2.2 Water - 2.2 Water Essential Idea Water is the medium of life Soniya Nahata IB Biology(SL Understandings U1 Water molecules are polar and hydrogen

2.2 Water - 2.2 Water Essential Idea Water is the medium of...

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Unformatted text preview: 26/09/15 2.2: Water Essential Idea: Water is the medium of life. Soniya Nahata IB Biology (SL) Understandings: U1. Water molecules are polar and hydrogen bonds form between them: • Water: o Made up of two hydrogen atoms covalently bound to an oxygen atom. o While this bonding involves the sharing of electrons, they are not shared equally – it is a polar covalent bond. o The number of protons in each atom is different; oxygen atoms have 8 while hydrogen atoms have just 1. o As the oxygen atom has more protons, they attract electrons more strongly. o This makes the oxygen end of the molecule slightly negative and the hydrogen end becomes slightly positive. • Covalently bonded molecules that have a slight potential charge are said to be polar: o The slightly charged regions of the water molecule can attract other polar or charged compounds. o Water molecules can associate via weak hydrogen bonds. o Hydrogen bonds are transitory in nature – they constantly form, break and re-­‐form. • Hydrogen Bonds: 12 26/09/15 2.2: Water Soniya Nahata IB Biology (SL) U2. Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water: • Cohesive properties: o Cohesion: § The binding together of two molecules of the same type. (The binding together of two water molecules.) o Useful for water transport in plants. § Sucked through the xylem vessels at a low pressure. § Can only work if suction forces don’t separate the water molecules: Ø This doesn’t happen because of the hydrogen of the hydrogen bonding. • Adhesive properties: o Adhesion: § The sticking together of molecules of different substances. o Water molecules tend to stick to other molecules that are charged or polar for similar reasons that they stick to each other. o Again similarly individual hydrogen binds are weak, but large number of bonds gives adhesive forces great strength. • Thermal properties: o High specific heat capacity: § Increases in the temperature of water cause hydrogen bonds to break: Ø The amount of energy needed to raise the temperature of water is relatively large. § To cool down water must lose relatively large amounts of energy: § Water is a thermally stable habitat for aquatic organisms. 13 26/09/15 2.2: Water Soniya Nahata IB Biology (SL) o High latent heat of vaporisation: § It is the heat needed to form a water vapour. § Considerable amounts of heat are needed to evaporate water, because hydrogen bonds need to be broken. § Water is a good evaporative coolant. o High boiling point: § The highest temperature a substance can reach at its liquid state. § • Water has a high boiling point from 0ºC to 100ºC Solvent properties: o Water forms hydrogen bonds with polar molecules. o The negative oxygen pole and the positive hydrogen pole attract the opposite poles making them dissolve. o Cytoplasm is a complex mixture of dissolved substances in which the chemical reactions of metabolism occur. • Adhesion + Cohesion Diagram: 14 26/09/15 2.2: Water U3. Substances can be hydrophilic or hydrophobic. • Soniya Nahata IB Biology (SL) Hydrophilic (water loving): o The term used to describe substances that are chemically attracted to water. § All substances that dissolve in water are hydrophilic, including polar molecules such as glucose, and particles with positive or negative charges such as sodium and chloride ions. § Substances that water adheres to, cellulose for example, are also hydrophilic. • Hydrophobic (water fearing): o This term is used to describe substances that are insoluble in water: § Molecules are hydrophobic if they do not have negative or positive charges and are nonpolar. § All lipids are hydrophobic, including fats and oils. § Hydrophobic molecules dissolve in other solvents such as propanone (acetone). Applications: A1. Comparison of the thermal properties of water with those of methane: A2. Use of water as a coolant in sweat: A3. Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water: 15 ...
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