Overview of Water — Properties & Importance to Life

Properties of Water

Water is the most abundant compound on Earth's surface. It exists in three forms: solid, liquid, and gas.

Water is a polar molecule, which means that it has poles, like a magnet and the Earth. It has a partial positive charge ($\delta+$) on the hydrogen atoms and a partial negative charge ($\delta-$) on the oxygen atom. The partial charges on a water molecule are similar to a bar magnet, where oppositely charged poles repel each other but opposite poles attract.

Water has many unique properties that are a result of its polarity. Hydrogen bonding is when the positively charged hydrogen atoms are attracted to the negatively charged oxygen of another water molecule. The poles make water molecules really like sticking to each other.

A useful mental picture of this is a pile of magnets. The positive and negative poles are attracted to each other.

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

$\color{#8925AE}{\text{Polarity}}$: 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

$\color{#8925AE}{\text{Surface Tension}}$: The attraction between different molecules on the surface of a liquid

$\color{#8925AE}{\text{Specific Heat}}$: 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

What Are the Properties of Water?

Water is everywhere. Many of the important, life-supporting qualities of water come from the fact that water is a polar molecule.

Water Is Polar

Oxygen is highly electronegative. This means that it is able to strongly attract electrons. The electrons shared in the bond between hydrogen atoms and the oxygen atom tend to stay closer to the oxygen. This uneven sharing is what causes partial negative and partial positive charges.

Because the hydrogen atoms have a partial positive charge, they tend to attract other water molecules by aligning with partially negative oxygen atoms.

Water Has Cohesion

This property that attracts water molecules to other water molecules is what causes cohesion. You can see cohesion in action when water spiders "walk" on water. The force of the water molecules sticking together pushes the spider upwards, keeping it from sinking into the water.

Water Has Adhesion

The polarity of water also contributes to adhesion. Adhesion is when water is attracted to a surface because of its polarity. If you look closely at water in a cup, you can see that the water on the sides of the glass is slightly higher than the water in the middle. This shape is called a meniscus and it is caused by the water being attracted to molecules in the glass.

Capillary Action of Water

When in a thin enough object, water can move upward almost indefinitely due to adhesion and cohesion. This is called capillary action. When you touch a spill with a paper towel, the paper towel quickly absorbs the liquid, sometimes even in parts of the towel that didn't even touch the water. This is because capillary action moves the water throughout the paper towel.

Water Is a Good Solvent

The polarity of water also makes it a powerful solvent. It can dissolve other polar molecules. The ocean is very salty because salt, which has a positive $Na$ and a negative $Cl$, dissociates in water.

Water Has a Unique Density

Because water becomes less dense as it freezes, it means that ice will float. This contributes to life because when ice remains on the surface of a body of water, it insulates the water underneath, allowing fish and other organisms to survive. If ice always sank as it formed, there is a good chance that all water would freeze during the winter, killing anything living in the water.

Water Has a High Heat Capacity

The heat capacity of water is also fairly high. It takes a lot of energy to raise or lower the temperature of water. This means that water doesn't freeze or vaporize very suddenly, which makes it more stable for use in living organisms than some other compounds.

Water and pH

Water has properties of both acids and bases because water can dissociate and create $H^+$, $H^3O^+$, and $OH^-$ ions. This makes water a weak buffer of other acids and bases as it is good at maintaining a neutral pH.

Summary of the Properties of Water

We need water for life, but why is it so important? Well, it actually has a lot of really unique properties. Check out this list to see why water is so cool:

• Cohesion: Water molecules really like to stick together. Think of puddles and water droplets.

• Adhesion: Water likes to stick to other things, like your dishes.

• Capillary Action: Water is able to flow up a small tube against external forces like gravity. Adhesion and cohesion work together. The attraction between the water and the tube, and the water to itself, causes the water to stick to the sides of the tube and move upwards.

• Surface Tension: Water has a strong top layer, which allows things to float on top of it. For example, a fallen leaf will float on top of a puddle instead of immediately sinking.

• High Specific Heat: Water resists changes in temperature. This is why it takes so long to boil.

• Universal Solvent: Water can dissolve many different compounds, from sugar to salts. This is why you add water to all kinds of recipes.

• Density: Water has an incredibly high density due to hydrogen bonding and its low molecular weight. This allows the molecules to be packed together tightly in the liquid phase, but more spread out in the solid phase.

• Insulator: Pure water contains no ions, so it can not conduct electricity. It is an insulator.

• Acid/Base: Water can dissociate, or break apart, into acidic ions and basic ions. The acidic ion is called the hydrogen ion, $H^+$, while the basic ion is the hydroxide ion, $OH^-$.

(Video) Water - Liquid Awesome

by CrashCourse

This video by Crash Course discusses some properties of water including hydrogen bonds, cohesion, capillary action, and ice density. You will also see some examples of these properties in action.