BIO - Water [LESSON]

Water

Structure

When it comes to water, we’re literally drowning in it, as water is by far the most abundant component of every cell. Everything that happens in cells and living organisms is influenced by water’s chemical structure. Water is a polar molecule and consists of an oxygen atom that shares electrons with two hydrogen atoms. Recall that this type of bond is a covalent bond. Since oxygen is more electronegative than hydrogen, it has the tendency to pull the electrons towards itself more than hydrogen does. Consequently, a partial charge results at each end of the water molecule. Oxygen has a negative partial charge (δ-) and hydrogen has a positive partial charge (δ+). This unequal distribution of charges as well as its bent shape gives water its polarity.

This picture likely looks familiar from the last lesson. That’s because the structure of water allows for the formation of multiple hydrogen bonds due to its polarity and subsequent partial charges. Notice that oxygen has a partial negative charge and can form two hydrogen bonds (labeled with a “1” in the image) with the neighboring hydrogens of other water molecules. Recall that hydrogen bonds are the strongest type of intermolecular force and they form when interactions occur between two partially charged atoms located on different molecules. 

Take a few minutes to investigate water’s molecular structure below (ensure you have the “show hydrogen bonding” and “show partial charges” checkboxes selected). You can also investigate the effect of temperature. Note how hydrogen bonding is temporary but the strong, covalent bonds between atoms always remain.

Properties

Cohesion/adhesion:

Cohesion is the attraction of water molecules to each other. Adhesion is the attraction of one substance to another; such as when water molecules are attracted to other types of molecules. The hydrogen bonding of water molecules to each other and other substances is important to biological functions. For example, the forces of cohesion and adhesion work together to produce capillary action, the force of water rising in a thin tube. This force makes it possible for water to travel up the thin tubes inside plants from roots to leaves. Adhesive forces cause the water molecules to be attracted to the insides of the tubes. The cohesive forces cause the water molecules to cling to each other.

High specific heat:

The capability of a molecule to absorb heat energy is called heat capacity. Water's high heat capacity is a property caused by hydrogen bonding among water molecules. When heat is absorbed, hydrogen bonds are broken and water molecules can move freely. When the temperature of water decreases, hydrogen bonds are formed and release a considerable amount of energy. Water has the highest specific heat capacity of any liquid. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie or 4.184 Joules. As a result, water takes a long time to heat and a long time to cool. 

Ice floats:

Water's lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes: the water molecules are pushed farther apart compared to liquid water. With most other liquids, freezing when the temperature drops include the lowering of kinetic energy between molecules, allowing them to pack even more tightly than in liquid form and giving the solid a greater density than the liquid. However, with ice, freezing causes density to decrease and subsequently float in liquid water. 

Universal solvent:

Water, which not only dissolves many compounds but also dissolves more substances than any other liquid, is considered the universal solvent. A polar molecule with partial positive and negative charges, it readily dissolves ions and polar molecules. Water is therefore referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules.

When ionic compounds are added to water, individual ions interact with the polar regions of the water molecules during the dissociation process, disrupting their ionic bonds. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. This process is shown in the image below.  Note the orientation of the water molecules around each solute – this depends on the charge.

 

 

Now that we have looked at water’s unique properties, click on each box in the Water Tabbed Activity to learn more about why that property is important for life on Earth.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION