Sprinkle, Splash, Submerge, Soak!
What are the physical and chemical properties of water that make it so unique and necessary for living things? Water remains liquid at a much higher temperatures than expected. It also boils and freezes at much too high, or low, of a temperature for a molecule of its size. Many of these unexpected properties of water can be explained by the structure of the water molecule and the characteristics of the atoms it contains. Pure water is virtually colorless and has no taste or smell, but the hidden qualities of water make it an interesting subject. In the activities that follow, students will discover cool ways that water interacts with various materials, all due to its special properties.
- Describe properties of the different states of water.
- Describe the cohesive and adhesive properties of water.
- Describe the relationship between capillary action, adhesion, cohesion, and surface tension.
Water is very important to life on Earth. Water is a major component of cells, typically forming between 70% and 95% of the mass of the cell. This means that we are made from approximately 80% water by mass (weight) and some soft bodied creatures such as jellyfish are made of up to 96% water. Water also provides an environment for organisms to live in: 75% of the earth is covered in water. Water circulates through the land just as it does through the human body; transporting, dissolving, and replenishing nutrients and organic matter while carrying away waste material.
Water is the only pure substance found naturally in all three states of matter: solid (ice), liquid (water), and gas (steam). This is possible due to its unusual properties.
Each water molecule is made up of two hydrogen atoms and one oxygen atom (H20). Water is not a linear molecule with the atoms all in a row. The two hydrogen atoms form a bond with the oxygen at an angle of 104.5o making the molecule V-shaped.
The two hydrogen atoms are attached to the oxygen atom by covalent bonds. This means that the hydrogen atoms share their electrons with the oxygen atom.
Although each water molecule is neutral, with the same number of negative electrons and positive protons, the electrons stay closer to the oxygen atom. The result is that the tips of the V (the hydrogen atoms) are more positive and the bottom of the V (the oxygen atom) is more negative. We describe a water molecule as "polar", which means that it has a positively charged "end" and a negatively charged end. This polarity helps make water both unusual and useful.
When water molecules are close together, the positively charged hydrogen atoms are attracted to the negatively charged oxygen atoms of another water molecule. These attractions are called "hydrogen bonds." They are weak individually but the sheer number of them can make the total force keeping the molecules together quite considerable.
Hydrogen bonding between water molecules leads to many interesting consequences. For example, the boiling point of water, its cohesion and surface tension, and its ability to dissolve salts are all related to hydrogen bonding.
The boiling point of water, 100°C, is unusually high for a molecule with such a low molecular weight. The high boiling point is due to hydrogen bonding. Boiling water means breaking up all of the hydrogen bonds in liquid water, so the molecules can move further apart and become steam. Breaking those bonds takes energy, thus the high boiling point for water.
Hydrogen bonds also give liquid water molecular cohesion and a high surface tension:
Because of the extensive hydrogen bonding in water, the molecules tend to stick to each other in a regular pattern. This phenomenon, called cohesion, is easily observed as you carefully overfill a glass with water and observe the water molecules holding together above the rim. Eventually gravity is stronger than the hydrogen bonds and the water molecules spill down the side of the glass. Likewise, the cohesive property of water allows tall trees to bring water to their highest leaves from sources below ground.
A special type of cohesion is surface tension. The tension on the surface of water occurs when water molecules on the surface are held together by hydrogen bonding. The result is that water behaves like it has a thin skin on the surface. When a water droplet is formed its spherical shape is the result of surface tension.
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