dielectrics. Liquid dielectrics need to remain chemically stable under the operational conditions they are subjected to. This stability ensures that the dielectric will not decompose or react with other materials, which could lead to a loss of insulating properties and potential failure of the electrical system. Chemically stable dielectrics are less likely to form conductive contaminants or gases that could impair their performance.

In high-voltage applications, the liquid dielectric must perform consistently over a wide range of temperatures and electrical stresses without breaking down or changing its composition. This stability is crucial for maintaining the insulating properties and ensuring the longevity and reliability of the electrical equipment.

Important Information:

To further understand the analysis, let’s evaluate the other options:

Option 1: Surface tension

Surface tension is a measure of the cohesive forces acting at the surface of a liquid. While surface tension can influence the behavior of a liquid dielectric, especially in terms of how it interacts with solid surfaces and how it forms droplets or films, it is not the primary factor in the selection of a dielectric for electrical insulation purposes. The primary concern is ensuring that the liquid can maintain its insulating properties under electrical stress, which is more directly related to its chemical stability.

Option 2: Solubility

Solubility refers to the ability of a substance to dissolve in a solvent. For liquid dielectrics, solubility is not a primary concern unless the dielectric is required to dissolve certain additives or contaminants. The main focus is on the dielectric's ability to insulate and remain chemically stable under operational conditions. Solubility might be a secondary consideration in terms of ensuring that the dielectric does not dissolve any undesirable materials from the system it is used in.

Option 3: Viscosity

Viscosity is a measure of a liquid's resistance to flow. While viscosity can affect the cooling properties and the ease of handling and pumping the liquid dielectric, it is not the primary criterion for its selection. The ability to provide effective insulation and remain chemically stable under electrical stress is more critical. Viscosity might be a secondary consideration in terms of ensuring that the dielectric can effectively circulate and remove heat from the electrical components.

Conclusion:

When selecting a liquid dielectric for electrical applications, chemical stability is of paramount importance. This property ensures that the dielectric will maintain its insulating capabilities and not decompose or react under the operational conditions it is subjected to. While surface tension, solubility, and viscosity are also important properties, they are secondary to the primary requirement of chemical stability in ensuring the reliability and longevity of electrical insulation systems.