Consider the relative amounts of energy in different phases Continuously add heat to change a solid into a gas Substances exhibit the following sequence of temperatures Distinguish between Sensible heat causes a measurable change in the temperature of a phase Latent heat cannot be felt, but is stored in the form of a material and can be used to warm or cool its surroundings

Sensible heat - causes a change in the temperature of a phase by changing the kinetic energy of atoms or molecules in the phase Use heat capacity to measure a phase’s ability to store sensible heat Latent heat - cannot be felt, but is stored in the state or form of a material Measure the latent heat of a phase change by measuring how much the phase transformation cools or warms the surroundings Amount of heat released or absorbed = the latent heat of the phase change

Pure water - the water molecule H2O Each molecule consists of two hydrogen atoms and one oxygen atom held together by chemical bonds Three atoms in molecule share outer shell electrons in a single hybrid orbital These intramolecular bonds are covalent bonds Atoms joined by covalent bonds usually have distinctive molecular geometry

Pure water - the water molecule H2O Geometry is such that lines connecting H & O atom centers make 105° angle Molecule shape = regular tetrahedron Hybrid orbital has four lobes of high electron density protruding from O atom H atoms sit in two lobes Other two lobes simply are regions of high electron density

Pure water - the water molecule H2O Sharing of outer shell electrons is not equitable O atoms exert a greater attraction on electrons, so electron density is higher in O lobes H atoms exert lower attraction on electrons, so electron density is lower in H lobes Molecule is an electrical dipole, with regions of weak positive charge and regions of weak negative charge

Polar water molecules are held together by an electrostatic force The weak intermolecular bond that holds water molecules together = hydrogen bond

Hydrogen bonds are responsible for many of water’s unusual physical properties

Physical properties of water, I Water melts at much higher temperature & boils at much higher temperature than other covalently bound but non-polar compounds with similar molecular weight CH4 - molecular weight = 16; melting temperature = -184°C; boiling temperature = -161°C H2O - molecular weight = 18; melting temperature = 0°C; boiling temperature = 100°C Hydrogen bonds cause molecules to cohere at higher temperatures

Physical properties of water, II Hydrogen bonds contribute to H2O’s large latent heat of fusion (= 80 cal/g) Hydrogen bonds contribute to H2O’s very large latent heat of vaporization = 540 cal/g Solar radiation (insolation) reaching earth may be stored as latent heat in water at earth’s surface On moon, noon temperature = ~135°C; on earth highest temperature = 57°C On moon, midnight temperature = ~-155°C; on earth coldest temperature = -68°C

Physical properties of water, III In liquid water, H2O molecules tend to cluster; in clustered state, H2O molecules are held into weak structure by hydrogen bonds Clusters form & break up very rapidly - 1010 or 1011 times a second When one adds heat to water, much of the energy goes to altering the inter-molecular structure rather than to increasing the kinetic energy of H2O molecules Water has an extremely high heat capacity = 1 cal/g/°C

Physical properties of water, IV In crystalline water, H2O molecules bond together in a structure characterized by open, six-sided rings; the shape is dictated by the tetrahedral shape of H2O molecule

Crystal structure of ice looking down its ‘c-axis’ (axis of hexagonal symmetry)

Ice structure looking oblique to c-axis

Physical properties of water, IV • In crystalline water, H2O molecules bond together in a structure characterized by open, six-sided rings; the shape is dictated by the tetrahedral shape of H2O molecule • The volume filled by 24 H2O molecules in ice = the volume filled by 27 H2O molecules in water Ice is less dense than water, & ice floats on water In fresh water, find that the maximum density of water occurs at 4°C; between 4°C and freezing temperature water becomes less dense as more of it becomes structured Ice is weak - bonds holding structure together are weak hydrogen bonds

Aqueous solutions Water is an excellent solvent Polar molecules of water can attract & hold ions & other polar molecules At any temperature or pressure, there is a limiting concentration of any dissolved component called the solubility of the component When the concentration reaches the solubility, we say that the solution is saturated with respect to that component

Aqueous solutions, II At saturation, molecules or ions may enter or leave the solution, but the total concentration does not change (have dynamic equilibrium) Most substances are more soluble, i.e. have higher equilibrium concentrations, at higher temperatures Gasses also dissolve readily in water Dissolved gasses will not escape from water unless we change the temperature or pressure of the water or alter the composition of the air above the water The solubilities of gasses are usually lower at higher temperatures

An interesting practical example, I Consider the solubility of carbon dioxide CO2 + 3H2O = HCO3–1 + H3O+1 + H2O = CO3–2 + 2H3O+1 Increasing pressure drives CO2 into solution i.e., HCO3–1 & CO3–2 are more stable at higher pressures Lower temperatures also lead to increase in the solubility of CO2 HCO3–1 & CO3–2 can attain higher concentrations in colder water

An interesting practical example, II Consider the solubility of calcium carbonate CaCO3 = Ca+2 + CO3–2 Concentration of carbonate anion (CO3–2) is buffered by the amount of CO2 in solution Increasing pressure enables sea water to have greater concentrations of HCO3–1 & CO3–2 (the products of the solubility reaction) CaCO3 is more soluble at higher pressure Similar effect occurs with decreasing temperature CaCO3 is more soluble in cool water than in warm water Altogether, CaCO3 is increasingly soluble at greater depths

Carbonate compensation depth (CCD) CaCO3 shells (tests) sink from surface waters Tests may reach a depth where water is significantly undersaturated with respect to CaCO3 At this depth, called the lysocline, shells begin to dissolve In the modern oceans, there is also a depth at which there is no longer any free CaCO3 This depth, called the carbonate compensation depth (CCD) = ~ -4 km CaCO3 tests accumulate only if they settle on sea floor above the CCD

Sea level * * * * * CCD Ocean floor

Effects of dissolved substances on water properties Little effect on heat capacity, latent heats of melting or evaporation, or water clarity Density of water with dissolved substances is greater than that of pure water Water with dissolved substances exhibits a freezing point depression & a boiling point elevation The greater the concentration of dissolved substances, the more pronounced these effects Water with dissolved substances freezes or boils over a range of temperatures rather than at a single temperature