Another name for this compound is…water. Water can create some absolutely beautiful sights. Iguassu Falls is the largest series of waterfalls on the planet, located in Brazil, Argentina, and Paraguay. And water is necessary for life. The importance of water to life cannot be emphasized enough. All life needs water. Life started in water. Essentially, without this simple three atom molecule, life would not exist.
Water, like carbon, has a special role in living things. It is needed by all known forms of life. Water is a simple molecule, containing just three atoms. Nonetheless, water’s structure gives it unique properties that help explain why it is vital to all living organisms.
Water is a common chemical substance on planet Earth. In fact, Earth is sometimes called the “water planet” because almost 75% of its surface is covered with water. If you look at the Figure below, you will see where Earth’s water is found. The term water generally refers to its liquid state, and water is a liquid over a wide range of temperatures on Earth. However, water also occurs on Earth as a solid (ice) and as a gas (water vapor).
No doubt, you are already aware of some of the properties of water. For example, you probably know that water is tasteless and odorless. You also probably know that water is transparent, which means that light can pass through it. This is important for organisms that live in the water, because some of them need sunlight to make food.
Chemical Structure of Water
To understand some of water’s properties, you need to know more about its chemical structure. As shown in the Figure below, each molecule of water consists of one atom of oxygen (red) and two atoms of hydrogen (white). The oxygen atom in a water molecule attracts negatively-charged electrons more strongly than the hydrogen atoms do. As a result, the oxygen atom has a slightly negative charge, and the hydrogen atoms have a slightly positive charge. A difference in electrical charge between different parts of the same molecule is called polarity, making water a polar molecule.
Opposites attract when it comes to charged molecules. In the case of water, the positive (hydrogen) end of one water molecule is attracted to the negative (oxygen) end of a nearby water molecule. Because of this attraction, weak bonds form between adjacent water molecules. The type of bond that forms between molecules is called a hydrogen bond. Bonds between molecules are not as strong as bonds within molecules. There are just many more hydrogen bonds in water (between water molecules) than there are covalent bonds within a molecule. The hydrogen bonds may not be strong, but in water they are strong enough to hold together nearby molecules.
Because of its polarity, water readily dissolves many substances including ionic compounds like table salt (NaCl). Water molecules stabilize the Na+ and Cl– ions that are formed after the ionic bond between them breaks.
Properties of Water
Hydrogen bonds between water molecules explain some of water’s properties. For example, hydrogen bonds explain why water molecules tend to stick together. This property is known as cohesion. Have you ever watched water drip from a leaky faucet or from a melting icicle? If you have, then you know that water always falls in drops rather than as separate molecules.
Another important physical property of water is adhesion. In terms of water, adhesion is the bonding of a water molecule to another substance, such as the sides of a leaf’s veins. This process happens because hydrogen bonds are special in that they break and reform with great frequency. This constant rearranging of hydrogen bonds allows a percentage of all the molecules in a given sample to bond to another substance. This grip-like characteristic that water molecules form causes capillary action, the ability of a liquid to flow against gravity in a narrow space. An example of capillary action is when you place a straw into a glass of water. The water seems to climb up the straw before you even place your mouth on the straw.
Adhesion, cohesion and capillary action are necessary to the survival of most organisms. It is the mechanism that is responsible for water transport in plants through roots and stems (see Figure below), and in animals through small blood vessels.
Hydrogen bonds cause water to have a relatively high boiling point of 100°C (212°F). Because of its high boiling point, most water on Earth is in a liquid state rather than in a gaseous state. Water in its liquid state is needed by all living things. Hydrogen bonds also cause water to expand when it freezes. This, in turn, causes ice to have a lower density (mass/volume) than liquid water. The lower density of ice means that it floats on water. For example, in cold climates, ice floats on top of the water in lakes. This allows lake animals such as fish to survive the winter by staying in the water under the ice.
The human body is about 70% water (not counting the water in body fat, which varies from person to person). The body needs all this water to function normally. Just why is so much water required by human beings and other organisms? Water can dissolve many substances that organisms need, and it is necessary for many biochemical reactions. The examples below are among the most important biochemical processes that occur in living things, but they are just two of many ways that water is involved in biochemical reactions.
6CO2 + 6H2O + Energy → C6H12O6 + 6O2
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Water is involved in many other biochemical reactions. As a result, just about all life processes depend on water. Clearly, life as we know it could not exist without water.
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