A liquid solvent is any type of liquid that serves to dissolve another liquid, gas, or solid material to create a mixture known as a solution. Water is the most common liquid solvent in nature, and the most common solvent used in industry. There are many other types of solvents used commercially as well, and most are organic, which means they are chemicals that are built upon molecular bonds of the element carbon.
Dipropylene glycol is an example of an organic solvent commonly used in industry. A chemical's degree of solvency, or ability to mix easily with other chemicals, often determines its uses as a liquid solvent. Commercially, solvents are widely used as plasticizers in the manufacture of various plastic compounds, where they serve to make the plastic somewhat flexible and soft, and this is where most organic solvents have industrial value. The more versatile a liquid solvent is, the more widely it is used, and dipropylene glycol is used as a mixture component in everything from dyes and paints to hydraulic fluids. Many chemicals meant to be applied in liquid form and then rapidly dry, such as solvent inks, often utilize intermediate chemicals that have high volatility and evaporate rapidly, such as dipropylene glycol.
Solvents can often pose health risks, as many of them contain chemical components of dangerous compounds such as benzene. The primary, broad definition of an organic solvent, however, is that it has at least one carbon and one hydrogen atom to its structure. This includes many solvents based on alcohols such as methanol and isopropyl alcohol. Acetates are another common form of liquid solvent, which are relatively non-toxic, and based on esters of acetic acid such as butyl acetate. They can have a fairly simple molecular structure, such as ethyl acetate, with a chemical formula of CH3COOCH2CH3.
Any liquid solvent produced in industry will share a class of common characteristics. These include that they have volatility, as they are often chemicals meant to facilitate a process such as cleaning through evaporation, lipophilicity, or the ability to dissolve fat-like compounds, and low molecular weight so that they mix easily with other chemicals. Organic solvents fit these categories well and are capable of dissolving a wide array of compounds from oils and fats to resins and rubber.
The coal-tar industry of the late 19th century kick-started the production of organic solvents. Coal-tar is a viscous black liquid produced from the distillation of coal that contains chemicals used in many solvents, such as benzene and phenols. Chlorinated solvents replaced many of these compounds in the 20th century, but they have equally toxic elements, and, when burned, can produce carcinogenic dioxin compounds.
In general, the nature of any liquid solvent can span a wide range of safe or risky chemical groups. Due to their propensity to evaporate easily into the air or be absorbed into skin, most pose some sort of health hazard to workers exposed to them and people who live in locations where they may be sources of ground water contamination or air pollution. Many thousands of types of liquid solvent have been produced as of 2011, but, as with most chemicals, only a very small minority of these have been tested individually or in concert for their inherent health risks.