Water as a Solvent

Take a glass of water, stir in a spoonful of sugar, and watch the granules slowly disappear. Take another glass of water, and this time, stir in a spoonful of salt. Just the same, the salt disappears, only this time you are left with a clear, salty liquid.

Sugar and salt are water-soluble substances, meaning they dissolve easily in water. There are many other substances that dissolve in water; in fact, water is considered as the universal solvent because it dissolves more substances than any other liquid.

In the following, we’ll be discussing what water solvent means, what properties make it useful as a solvent, and its importance to biology.

Role of Water as a Solvent

The term solution pertains to a homogenous mixture of one or more substances. It is composed of a solvent, a substance that has the capacity to break apart another molecule or compound known as solute.

Water is generally considered the “universal solvent” as it dissolves more substances than any other liquid and is also widely accessible compared to its counterparts. How does water do this?

Water has the ability to act as a solvent due to its polarity, the unequal sharing of electrons within a molecule where one end possesses a partially negative charge and the other end, a partially positive charge.

Because water is composed of one oxygen atom (which is partially negative) and two hydrogen atoms (which are partially positive) water is considered to be a polar solvent (Fig. 1).

This polar nature also allows water to cause hydrogen bonding. Hydrogen bonds form as a result of intermolecular forces between and among neighboring water and other polar molecules: the positive hydrogen of one water molecule will link with the negative oxygen of the next molecule, whose hydrogen atoms will then be attracted to the next oxygen, and so on. Hydrogen bonding is useful in this case because it can also occur both in between water molecules and different substances that are polar or ionic.

In simple terms, the molecules in a water solvent and in polar or ionic solutes attract each other through their opposite charges. This attraction enables the solute particles to be pulled apart and eventually dissolve. The rule of thumb is that “like dissolves like” and so a polar solvent like water can only dissolve polar and ionic solutes.

Water as a Solvent Examples

Water has the ability to dissolve solid, liquid, and gas substances. Here are some examples of as seen in our daily lives:

• Carbon dioxide (gas solute) dissolved in water (liquid solvent) produces carbonated water. This is what makes your soda fizzy!

• Acetic acid (liquid solute) dissolved in water (liquid solvent) results in vinegar. You’ve probably eaten a dish or two with vinegar.

• Salt (solid solute) dissolved in water (liquid solvent) results in a saline solution. You’ve probably stored contact lenses, healed a piercing, or treated a runny nose with this solution.

Functions of Water as a Solvent in the Body

Water’s function as a solvent is not only observable around us; it can also be observed within our own bodies!

For example, the liquid part of our blood–called plasma–is made up of over 90% water.

The solvent function of water enables blood to dissolve and transport substances to and from various parts of our body. These substances include:

• Nutrients such as glucose, which acts as our body’s main energy source.

• Hormones, which acts as our body’s chemical messengers.

• Electrolytes–such as sodium and potassium–which are vital to our bodily functions.

• Gasses such as oxygen and carbon dioxide.

Our kidneys also require the solvent property of water to filter out chemicals that enter our bodies through the meals and beverages we consume. Being an excellent solvent, water passing through the kidneys is able to dissolve these compounds and transport them out of our bodies. Wastes that are eliminated through our kidneys include ammonia, urea, and creatinine.

Uses of Water as a Solvent in Plants

Water also plays an important role as a solvent in plants. All plants need 17 essential elements to grow and develop, and 13 of these are in ionized, polar forms that are easily soluble in water, enabling plants to take them in through the soil.

Under typical soil and water conditions, the concentration of dissolved elements inside the plant is higher than in the soil. Through osmosis, the solution consisting of water and the dissolved essential elements passes through the root membranes and into the plant. Another important property of water called capillarity (or the capacity of water to climb up a surface against the pull of gravity) enables it to take up the solution and bring it to other parts of the plant.

The Biological Importance of Water as a Solvent

Why is the solvent property of water so important to life on Earth? All living things are made up of carbohydrates, lipids, proteins, and nucleic acids; these four so-called biological macromolecules serve as the building blocks of life.

Water and Nonpolar Molecules

Take a glass of water and stir in whatever type of oil is at your disposal. At some point, you might think that you have successfully mixed the two substances together and formed a homogeneous mixture, but leave it alone for a minute or two, and you would notice the two substances forming separate layers in the glass.

While water is regarded as a “universal solvent,” not everything can dissolve in water. Because water molecules are polar, they are attracted to ionic or polar compounds, which are charged substances. Nonpolar substances such as lipids have no charge, so water is not attracted to them. Even when adequately mixed, molecules of nonpolar substances tend to separate from the water when the mixing ceases.

Oil does not not dissolve in water, but it can dissolve in other nonpolar solvents such as gasoline. This is because gasoline and oil are both nonpolar. 'Like dissolves like', right?

So what about soaps and detergents? You wash dishes using soap because it is able to dissolve oil and grease. So if like dissolves like, why does soap dissolve in water, too?

Soaps and detergents are special because they are amphipathic molecules, meaning they contain both polar and nonpolar groups. Their polar ‘head’ can form hydrogen bonds with water molecules while their long, nonpolar ‘tails’ can interact with other nonpolar molecules (Fig. 3).

When a soap molecule is in contact with a nonpolar substance like oil, its nonpolar ends slide in between the nonpolar molecules while its charged head faces outwards and attracts water molecules. As detergent molecules bind to the nonpolar substance, they enclose it, making it easy for the substance to be carried into the water solution. This is how we end up with clean dishes!