Properties of Water

Water is one of the most important molecules on the planet. In fact, it is so important that scientists are on the hunt for finding water on other planets. Our bodies are 60-70% water, our cells are composed mostly of water.

Below are the six properties of water that make it unique and crucial to life on Earth.

1. Water is a polar molecule

Polarity means to have a difference in charge; one side is positive and the other is negative. Water is composed of two hydrogen atoms and one oxygen atom. There is a shared pair of electrons between the two hydrogen and oxygen O-H bonds. 

Water molecule showing a pair of electrons on each O-H bond

Oxygen is more electronegative than hydrogen. An electronegative atom will attract the shared electron pair closer to itself that the other atom will. Think of it as greediness. The more electronegative an atom is the more greedy it is to keep the electron pair closer to itself. In the case of water, oxygen pulls the shared electron pair closer to itself. Remember that electrons are negatively charged. So when the electron pair moves closer to the oxygen atom, oxygen gains a slight negative charge. And hydrogen gains a slight positive charge. Hydrogen has a positively charged proton which gives it the positive charge.

This means that in a water molecule, oxygen will have a slightly negative charge and the two hydrogens will have a slightly positive charge. 

Why is the polarity of a water molecule important? Remember that negative charges are attracted to positive charges. All water molecules are polar and so they attract each other. A hydrogen atom from one water molecule will attract the oxygen of another and vice versa. This is called hydrogen bonding. Hydrogen bonds are the reason why the water in your glass has all the water molecules stuck (or attracted) to each other and therefore in liquid form.

Similarly, water also attracts other polar molecules like sugar. Like dissolves like. A polar molecule will dissolve in water. A substance that readily dissolves in water is called hydrophilic (i.e. water-loving). Oils and fats are non-polar and do not dissolve in water. You can see oils floating on top of water. Such substances that are non-polar and don’t dissolve in water are called hydrophobic (i.e. water-hating).

2. Water is an excellent solvent

A solvent is a substance in which other substances dissolve. Water is called the “universal solvent” because it dissolves many substances. Other than polar molecules, water also dissolves ionic compounds.
A salt molecule is polar, ionic, and is made of sodium chloride. Sodium (Na) is slightly positively charged, and chloride (Cl) is slightly negatively charged. Na will be attracted to the negatively charged O of water, and Cl will be attracted to the positively charged H of water. When you put salt in water NaCl dissociates (breaks up) into Na and Cl and bonds with O and H respectively we say that a sphere of hydration or hydration shell is formed. 

Spheres of hydration form when salt dissolves in water.

3. Cohesion

Cohesion means a molecule is attracted to itself. Water molecules are attracted to each other since they’re polar and end up forming hydrogen bonds with each other. This attraction results in surface tension. If you drop a small needle in water it will “float” and not sink. This is because the water molecules are holding on to each other and the needle doesn’t break through and sink.

Similarly, if a very small insect lands on water it can “stand” on water.

A paperclip "floats" on water

4. Adhesion

Adhesion means that a molecule is attracted to other molecules. If you fill a narrow glass or test tube with water you will notice that the edges of the water are higher up.

This is called the meniscus and it occurs because water molecules like to stick to other objects. 

When we water plants, the water climbs up from the roots all the way to the stems and leaves due to the property of adhesion.

5. Water has a high heat capacity

Heat capacity refers to how much heat a substance can absorb before its own temperature rises. Water has a high heat capacity and so it can absorb a lot of heat before its temperature rises. 

Imagine yourself going to the beach on a hot summer day. You take your shoes off as you get to the beach, you walk through the sand, and then into the water. Which felt cooler, the sand or the water? The water did. The same sun and heat are beating on both the water and the sand, however, as the sand absorbs heat its temperature rises. Whereas water absorbs the same heat but its temperature does not rise as much.
This property is very important for marine life. If water did not have a high heat capacity, hot summers would kill off a lot of marine life as water temperatures would drastically rise.

6. Solid water (ice) is less dense than liquid water

When water freezes to ice, water molecules form a beautiful rigid, lattice-like structure. This structure is lighter (because the water molecules are farther apart) and so ice floats on water.

Ice floats on water

This property is important for the survival of marine life. Floating ice insulates the water underneath. If ice did not float on water, the animals in the water would freeze during the harsh winter months. 

Amna Qureshi