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Waterand

Seawater

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Bohr atom interpretation

a quantum mechanical interpretation of an atom’s energy state

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An Example of Molecular Structure

4Geometry of the Water Molecule

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What is a hydrogen bond ?This kind of bond forms in water because of the

polarity of water molecules. It is a bond between water molecules and is the basis for much of their

anomalous behavior.

What is a covalent bond ?Covalent bonds are chemical bonds formed by the

sharing of electrons between atoms. These bonds are generated between the oxygen and hydrogen atoms and are responsible for the polarity of the water molecule.

Stronger covalent bonds hold water molecules together while weaker hydrogen bonds give

water its uniquely different properties.

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Water Molecule Representations

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Water molecules actually arrange themselves in groups. Hands represent hydrogen atoms and feet are the electron pairs. The hand-grab-foot is a hydrogen bond between two molecules. These lead to a tetrahedral arrangement of neighbors around any single molecule and this brings on all of water’s anomalous properties. But there still is a twist to this that is not evident in the drawing above.

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Hydrogen atoms in hydrogen bonds just don’t stick anywhere on an oxygen atom. Since the hydrogen atom is positively charged it goes to one of an oxygen atom’s lone electron pairs, so the hydrogen bond is actually a bond between a hydrogen atom and a lone pair. This means each oxygen atom can form four hydrogen bonds and the structure builds on this basic capability.

Try impersonating a water molecule : your hands are hydrogen atoms and your ankles are each one of oxygen’s lone electron pairs. Stand with your legs apart about 104.5 degrees, careful there. Now twist 90 degrees at the waist and stretch out your arms … you are a water molecule. Pick four of your classmate molecules that are floating above the seats [ with you ] and with your hands grab an ankle of two different classmates, while two other classmates do the same to you. It will take five of you and we have what this picture [below] is trying to show you, the basic water structure. Try this at home for your next party, it’s fun, really.

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In its ice phase, each water molecule is dutifully hydrogen-bonded to four other molecules and also exists in crystalline matrix that displays hexagonal sym-metry. This creates an open lattice with a generous amount of space between molecules. They cannot be packed closer together and still be in the right places to make hydrogen bonds. Thus we have a paradox : the hydrogen bonds hold the crystal structure together giving it high strength and rigidity while simeltaneously being responsible for its low density. So, solid water is strong and it floats.

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What is temperature ? What is heat ?

Kinetic energy is the energy that a molecule has due to its speed.

Temperature is a measure of the average kinetic energy of the molecules of a substance.

Heat is the word we use to describe the transfer of the energy of molecular motion from one

substance to another.

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The mutual attraction of a hy-drogen atom in a water molecule for an oxygen atom in another is known as a hydrogen bond. These bonds occur because of the polar configuration of water molecules. The bonding depends on electrostatic forces that exist between positive hydrogen and negative oxygen atoms of adjacent water molecules.

The extraction or addition of the amounts of energy required to forge or dissociate these bonds are responsible for water’s high heat capacity and its ability to moderate temperature fluctuation at the earth’s surface.

Removal or addition of thermal energy is normally accompanied by measurable changes in temperature. However, during changes in state, say fusion or vaporization, energy transfers are not accompanied by sensible temperature changes because heat is involved in altering the hydrogen bonds. The amounts of energy involved are known as latent heats and these are not equivalent. The latent heat of vaporization, 540 cal/g, is much greater than the latent heat of fusion, 80 cal/gm, because all the bonds must be broken during evaporation.

One more time !

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Latent heat is a quantity of thermal energy [calories] gained or lost as a substance undergoes

a change of state at a given temperarture or pressure

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There is a map that shows the relationship between the three states of matter : solid, liquid and gas. The borders between each are defined by conditions of temperature and pressure at which transitions such as melting and freezing occur. We often think that a liquid will evaporate when it gets warmer. The map says that, along the liquid-gas border, a liquid can change to gas at the same temperature [ move down vertically on the map ]. The change of state is not produced by change in temperature but rather by a change in the amount of energy in the liquid, that is,

the change of state is driven by an addition of thermal energy. The substance alters its heat content, and so the arrangement of its particles, without a change in its temperature.

The heat absorbed in a gas as it evaporates from a liquid is called the latent heat of evaporation and this energy will be released when the gas condenses to a liquid. We will see that this is critically important to the equilibrium of the earth’s climate.

The point at which the melting and boiling lines come together. Below this point, the triple point, liquid does not exist and gas and solid are the only stable states. At the triple point all three states can exist simeltaneously. What is the process of solid passing directly to gas called ? See this happen at Vail every winter.

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A typical map of the state of a substance

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This map is for water and is slightly different than the previous map. However, the difference is very important. The border between solid and liquid slopes backwards toward the pressure axis with decreasing temperature. Actually, the map is this way because water is less dense when it freezes. We know that the crystalline structure of solid water (ice) is such that a given mass of ice takes up more volume than the same mass of water. Remember, mass per unit volume is density, and the map knows : ice will float on water.

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