A solvent is a molecule that has the ability to dissolve other molecules, known as solutes. A solvent can be solid, liquid or gas. The molecules of the solvent work to put the solute molecules apart. Eventually, the molecules of solute become evenly distributed in throughout the solvent. This homogenous mixture is perfectly even, and cannot be separated physically. Heat or another chemical process must be applied to the solution to separate the solvent and solute.
Types of Solvents
Molecules in general have two classes, polar and nonpolar. Polar molecules have separated electrical charges on different sides of the molecule. Nonpolar molecules, while they can fluctuate in charge, do not carry a static charge. Both types of molecules can act as solvents, as described below.
Polar solvents work through the actions of the positive and negative ends of each atom interacting with each other, and the solute. A polar solvent dissolves a solute by the electrical charges pulling on different parts of the solute molecules. Polar solvents can dissolve ionic compounds, like salt, by pulling on the oppositely charged molecules. The negative side of solvent molecules pull on the positive ions in the compound. The positive side of other solvent molecules pull on the negative ions. In this way the ions become evenly distributed throughout the solvent.
Nonpolar solvents work in a similar way to polar solvents. Nonpolar molecules that act as solvents are usually spontaneous dipoles, in that they occasionally form opposite electrical charges between bonds. These momentary electrical dipoles cause nearby solvent molecules to also form dipoles. These fleeting interactions can dissolve other nonpolar compounds. However, polar compounds typically have stronger interactions between each other than with the momentary dipoles of nonpolar molecules. This is why nonpolar and polar solvents, like water and oil, do not mix.
Examples of Solvent
Water is the most important biological solvent. All cells, regardless of domain or species, rely on water. H2O has a very unique structure when it comes to molecules. The large oxygen pulls the electrons closer, and thus becomes more negative electrically. The hydrogen atoms each get a smaller share of the shared electrons, and become more positive. This makes water a very strong dipole molecule. These opposite electrical charges can dissolve a large variety of substances. Water is a polar solvent, meaning it can easily dissolve the ions and molecules created by a cell.
Some substances produced by cells are non-polar and tend to cluster together away from water. All cells use this property of water as a solvent to create membranes out of lipids. Phospholipids are large molecules that have a polar head and a nonpolar tail. When two sheets of phospholipids are placed together, the nonpolar tails are attracted to each other and the polar heads are attracted to the water. This creates a water barrier between the two reservoirs. Water acts as a solvent on the molecules inside and outside of the cell, but the cell can use special proteins to transfer important molecules inside and waste molecules out. These solute molecules are quickly moved throughout the cell as they will follow a diffusion gradient, or go from areas of high concentration to areas of low concentration with the help of the solvent. Solvents can also become saturated with solute, a situation leading to no more solute being dissolved.
On a bigger scale, the entire ocean is a giant solution of different salts and chemicals. When it rains, the rain falls on the ground, dissolving solid solutes. These solutes are carried into the river, and flow downstream. All rivers flow towards the ocean, and these solutes are carried into the ocean as well. Various organism rely on these solutes as nutrients or important metabolic salts. Oftentimes in the search for life on other planets, water is considered a key component because it is such an important and diverse solvent.
Solvents in Everyday Cooking
A common procedure in cooking, deglazing, is when the sticky and caramelized bottom of a pan is dissolved into a solvent. Because heat is being used, both nonpolar solvents and polar solvents work to dissolve the sticky and burnt substance on the bottom of the pan. Nonpolar substances, like oil can be used to create a hot solution in which other foods can be fried. This partly infuses some of the solutes dissolved in the solvent into the food being cooked. Chefs use this to add a kick to fried foods. Water can also be used to deglaze a pan, which can create a stock for soup, gravy, and a variety of other sauces.