This means that one portion of a molecule is slightly positive or slightly negative in relation to another portion of the same molecule. When such an uneven distribution of charge occurs, the molecule is said to exhibit polarity.
The molecule has a positive and a negative end, separated from eachother like the poles of a bar magnet. Water is a good example to illustrate this point. Although, as a whole, the water molecule is electrically neutral, it does have a positive and a negative end. The geometric configuration of the molecule places both hydrogen atoms at one end. The nucleus of the oxygen atom attracts electrons more than the nuclei of the hydrogen atoms.
This results in two slightly positively charged regions on one end of the molecule and a single slightly negatively charged region on the other. The molecule thus has a positive and a negative end, or two poles. The water molecule is a polar molecule. Moreover, water has thermal properties known as a high specific heat capacity and high latent of heat of vaporization.
The latter means water takes significant energy to evaporate, allowing it to act as a coolant, which is important in the thermoregulation of mammals through sweating. On the other hand, the high specific heat capacity means water requires a lot of energy to change state, making it a stable environment for aquatic species.
Cohesion is the property of water which makes water molecules attracted to each other. Since the hydrogen atoms are positively charged and the oxygen atom is negatively charge, when various water molecules in the same place, the unlike charges will attract. This forms hydrogen bonds, which hold water together in drops. When table salt NaCl is mixed in water, spheres of hydration form around the ions.
A positively charged sodium ion is surrounded by the partially negative charges of oxygen atoms in water molecules. A negatively charged chloride ion is surrounded by the partially positive charges of hydrogen atoms in water molecules. These spheres of hydration are also referred to as hydration shells.
The polarity of the water molecule makes it an effective solvent and is important in its many roles in living systems.
Figure 5. The weight of a needle on top of water pulls the surface tension downward; at the same time, the surface tension of the water is pulling it up, suspending the needle on the surface of the water and keeping it from sinking.
Notice the indentation in the water around the needle. Have you ever filled up a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water actually forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other because of hydrogen bonding , keeping the molecules together at the liquid-air gas interface, although there is no more room in the glass.
Cohesion gives rise to surface tension, the capacity of a substance to withstand rupture when placed under tension or stress. When you drop a small scrap of paper onto a droplet of water, the paper floats on top of the water droplet, although the object is denser heavier than the water. This occurs because of the surface tension that is created by the water molecules. Cohesion and surface tension keep the water molecules intact and the item floating on the top.
You will notice that the water appears to be higher on the sides of the straw than in the middle. This is because the water molecules are attracted to the straw and therefore adhere to it.
Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in , offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.
How is a Water Molecule Like a Magnet? The Formation of Hydrogen Bonds. What Are the 5 Emergent Properties of Water? Is Methane Nonpolar? How Does Water Freeze? How to Rationalize the Difference in Boiling Points. Special Properties of Hydrogen.
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