Back To CourseGED Science: Help and Review
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Water, water, everywhere. Whole oceans of it cover most of the earth. If we piled all the water in the world up on the United States, it would cover the entire country to a depth of over a hundred miles, an amount approximately 332,500,000 cubic miles in volume. Water vapor in the atmosphere keeps the planet much warmer than it would otherwise be. The human body is more than half water, more for babies, less for adults, and virtually every biochemical reaction necessary for life runs in water. We use the power in swiftly moving water to produce electricity for cities; this is known as hydropower. We grow plants, wash dishes, and put out fires with water. We ski on it, cook with it, and drink it. We even complain about it - particularly when it snows or rains too much - but we need it.
We think of water as common and familiar, but chemically it is one strange molecule. Most chemicals shrink and sink when they freeze, but water expands and floats. Most lightweight molecules are gases at room temperature, but water is a liquid. It has an unusually high boiling point, 100 degrees Celsius or 212 degrees Fahrenheit, and freezing point, 0 degrees Celsius or 32 degrees Fahrenheit. Water also dissolves a large number of different substances, from solid table salt to liquid alcohol, and to a lesser extent gases such as oxygen and carbon dioxide.
All of these properties result from water's structure. An atom is the smallest piece of an element, and it can bond with other atoms to make a molecule, which in turn is the smallest piece of a compound. Water is a compound, and each of its molecules has two hydrogen atoms bonded to a single oxygen atom.
Notice the bent shape of the water molecule. All atoms have negatively charged particles called electrons circling around a positively charged nucleus. The electrons arrange themselves in shells and are most stable if paired, and the electron pairs are as far apart as possible. In water, only the outermost electrons, those in the outer shell, participate in bonding. An oxygen atom alone has six electrons in its outer shell, and each hydrogen atom initially has one. When the hydrogen atoms share their electrons with two of the oxygen atom's outermost shell electrons, two bonds form, one each between a hydrogen atom and the oxygen.
Now the oxygen atom has eight electrons in its outer shell - its own six electrons plus the two from the hydrogen atoms. This makes two bonding pairs and two non-bonding pairs. The four pairs of electrons around oxygen all take up space, and the most stable arrangement is at the corners of a tetrahedron, a pyramid shape with oxygen at the center and the four electron pairs pointing toward the four corners. As a result, the water molecule looks bent, with the hydrogen atoms on one side of the oxygen atom and the unpaired electrons on the other side.
Additionally, oxygen draws electrons to itself very strongly; a chemist would say it is highly electronegative. The hydrogen atoms, which are much less electronegative than oxygen, lose some electron density to the oxygen, so the electric charge inside the molecule becomes lopsided. The oxygen side of water is more negative and the hydrogen side more positive, forming a dipole, which means the molecule is positive on one side and negative on the other, even though the molecule overall is neutral. Most molecules have weak partial charges like this, which are written as delta plus and delta minus, but only in water and a few other molecules are the partial charges this strong and this fixed. We say that a molecule with a permanent dipole is polar.
Think like a water molecule. There you are, floating around with a lot of other water molecules in a beaker. The partial negative charge on your oxygen side will attract the partial positive charges from the hydrogen atoms on other water molecules. The partial positive charge on your hydrogen side will attract the oxygen atoms of other water molecules. That will be true for all the other molecules, too. In liquid water, molecules jostle and move around in a disorderly fashion, but they also tend to stick to each other, an unusually strong attraction called hydrogen bonding, which refers not to the hydrogen bonded to oxygen inside each molecule, but rather to the attraction of partial charges between the molecules.
Hydrogen bonding explains water's unique characteristics. Water remains a liquid at room temperature because the molecules attract each other so strongly. Water can soak up a lot of energy before its temperature goes up, and it takes a lot of energy to break hydrogen bonds connecting a water molecule to the surface of liquid water, so it evaporates slowly and has a high boiling point. Water vapor in the atmosphere keeps the earth warm because the molecules had to absorb so much energy to become vapor in the first place.
Water expands when it freezes because the frozen water, or ice, crystal is less dense than water itself. Part of that is due to hydrogen bonding, and the other part is due to water's shape, which allows an orderly, open lattice structure to form upon freezing. Good thing, too. Many organisms live in water, and the ice cover in winter allows them to survive severe cold.
Water is sometimes called the universal solvent because it dissolves so many other substances. Table salt, or sodium chloride, for example, dissolves in water because several water molecules surround both the sodium ion and the chloride ion, breaking them off from the NaCl crystal. Any substance with a dipole will dissolve in water at least a little because of hydrogen bonding.
Water is plentiful, useful, and necessary. Water's properties result from its molecular structure: two hydrogen atoms bonded to a single oxygen atom. Electrons pair up and can be shared among atoms. In the water molecule, there are a total of four pairs of electrons in the outer shell: two lone pairs and two pairs that bond each hydrogen atom to the oxygen atom.
The water molecule is bent in shape because the lone pairs of electrons take up space on one side of the molecule, with the hydrogen atoms on the other side, and polar because oxygen's electronegativity draws electrons strongly to itself, creating a strong overall dipole. The shape and the dipole both contribute to hydrogen bonding as the partial charges on each side of the molecule attract other molecules in the vicinity, or other polar materials. Water's high boiling point, expanded ice structure, and ability to dissolve other polar substances can all be explained by hydrogen bonding.
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Back To CourseGED Science: Help and Review
35 chapters | 505 lessons