Good Solvent (Water) — Properties & Examples

Why Is Water a Good Solvent?

Water is known as the universal solvent. (Here's a good review of what a solvent is.) Water's ability to dissolve is again due to its polarity. The partial positive and negative charges of water allow it to disrupt the structure and configuration of many other molecules, such as salt.

Example: NaCl has one positively charged (Na$^+$) and one negatively charged (Cl$^-$) ion. These two ions will dissolve away from each other in water. Water's positive hydrogens surround the negatively charged Cl and the negatively charged oxygen surround the positive Na.

Water's ability to disrupt molecular structures makes it a valuable tool for organisms at the cellular level. Water's polarity allows it to carry many molecules into and out of cells. Blood is a great example. If the majority of our blood was not water, it couldn't carry all of the red blood cells, white blood cells, sugars, and oxygen around our body that we need to function.

The Universal Solvent

Water is commonly referred to as the universal solvent.

Let's review what a solvent is before we dig deeper into this:

So, when we say that water is the universal solvent, we are saying that it can dissolve more solutes than pretty much any other liquid. It's important to note that, while water is a potent solvent, it cannot dissolve all solutes.

To understand why water is such an amazing solvent, we need to think back to the structure of each water molecule.

Recall that water ($H_2O$) is a polar molecule with two distinct ends: one with a positive charge and one with a negative charge.

The importance of water's polarity can be boiled down to the phrase "opposites attract". Any other molecule that is polar (has a positive end and a negative end) can be dissolved by water.

To talk about why, let's use a polar molecule we all know can dissolve in water: salt. The chemical formula for salt is $NaCl$, or one sodium (Na) atom bonded to one chlorine (Cl) atom.

Here's a look at each type of molecule on its own. Note the charged ends of each.

Now let's look at what happens when we mix water and salt:

And finally:

So, any molecule that is polar (has a positive end and a negative end) can be ripped apart and dissolved by water, just like salt was.

For the same reason, water is a poor solvent of substances that are non-polar because these substances don't have any positive or negative ends. With nothing for the water molecules to be attracted to, the non-polar substance sort of just sits there; water cannot rip the non-polar molecules apart.

A good example of a non-polar substance is wax. Imagine putting some candle wax into regular water. It's not going to dissolve and will just sit there.

Key Vocabulary:

1. Solute
2. Solvent
3. Polar
4. Universal solvent

Like Dissolves Like

A solvent is a substance that dissolves other substances. Water is known as a universal solvent. Now, this doesn't mean that it dissolves everything, but it does dissolve more types of solute than any other liquid. Solutions where water is the solvent are called aqueous solutions.

You've probably heard that "like dissolves like". This is very true for water; it only dissolves polar or ionic substances. You have definitely seen this before if you've added oil or butter to water: the fats always floats to the top. This is because oils and fats (also known as lipids) are non-polar and don't mix with water.

For instance, adding salt to water causes the salt ions to break up. For table salt (NaCl), the negative oxygen atoms attract the positive sodium ions and the positive hydrogens attract the negative chloride ions.

The Structure of Water

A single water molecule is polar and made up of 2 hydrogens and 1 oxygen. But what about your glass full of water? Individual $H_{2}O$s connect with each other with hydrogen bonds. These hydrogen bonds are an electrostatic attraction between the positive pole of one water molecule and the negative pole of another water molecule.

What does a hydrogen bond have to do with water's strong solvent abilities? Water molecules attract each other and become laid out in a spherical shape that can surround solutes. This is called a hydration shell. Hydration shells surround parts of broken-down polar solutes, keeping them apart and dispersing them evenly throughout the aqueous solution!

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Polarity}}$ : Partial positive and negative charges caused by some atoms wanting electrons more than other atoms in a molecule

$\color{#8925AE}{\text{Hydrogen Bond}}$ : Positively charged hydrogens of one molecule are attracted to the negative charge on another molecule

$\color{#8925AE}{\text{Cohesion}}$ : The intermolecular (between-molecules) attraction between molecules of the same type

$\color{#8925AE}{\text{Adhesion}}$ : The intermolecular attraction between different types of molecules

$\color{#8925AE}{\text{Density}}$ : Water's liquid state is much denser than its solid state, causing ice to float on top of the water. Calculated by dividing an object's mass by its volume

$\color{#8925AE}{\text{Specific Heat}}$ : The amount of heat energy needed to increase the temperature of a specific amount of a substance to a specific temperature. The lower the specific heat, the easier it is to heat a substance up

$\color{#8925AE}{\text{Solvent}}$ : Water is a universal solvent. Polar molecules dissolve very quickly in it

(Video) Water–Liquid Awesome

by CrashCourse

What does it mean to say that water is the universal solvent? No, water isn't going to slowly dissolve the whole universe. In this video, Hank Green explains water's life-supporting properties, including how good of a solvent it is.