The Chemistry of Life Properties of Water

Unique Properties of Water Water has both a high boiling point and a low freezing point Liquid water changes temperature very slowly Water exists in three forms: solid, liquid and gas Molecules in liquid water are attracted to one another and as a result stick very close together

Unique Properties of Water Liquid water is an excellent solvent Liquid water expands when it becomes a solid Water is colorless and allows light to shine through it

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The Water Molecule Like all molecules, a water molecule is neutral. The positive charges on its 10 protons balance out the negative charges on its 10 electrons.

The Water Molecule Negative The oxygen end of the molecule has a slightly negative charge and the hydrogen end of the molecule has a slightly positive charge. Oxygen Hydrogen Positive

The Water Molecule With 8 protons in its nucleus, an oxygen atom has a much stronger attraction for electrons than either of the hydrogen atoms with their single proton each. Therefore, there is a higher likelihood that the shared electrons will be closer to the oxygen atom than the hydrogen atom at any given time.

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A Polar Molecule A molecule in which the charges are unevenly distributed is called a polar molecule, because the molecule is like a magnet with poles.

Hydrogen Bonds With partial positive and negative charges, polar molecules such as water can attract each other. Hydrogen bonds are not as strong as covalent or ionic bonds, but water’s ability to form multiple hydrogen bonds is responsible for many of its special properties.

Hydrogen Bonds A single water molecule man be involved in as many as four hydrogen bonds at the same time. This ability to form multiple hydrogen bonds is responsible for many of water’s properties.

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Cohesion Cohesion is an attraction between molecules of the same substance. Water is extremely cohesive due to its hydrogen bonding. Water’s cohesion causes molecules on the surface of water to be drawn inward. Cohesion also explains why some insects and spiders can walk on a pond’s surface. Also called Surface Tension.

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Adhesion Adhesion is the attraction between molecules of different substances. The surface of the water in the graduated cylinder dips slightly in the center because the adhesion between the water molecules and the plastic or glass is stronger than the cohesion between water molecules.

Capillary Action Adhesion between water and glass causes water to rise against the force of gravity. This effect is called capillary action. Capillary action is one of the forces that draw water out of the roots of plants and up into its stems and leaves. Cohesion holds the column of water together as it rises.