Chapter 10 - Water: Essential to Life

1. Week 11, Lesson 2 Chapter 10 - Water: Essential to Life

2. A Molecule Essential for Life • Water is the most abundant liquid on Earth. • It covers 70% of our planet. • It moderates our weather, shapes our land and is essential to the existence of life. • The Earth’s position in the solar system means that it is not close enough for the water to boil, nor is it too far away for it to freeze.

3. The World’s Water

4. The Water Cycle • Water on Earth exists in solid, liquid and gaseous states and can be readily changed from one state into another. • The water cycle could be said to begin with evaporation from the oceans, solar energy being the main energy source. • Evaporation also occurs from lakes and rivers. • Human activities such as the combustion of fossil fuels, which produces steam, also contribute to the water cycle. • Water vapour in the air is transported around the globe until it condenses to form clouds. • It precipitates as rain water, or occasionally, as ice crystals in hail or snow and fall to the ground.

5. The Water Cycle cont…

6. Water and Living Things • Water is involved in the reactions of life: photosynthesis and respiration. • The overall process of photosynthesis can be represented by: sunlight 6H2O(l) + 6CO2(g)  C6H12O6(s) + 6O2(g) chlorphyll glucose

7. Water and Living Things cont… • In this essential reaction, water is absorbed into the cells of plants of algae and in the presence of chlorophyll and sunlight it is used to make glucose. • Cells of all oxygen-using life forms use glucose as their energy source. • The reaction between glucose and oxygen produce carbon dioxide and water. • In this way, water that was removed during photosynthesis is returned to the cycle. • The process of respiration is: C6H12O6(s) + 6O2(g)  6H2O(l) + 6CO2(g)

8. Functions of Water • Provides a system to transport nutrients and soluble wastes: water fulfils a similar transport role in animals and plants. The ability of water to act as a solvent and dissolve a wide range of materials is central to its functions in transporting materials in living things. • Provides a system to transfer heat: Water has the capacity to store a large amount of heat energy. Water transfers heat energy from the cells where it is produced in respiration, to the body’s surface where it is lost to its surroundings. • Provides a system to cool the body: skin is always moist. When water evaporates from the skin, it absorbs a relatively large amount of heat energy from the body and so cools it efficiently. Plants lose water from the surface of their leaves in a process known as transpiration.

9. Water and Climate • Water vapour, from the evaporation of waterways, precipitate to Earth as rain or snow. • Water can penetrate the surface and become groundwater, where it dissolves minerals from rocks and transports them to the oceans. • The balance of water on the surface runs off and empties back into rivers, lakes and oceans, where the cycle is completed. • The evaporation stage produces water free of dissolved substances. • Water is also a major means of distributing heat around the planet. • Warm water-laden tropical winds blow towards the poles, where they warm the rest of the Earth. • The ocean currents also transfer heat and disruptions in typical currents, such as the El Nino effect, can have disastrous effects, causing drought or major floods.

10. Week 11, Lesson 3 The Properties of Water • Water is described as a covalent molecular compound, with a molecular formula of H2O. • The oxygen-hydrogen bonds are polar, with the oxygen atom having the larger share of the bonding electrons, due to its higher electronegativity. • The forces between water molecules are hydrogen bonds, which are relatively strong on comparison to other types of intermolecular bonds.

11. High Melting and Boiling Temperatures • Water is the only substance which is commonly found in three states. • The graph below gives the melting and boiling temperatures of some other molecular substances that hand the same number of electrons.

12. High Melting and Boiling Temperatures cont… • Two observations can be made from the data in this figure: • Water exists as a liquid over a temperature range commonly found on the Earth. • The melting and boiling temperatures of water are significantly higher than those of other molecular substances of similar size.

13. Using the Bonding and Structure to explain this property… • In an ice crystal, the water molecules are arranged in a structured and regular manner, with each molecule forming hydrogen bonds directly to four other molecules. • For ice to melt, energy must be provided to increase kinetic energy of the water molecules to the point where they break free of the crystal lattice. • In the melting process, some of the hydrogen bonds break as the ice structure collapses to produce liquid water, which is roughly 10% denser than ice. • When water is boiled, the water molecules separate from each other and all hydrogen bonds are broken. • Water requires a significant amount of energy to overcome these bonds, thus the high melting and boiling temperatures.

14. High Latent Heat Values • Latent heat measure the energy needed to change the state of a substance at its melting or boiling temperature. • The latent heat of fusion of water is the amount of energy needed to change a fixed amount of water from solid to liquid at 0 degrees. • The latent heat of evaporation of water is the amount of energy needed to change a fixed amount of water from liquid to gas at 100 degrees.

15. Latent Heat Values cont… • These values of water are considerably higher than those of most other common molecular substances. • This means much more heat is needed to melt or boil water than for equivalent amounts of other substances. • The high latent heat values of water can also be explained in terms of the hydrogen bonds between molecules. • A relatively large amount of energy is needed to disrupt the lattice when some hydrogen bonds are broken. • An even larger amount is needed to break all of the hydrogen bonds and separate the water molecules to form a gas. • The high latent heat of vaporisation makes water a good coolant. • When you perspire, the water evaporates by absorbing heat energy from your skin. • This heat energy, 44kJ/mol of water, is transferred from your body to the evaporating water and you feel cooler.

16. High Heat Capacity • Several uses of water depend on its capacity to store large amounts of heat energy. • The specific heat capacity of a substance measures the amount of heat energy needed to increases the temperature of a certain amount (usually 1 gram) of that substance by 1 degree. • Water has a specific heat capacity of 4.2G/g/°C. • This means that 4.2 joules of heat energy is needed to increase the temperature of 1 gram of water by 1°C. • The higher the specific heat capacity of a substance, the more effectively it will store heat energy.

17. Finding Heat Energy • The equation below can be used to answer questions on heat energy: Heat Energy = specific heat capcaity (J/g/°C) x mass(g) x temperature change (°C) OR Heat Energy = SHC x mass x ΔT Heat energy is measure in Joules and is usually converted into kJ by dividing by 1000.

18. Expansion on Freezing • As liquid water is cooled, the water molecules move more slowly. • On approaching the freezing temperature of water, the molecules take up an arrangement typical of an ice crystal. • Each water molecule forms bonds with four other molecules. • This is a very open arrangement – the water molecules in ice are more widely spaced than in liquid water. • When ice melts, the organised structures are lost, and the molecules become free to change places, they move closer together and the density increases.

19. Density Density =

20. Week 11, Lesson 4 Water as a Solvent • Almost all biological and industrial processes occur in an aqueous environment, in the presence of water. • When substances are dissolved in water, the particles move freely and so interactions between reactants are very effective. • The polar nature of the water molecule enables water to dissolve a large number of substances.

21. Solutions • Solutions have the following important characteristics: • The are homogenous, that is the solute and solvent cannot be distinguished from one another. • The dissolved particles are too small to see. • The proportion of dissolved solute varies from one solution to another.

22. Water • More than half of the naturally occurring elements are found dissolved in the oceans of the world. • Seawater also contains smaller quantities of at least 40 other elements, including silver and gold. • It is estimated that the oceans of the world contain 10 billion tonnes of dissolved gold.

23. Why do substances dissolve? • When one substance dissolves in another, the following processes occur: • The particles of the solute are separated from one another. • The particles of the solvent are separated from one another. • The solute and solvent particles attract each other.

24. Why do substances dissolve? Cont… • A solute will dissolve if the attraction between the solute and solvent particles is strong enough to compete with the solute-solute and solvent-solvent forces of attraction. • Substances that dissolve readily in water have ionic or polar covalent bonding. They fall into one of the following three groups: • Polar covalent compounds that can form hydrogen bonds with water. • Polar covalent molecular compounds that ionise. • Ionic Compounds

25. Polar Covalent Compounds that can form Hydrogen Bonds • Some covalent molecular substances such as sugar and ethanol, dissolve easily in water. • These molecules are polar in character, usually through one or more –OH groups. • Ethanol contains a polar O-H bond, so there are hydrogen bonds between the molecules in pure ethanol.

26. Forming Hydrogen Bonds cont… • When ethanol is added to the water, hydrogen bonds form between the ethanol and water molecules and the ethanol dissolves. • Water and ethanol molecules mix freely with each other, held together in the solution by hydrogen bonds. • As the ethanol molecules are now surrounded by water molecules , the solution forming process can be represented by: H2O C2H5OH(l)  C2H5OH(aq)

27. Forming Hydrogen Bonds cont… • In summary, when ethanol bonds with water: • Hydrogen bonds between water molecules are broken • Hydrogen bonds between ethanol molecules are broken • Hydrogen bonds between water molecules and ethanol molecules are formed.

28. Forming Hydrogen Bonds cont… • The larger the non polar section of the molecule, the less soluble it is in water. • Non polar substances like oil, are insoluble in water because water molecule form hydrogen bonds with each other in preference to the weak attraction with non-polar molecules. • A way of recalling which substance dissolve is the rule ‘like dissolves like’. • Meaning, polar solutes will dissolve polar solvents and non-polar solutes will dissolve non-polar solvents.

29. Polar Covalent Molecular Compounds that Ionise • Some compounds have one or more covalent bonds that are so highly polarised that they break when the compound is placed in water. • These bonds break because of the electrostatic forces of attraction between the solute molecules and the water molecules. • For example, the polar hydrogen chloride molecules attract water molecules.

30. Compounds that Ionise cont… • This attraction results in the hydrogen-chlorine bond breaking. • Both bonding electrons go with the chlorine atom and an H+ forms a covalent bond with a water molecule. • In this way two ions are formed – the H3O+ and the Cl- ion • H3O+ is known a the hydronium ion. • This hydrolysis process is known as ionisation. • The HCl molecules have ionised in water. • The chloride ions and the hydronium ions will be surrounded by water molecules. • They are hydrated ions. • The ionisation process can be represented by the equation on the previous slide.

31. Compounds the Ionise cont… • In summary, when a polar covalent molecular substance that ionises in water: • Polar covalent bonds within molecules are broken, producing hydrogen ions and anions. • A covalent bond forms between each H+ and an H2O molecule, giving H3O+ ions. Ion-dipole attractions between the newly formed ions and the polar water molecules are formed. Other polar covalent molecular compounds that ionise in water include the common acids nitric acid (HNO3), sulfuric acid (H2SO4) and ethanoic acid (CH3COOH).

32. Ionic Compounds • Ionic solids contain positive and negative ions held in a 3D lattice by strong electrostatic forces. • When an ionic solid such as NaCl is placed in water, the positive ends of the water molecules are attracted to the negatively charged chloride ions. • The negative ends of the water molecules are attracted to the positively charged sodium ions. • Because water molecules are in a continuous state of random motion, they pull the sodium and chloride ions on the outer part of the crystal out of the lattice and into the surrounding solution.

33. Ionic Compounds cont…

34. Ionic Compounds cont… • When the sodium chloride has dissolved in the water, the sodium ions and chloride ions are surrounded by water molecules. • The ions are hydrated. • The attraction between an ion and a polar molecule such as water is described as an ion-dipole attraction. • When an ionic compound dissolved in water, positive and negative ions in an ionic lattice are separated from one another. • This process is known as dissociation.

35. Dissociation of Sodium Chloride

36. Ionic Compounds cont… • When writing equations for the dissociation of ionic compounds in water, the rules for balancing equations apply. • The charges on the right hand side of the equation must also balance. • For example, the equation for the dissociation of potassium carbonate in water is: H2O K2CO3(s)  2K+(aq) + CO32-(aq)

37. Ionic Compounds cont… • In summary, when an ionic solid dissolves in water: • Ionic bonds within the solid are broken • Hydrogen bonds between water are broken • Ion-dipole attractions between the ions and the polar water molecules are formed.

38. Solubility • Not all ionic compounds are water soluble. • For insoluble substances, the energy required to separate the ions from the lattice is much greater than the energy released when the ions are hydrated. The ions therefore stay in the lattice. • Although substances are described as ‘soluble’ or ‘insoluble’ that is a generalisation. • Substances described as insoluble tend to dissolve sparingly. • Those that are described as soluble dissolve to varying degrees. • The solubility of a substance also varies based on temperature.

39. Solubility Guide

40. Relationship between Solubility of Substances in Water and Bond Type