Water is an unusual compound with unique physical properties. As a result, its the compound of life. Yet, its the most abundant compound in the biosphere of Earth. These properties are related to its electronic structure, bonding, and chemistry.
What are we going to learn in this topic ?
1. water structural molecule
2. hydrogen bond in water molecule
3. thermal and solvent properties in water
4. ionization of water
The chemical formula for water is H2O which means that every molecule of water has 2 atoms of hydrogen (H) and one atom of oxygen (O). Here comes the key part. From the Periodic Table of Elements, one sees tha tone mole of hydrogen atoms weighs 1 gram while one mole of oxygen atoms weighs 16 grams.
COHESION
Cohesion refers to hydrogen bonds that holds the molecules of water together. This contributes to the transportation of water and dissolved minerals against gravity in plants.
ADHESION
Adhesion refers to hydrogen bonds that hold water molecules and other polar molecules together. It means the clinging of one substance to another.
What are we going to learn in this topic ?
1. water structural molecule
2. hydrogen bond in water molecule
3. thermal and solvent properties in water
4. ionization of water
The chemical formula for water is H2O which means that every molecule of water has 2 atoms of hydrogen (H) and one atom of oxygen (O). Here comes the key part. From the Periodic Table of Elements, one sees tha tone mole of hydrogen atoms weighs 1 gram while one mole of oxygen atoms weighs 16 grams.
- Water molecules are tiny and V-shaped with molecular formula H2O a and molecular diameter about 2.75 Å. g
- the density of water vapor being just 62% the density of dry air
- Two hydrogen atom are bonded to one oxygen atom by strong covalent bond.
- The bond angle is 104.5.
- Water molecule is in a bent shape which formed because the oxygen atom compressed the bond higher than hydrogen atom.
COHESION
Cohesion refers to hydrogen bonds that holds the molecules of water together. This contributes to the transportation of water and dissolved minerals against gravity in plants.
ADHESION
Adhesion refers to hydrogen bonds that hold water molecules and other polar molecules together. It means the clinging of one substance to another.
THERMAL PROPERTIES
Water has a very high specific heat capacity – the second highest among all the heteroatomic species (after ammonia), as well as a high heat of vaporization (40.65 kJ/mol or 2257 kJ/kg at the normal boiling point), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climateby buffering large fluctuations in temperature.
The specific enthalpy of fusion of water is 333.55 kJ/kg at 0 °C, i.e. melting ice absorbs the same energy as ice warming from -160 degrees Celsius up to its melting point. Similarly the heat needed to melt ice at 0 °C, would heat the same amount of water by about 80 °C. Of common substances, only that of ammonia is higher. This property confers resistance to melting on the ice of glaciers and drift ice. Before and since the advent of mechanical refrigeration, ice was and still is in common use for retarding food spoilage.
SOLVENT PROPERTIES
Water is also a good solvent, due to its polarity. Substances that will mix well and dissolve in water (e.g. salts) are known as hydrophilic ("water-loving") substances, while those that do not mix well with water (e.g. fats and oils), are known as hydrophobic ("water-fearing") substances. The ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between other water molecules. If a substance has properties that do not allow it to overcome these strong intermolecular forces, the molecules are "pushed out" from the water, and do not dissolve. Contrary to the common misconception, water and hydrophobic substances do not "repel", and the hydration of a hydrophobic surface is energetically, but not entropically, favorable.
Water has a very high specific heat capacity – the second highest among all the heteroatomic species (after ammonia), as well as a high heat of vaporization (40.65 kJ/mol or 2257 kJ/kg at the normal boiling point), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climateby buffering large fluctuations in temperature.
The specific enthalpy of fusion of water is 333.55 kJ/kg at 0 °C, i.e. melting ice absorbs the same energy as ice warming from -160 degrees Celsius up to its melting point. Similarly the heat needed to melt ice at 0 °C, would heat the same amount of water by about 80 °C. Of common substances, only that of ammonia is higher. This property confers resistance to melting on the ice of glaciers and drift ice. Before and since the advent of mechanical refrigeration, ice was and still is in common use for retarding food spoilage.
SOLVENT PROPERTIES
Water is also a good solvent, due to its polarity. Substances that will mix well and dissolve in water (e.g. salts) are known as hydrophilic ("water-loving") substances, while those that do not mix well with water (e.g. fats and oils), are known as hydrophobic ("water-fearing") substances. The ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between other water molecules. If a substance has properties that do not allow it to overcome these strong intermolecular forces, the molecules are "pushed out" from the water, and do not dissolve. Contrary to the common misconception, water and hydrophobic substances do not "repel", and the hydration of a hydrophobic surface is energetically, but not entropically, favorable.
IONIZATION OF WATER
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−.
The hydrogen nucleus, H+, immediately protonates another water molecule to form hydronium, H3O+. It is an example of autoprotolysis, and exemplifies theamphoteric nature of water.
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−.
The hydrogen nucleus, H+, immediately protonates another water molecule to form hydronium, H3O+. It is an example of autoprotolysis, and exemplifies theamphoteric nature of water.