Solvents used in industries are classified as follows:
1- Aqueous solvents
2- Organic solvents

Typically, when selecting a solvent for a system, there are two choices: an aqueous solvent or an organic solvent. Common organic solvents have a lower viscosity compared to water. Organic solvents typically exhibit higher vapor pressure, lower latent heat of vaporization, lower boiling point, and lower surface tension compared to water. These characteristics arise from the presence of robust hydrogen bonds between water molecules. The evaporation rate of a solvent is typically assessed based on its latent heat of evaporation, although occasionally the boiling point is employed as an approximation for the evaporation rate. The detrimental effects of organic solvents manifest in indoor spaces due to their high volatility, which renders them toxic to humans and other organisms. One of the fundamental principles of working with solvents is to utilize less hazardous solvents. In laboratory settings, it is usually recommended to avoid the use of hazardous solvents for dissolving substances involved in chemical reactions. The use of certain solvents in large quantities in industry can pose significant risks to human health and safety, as well as potential hazards like fire and explosion. Some of the solvents that are quite harmful to human health are carbon tetrachloride, chloroform, and perchloroethylene.

However, there is a better classification of solvents based on the polarity of molecules:
1- Non-polar solvents
2- Polar solvents
3- Hydrogen solvents

Polar solvents consist of polar constituent particles that exhibit electrostatic attraction towards each other. Water is the primary polar solvent. This category encompasses various types of acids, including sulfuric acid (H2SO4) and hydrogen fluoride (HF). The particles in non-polar solvents lack polarity, resulting in a relatively weak van der Waals force of attraction between them. That is why these solvents often have very low boiling points, are highly volatile, and evaporate quickly.

Organic solvents exhibit lower polarity compared to inorganic solvents or mineral solvents.Consequently,these solvents typically better dissolve non-polar substances. Liquidity is adesirablequality in solvents. Water exhibits a liquid state within the temperature range of 0°C to 100°C.Consequently, it is only suitable for use as a solvent within this specifictemperature range. If thesoluble substances are at temperatures that are either lower orhigher, there is no choice but to employalternative solvents.The intermolecular forces in a non-polar liquid are negligible, resulting in theabsence ofany distinct arrangement or orientation of molecules. The magnitude of the electric field ofpolar molecules is higher when the atoms involved are located further apart in the periodictable. Polar molecules tend to align or position themselves in relation to each other, and the formation of these assemblies, characterized by the least energy, is directly related to their geometry and thermal oscillations. Polar groups exhibit greater reactivity than non-polar groups when in contact with a charged surface. Polar groups adsorbed onto particle surfaces result in the formation of a non-polar surface, which can further adsorb an additional layer. The layering process persists until the thermal vibrations surpass the forces that influence the alignment of the particles. Hydrogen bonding occurs in water when a hydrogen atom is near an atom that possesses a non-bonding electron pair. Non-polar solvents have poor spray ability. These solvents encompass various types such as aromatic and aliphatic hydrocarbons, chlorinated solvents, and terpenes. The electrostatic spray ability of primary coating materials can be enhanced by incorporating compatible polar solvents. Polar solvents encompass a range of substances such as ketones, alcohols, glycol ethers, esters, and nitroparaffins.