Biology Unit 3 – Water Aims: Aims: Must be able to describe and explain the key properties of water. Must be able to describe and explain the key properties of water. Should be able to explain the importance of water properties to cellular and organism function. Should be able to explain the importance of water properties to cellular and organism function. Could be able to properties of water to importance in living organisms. Could be able to properties of water to importance in living organisms.

Water The atoms in a water molecule (H 2 O) are not in a straight line – They are bent. The atoms in a water molecule (H 2 O) are not in a straight line – They are bent. The smaller hydrogen atoms are at about 5 and 7 O’clock on the larger Oxygen atom. The smaller hydrogen atoms are at about 5 and 7 O’clock on the larger Oxygen atom. Physical characteristics Physical characteristics 0°C - 100°C = Liquid  0°C = Solid  100°C = Vapour Liquid over a wide range compared to molecules of a similar weight. Liquid over a wide range compared to molecules of a similar weight.

The most important feature of the chemical behavior of water is its dipole nature (It has TWO charges). There is a small positive charge on each of the two hydrogens. There is a small negative charge on the oxygen. Small positive charge ++ Small negative charge – O HH A water molecule has the molecular formula H 2 O The Water Molecule

Covalent bonds Atoms of non-metals combine with other non- metal atoms by sharing pairs of valence electrons. Atoms of non-metals combine with other non- metal atoms by sharing pairs of valence electrons. The sharing of electrons form what we term a covalent bond. The sharing of electrons form what we term a covalent bond. Covalent bonds are strong and make the molecule robust. Covalent bonds are strong and make the molecule robust. Covalent Bond

Hydrogen Bonds The water molecule is polar – it has a pair of equal and opposite charges. The water molecule is polar – it has a pair of equal and opposite charges. ‘Weak’ Hydrogen bonds form between Hydrogen atoms of one molecule and oxygen atoms of another. ‘Weak’ Hydrogen bonds form between Hydrogen atoms of one molecule and oxygen atoms of another. ALL properties of water can be explained in terms of polar nature: ALL properties of water can be explained in terms of polar nature:

Importance of Water Solvent Solvent High Specific heat Capacity High Specific heat Capacity Latent Heat of Vaporisation Latent Heat of Vaporisation Latent Heat of Fusion Latent Heat of Fusion Density Density Cohesion Cohesion Ionisation – Salts dissolve ionising into positive and negative ions. Ionisation – Salts dissolve ionising into positive and negative ions. pH – Water is partly ionised providing protons for chemical reactions. pH – Water is partly ionised providing protons for chemical reactions.

Water as a Solvent Important for: Important for: –Chemical Reactions –Transport ALL polar (including IONIC) substances dissolve in water by hydrogen bonding to the water molecules. ALL polar (including IONIC) substances dissolve in water by hydrogen bonding to the water molecules. Large molecules (e.g. Proteins) form COLLOIDS (e.g. Cytoplasm). A colloid is a permanent suspension. Large molecules (e.g. Proteins) form COLLOIDS (e.g. Cytoplasm). A colloid is a permanent suspension. Non-polar molecules do not dissolve in water (e.g. fats, oils, phospholipids). The molecules either form small droplets (MICELLES) suspended in water (EMULSION) OR they form compartments (phospholipids form LIPOSOMES). Non-polar molecules do not dissolve in water (e.g. fats, oils, phospholipids). The molecules either form small droplets (MICELLES) suspended in water (EMULSION) OR they form compartments (phospholipids form LIPOSOMES). Water acts to transport solutes around the body (Animals and plants). Water acts to transport solutes around the body (Animals and plants).

High Heat Capacity of Water It requires a large amount of energy to raise the temperature of water (4.2Joules of energy to heat 1g by 1°C). It requires a large amount of energy to raise the temperature of water (4.2Joules of energy to heat 1g by 1°C). Water acts as a thermal buffer – minimises temperature changes due to the atmosphere. Water acts as a thermal buffer – minimises temperature changes due to the atmosphere. Provides a stable external environment (e.g. fish), and a stable internal environment (Human bodies stay warm for a long time). Provides a stable external environment (e.g. fish), and a stable internal environment (Human bodies stay warm for a long time). The polarity of the molecules holds them together and therefore resists rises in temperature. The polarity of the molecules holds them together and therefore resists rises in temperature.

High Latent Heat of Vapourisation Water requires a relatively high amount of energy to change state – Remains liquid over a wide temperature range. Water requires a relatively high amount of energy to change state – Remains liquid over a wide temperature range. This is exploited by animals in cooling mechanisms – e.g. sweating and panting. This is exploited by animals in cooling mechanisms – e.g. sweating and panting. As sweat goes from liquid to vapour energy (heat) is removed from the animal, cooling the body. As sweat goes from liquid to vapour energy (heat) is removed from the animal, cooling the body. This is again as a result of the hydrogen bonds between molecules. This is again as a result of the hydrogen bonds between molecules.

High Latent Heat of Fusion Large amount of energy is required to freeze water. Large amount of energy is required to freeze water. Very important as ice crystals damage cells. Very important as ice crystals damage cells.

Density Water is most dense at 4°C. Water is most dense at 4°C. The bottom of lakes rarely freeze. The bottom of lakes rarely freeze. Diagram: Diagram:

Cohesion and Surface tension Cohesion – Strong attraction of water molecules to one another. Cohesion – Strong attraction of water molecules to one another. Surface Tension - Water forms a skin at the surface - Diagram: Surface Tension - Water forms a skin at the surface - Diagram: Exploited in a number of ways: Exploited in a number of ways: –Environment for animals to live on (pond skaters). –Some animals hold to the skin (malarial parasites). Surface tension and cohesion is exploited by plants in water and nutrient transport. Surface tension and cohesion is exploited by plants in water and nutrient transport. Polar water molecules bind to the xylem walls in transpiration – cohesion and adhesion create a continuous column of water – Diagram: Polar water molecules bind to the xylem walls in transpiration – cohesion and adhesion create a continuous column of water – Diagram:

Water as a Reagent Water takes part in many reactions: Water takes part in many reactions: E.g. Water acts as a source of Hydrogen in Photosynthesis. E.g. Water acts as a source of Hydrogen in Photosynthesis.

Water Copy and complete a table like this using your notes and the textbook. Copy and complete a table like this using your notes and the textbook. Property of Water How it Helps Life Exist

Questions Pages 16 and 18 in the Biozone Student workbook. Pages 16 and 18 in the Biozone Student workbook.